TAKAHISA NEMOTO
The genus Thysanoessa was established by Brandt for Thysanoessa longipes. These diagnoses were again well examined by Hansen in 1911, who gave the refined generic description (Hansen, 1911). There have been nine or ten species of Thysanoessa euphausiids considered up to these days, and the morphology and ecology on those species have been contained fragmentaly in many previous papers on euphausiids (Einarsson, 1945:  Boden, Johnson & Brinton, 1955 :  Brinton, 1962b :  Ponomareva, 1962 :  Nemoto, 1959 : 1962 : etc). This study is mainly concerned with the comparative morphology, relative growth (Allomorphosis or interspecific relative growth allometry) and discussions on the characteristic in distributions and systematic relationships of each species among the genus and other euphausiids. In the morphological study, general external points considered as important for taxonomies and other studies are examined, and some other internal character- istics such as structures of the stomach, are also investigated. The allomorphosis among different species has been examined to consider the ecological and phylo- genetic characters of each species too.
MATERIALS AND METHODS
MATERIALS
The materials of Thysanoessa have been collected through the following expeditions in the Antarctic and the North Pacific as shown in Table 1. They are mainly composed of two different lines of works, namely plankton net samplings and collec-
** Not all samples contained Thysanoessa euphausiids.
109
tions of euphausiids from the stomach contents of baleen whales. MEASUREMENT OF BODY PARTS The measuring positions are illustrated in Fig. 1, in which each indication is measured  with  built-in  micrometer  in  binocular  microscope.   As for the length of carapace the cephalothorax length is considered as the common measureing posi¬ tion. But it is rather convenient to measure the edge along the lower margin of the carapace, to compare the ratios of length between anterior top and carapace denticle with the length of the carapace, and without dissection of the body. It may be called ' carapace margin length ' in this report. The mature specimens are measured for the relative growth study. Those are males having fully developed external copulatory organs and distinct spermato- phores in the reproductive organs or ducts and females with spermatophores in their thelycums or having fully developed thelycums (Einarsson, 1945). From 5 to 15 specimens of males and females have been measured in each body length group for the allomorphosis study. About 100 specimens are selected from the mature or copulated samples, for the measurement of body length and growth, and the geographical cline examination in T. inermis.
KEY TO THE ADULT THYSANOESSA EUPHAUSIIDS
The following simple key may be applied as expressing the taxonomical characters for Thysanoessa euphausiids.
T. longipes
Eye has clear transverse constriction above the middle. Second pair of legs elongated. 2a Lateral  margin of carapace has denticle. 3a Lateral denticle in the margin of carapace situated approximately at the middle of the carapace. 4a Strong dorsal keel and spines present at third and fourth abdominal segments. la 3b Denticles situated considerably posterior part of the lateral margin. 5a Keels absent in the dorsal segments. 6a Sixth abdominal segment is much shorter than the sum of preced- ing two segments. 7a Setae on carpus and propodus of the first to third thoracic legs are as long as those of dactylus. T. gregaria Setae on carpus and propodus of the first to third legs are 7b short and fine compared with those of dactylus. T. parva Sixth abdominal segment is longer than or as long as the sum of 6b the two preceding abdominal segments. The end of terminal process of male copulatory organ is 8a serrated. Antennulae with the upper flagellum somewhat or considerably longer than the sum of distal peduncular joints. T. vicina The end of terminal process of male copulatory organs is not serrated. Antennulae with the upper flagellum some¬ what or considerably shorter than the sum of distal two joints. T. macrura 8b 5b Keels present on third to fifth segments. T. inspinata T. longicaudata T. raschii Lateral margin of the carapace lacks the denticle. 2b Eye has only slight transverse constriction or without clear constriction. Second pair of legs are not so elongated or not elongated. 9a Lateral margin of carapace with lateral denticle. lb 9b 10a segments. Lateral margin of carapace without lateral denticle. Dorsal keels present from first to fifth abdominal T. spinifera Dorsal keel absent, only spines on fifth and sixth or sixth abdominal segments. T. inermis 10b
FIRST ANTENNAE The first antenna, the peduncle and flagellum are often considered as sexual and taxonomical characteristics. In Thysanoessa euphausiids, they are rather simple TABLE 2.  RATIO AND NUMBER OF ARTICULATION OF ANTENNULAR FLAGELLUM IN THYSANOESSA EUPHAUSIIDS
and Hansen (1911) used the length of antennule with the upper flagellum as the taxonomic character for the closely related Thysaneossa macrura and T. vicina. Three types of flagellum, longer, intermediate and shorter types are observed as shown in Fig. 2. The number of the articulations in upper and lower flagella are also divided into two or three groups. The flagellum of longer group is rather tough in preserving, but those of In the shorter group are fragil especially in the intermediate species T. longicaudata. longer groups, the articulations are ranging from 20 to some 60 and lengths of longer flagellum are almost the same or longer than the peduncles. On the other hand, the shorter group has less than 20 articulations and the flagella are shorter than the peduncles. The distal fringes of flagellum are almost the same among specimens of Thysanoessa cuphausiids. They have usually four small setae in the distal ends of the upper and lower flagella. One of the four setae has broad and tough base which sometimes two storied structure. The sexual dimorphism in the peduncles of Thysanoessa euphausiids is typical in T. inermis and T. spinifera. UEinarsson (1945) considered the possible relationships of T. inermis to the Pacific species T. spinifera which has the antennule armature of which bears close resemblance to that of T. inermis. The male T. inermis has a rounded lobe on its inner side, in the first antennular segment, bearing slightly curved spines (Einarsson, 1945). The relation between the longer flagellum and the carapace length as allomor- phosis gives three allometric tribes as shown in Fig. 3. The longer group, namely T. inermis, T. spinifera and T. raschii form one tribe, and T. longicaudata which has intermediate number of articulation between shorter and longer groups, forms another point alone. The shorter group of other species runs the same re- T. longicaudata ecologically bears the wider gression both in males and females. distribution from temperate and boreal Atlantic Ocean to the Arctic, from the atlantic by 80°N (Einarsson, 1945), than other Thysanoessa euphausiids belonging to the shorter group. On the other hand, it also covers the neritic and other shallow waters like the longer group.
CARAPACE
The carapace of the Thysanoessa group is various, but the rostrum is always well developed in every species, which is one of the generic character of the Thysanoessa (Hansen, 1911).  All species have keels in the center of rostrum part of the carapace, longer or short, and seven species out of ten have denticles in the lateral margin of the carapacc. The general outlines of each species are illustrated in Fig. 4 which do not differ from those of other euphausiids. Rostrum and keel All Thysanoessa species have well developed spear pointed rostrums, but T. Other Thysanoessa gregaria, T. raschii and T. inermis have rather broad oblong tops. have acute distal ends when they are pressed on the deck glass for observations. Thysanoessa spinifera have the most acute triangular, and longest rostrum, which is narrower than other Thysanoessa euphausiids.   Three species, T. longipes, T. vicina and T. macrura have broader tryangular rostrums. The tip of the rostrum of T. The general shape longipes is somewhat slender and differes from other two species. And it is also inter- of the rostrum of T. macrura is closely related to that of T. vicina. esting that the rostrum of T. raschii is similer to T. gregaria, which is different cons¬ siderably in other external points. T. gregaria has oblong and broader rostrum, and the shape of the keel in rostrum is different from T. raschii. The vertical sections of the keel in the rostrum and the carapace show also two types. The keels in the rostrum of T. inermis and T. raschii are rather gentle crest ridges, but T. longipes, T. The gregaria, T. vicina and T. macrura have the sharp ridges as shown in Fig. 5. geographical variation in rostrum of carapace in Euphausia diomediae is reported by Hansen (1911). Some taxonomical importance in the strech of rostrum in Thy- sanopoda, T. aequalis and T. subaequalis, is also obtained (Boden & Brinton, 1957).    No geographical difference is observed in Thysanoessa euphausiids in rostrums in this studies. The edge in the cove between rostrum and the anterior top of the margin of the carapace swells out in the larger Thysanoessa, T. macrura, T. inspinata, T. raschii and T. inermis. T. spinifera has a But it is not so clear in T. gregaria and T. vicina. The sexual dimorphism is prominent supraorbital spine in the top of the swell. not observed in all species but T, raschii, the male of which has broader rostrum than females. In the younger stages of Thysanoessa, the rostrums are also well developed. But their tip ends are not so pointed and they have rather blunt ends. The anterior top of lateral margin of the carapace Thysanoessa euphausiids have pointed ends except T. spinifera which has two points in the anterior top of the carapace along the supraorbital cove. As illustrated in Fig. 6, T. gregaria, T. longipes and T. raschii have all pointed ends, the group T. spinifera has one keel like spine also includes other species but T. spinifera. The end of the slightly upper positions of the end of the margin of the carapace. margin of the carapace also makes point, from which two streaks run along the edge of the lateral margin of the carapace.
Lateral denticle
Seven species out of ten in Thysanoessa have lateral denticles in the lower margin of the carapace, and other three species completely lack the denticles. Lateral denticle present Thysanoessa raschii T. T. longipes, T. gregaria, T. macrura, vicina, T. inspinata, T. parva. Lateral denticle absent T. inermis, T. spinifera, T. longicaudata. All species are bearing only one denticle in one side margin of the carapace. The anterior top of the lateral margin of the carapace in Thysanoessa euphausiids. A-T. gregaria, B-T. longipes C-T. raschii, D-T. spinifera. There are two types of lateral denticles of carapace. One is Thysanoessa raschii bearing a acute spine in the anterior part of which is also rather completely free from the margin of the carapace. On the other hand, T. longipes has the acute but narrower base spines which directly rise up from the margin of the carapace in the posterior position. T. gregaria, T. macrura, T. vicina and T. inspinata all show latter characteristic and T. parva also possibly belong to this group. In one specimens of T. raschii a deformed denticle which has two character of them is observed as illustrated in Fig. 7. T. gregaria, T. vicina and T. inspinata have sometimes very incomplete lateral denticles. Especially, T. gregaria sometimes completely lacks them.   Samples of T. gregaria collected in KT-64-17 and T. macrura collected in the Antarctic also lack the lateral denticle in the lower margin of the carapace. In other species of euphausiids, the local difference of the presence of this denticle is reported on Nematoscelis tenella. The male Nematoscelis has this denticle in the Atlantic, but it is absent in Pacific specimens. (Boden, Johnson & Brinton, 1955). Recently Mclaughlin (1965) described two forms, denticled and original forms of Nematoscelis difficilis.  Mclaughlin described that the prominent lateral denticle, varying somewhat in size, occurres on the posterior third of the inferior margin of the carapace about Nematoscelis difficilis denticle form, (Mclaughlin, 1965), but it has another ' no-denticle ' form.
Position of lateral denticle
The position of lateral denticle is a very imporfant taxonomic characteristic especially on Thysanoessa longipes and T. inspinata (Nemoto, 1963). Hansen also used the position of the lateral denticle for the taxonomic considerations on Thysanoessa groups. He (1911) stated that lateral margins of the carapace with a denticle at the middle on Thysanoessa longipes and denticles situated before the middle of the lateral margin of the carapace on T. raschii. As for Thysanoessa gregaria group, that the denticles is conspicuously posterior the middle of the lateral margin, is also noted as the taxonomic character by him. deviations are illustrated. The positions of lateral denticles expressed as ratios, denticle length / carapace margin length are shown in Fig. 8. F There are fairly good expressions of the species range, which also confirmes the taxonomic importance of them. And there is few difference between males and females in all species. The denticles of T. longipes is situated at the middle of the lateral margin, but T. inspinata, the closely related species of the T. longipes, has its denticle in far pos¬ terior position as illustrated in Fig. 8. The position of the denticles in T. macrura and T. vicina are rather similer in their positions, in which T. vicina shows slightly posterior position of denticles. In other closely related euphausiids species, Euphau- sia pacifica and E. nana established by Brinton (Brinton, 1962a) also show such tenden- The larger E. pacifica has the comparatively anterior lateral denticles as shown cy. in Fig. 9, and E. nana from the East China sea has posterior lateral denticles. There is very clear difference between typical E. pacifica in the northern waters and E. nana in the Eastern China sea, however, some vague samples have been collected from the intermediate regions in the southern waters of the sea of Japan. These samples collected from Tsushima waters are now under examination. Allomorphosis in the position of denticles The allomorphosis in the relation between denticle length and carapace length There is no difference between males and for Thysanoessa group is given in Fig. 10. females in all species. In a previous report (Nemoto, 1963), I noticed the clear different allometry lines in T. longipes and T. inspinata which is one of the critical point for dividing those two species. The relative growth coefficient for T. longipes is 0.973 and T. inspinata 1.19. Thysanoessa macrura and T. vicina draw the same al- lometry line, the growth coefficient of which is 1.000. Thysanoessa gregaria shows growth coefficient 0.925 which differs from those Thysanoessa raschii draw the line apart from other species as of other Thysanoessa. shown in Fig. 10. Those growth coefficients are as follows. Thysanoessa raschii 1.034 longipes (adult) 0.973 1.000 macrura & vicina 0.925 gregaria
EYE
Shape and arrangement The eye of Thysanoessa euphausiids are usually higher than broad with trans¬ verse constrictions or sometimes almost circular (Hansen, 1911 :  Bodon, Johnson & Brinton, 1955). The ratio, lateral length/vertical length and the general shape of eyes are illustrated in Fig. 11. Thysanoessa raschii has almost completely circular and broad eye, and the ratio of lateral and vertical length is about 95%. But from the side view, the upper part is rather compressed as compared with Euphausia which is almost circular. T. spinifera agrees with T. raschii in this point showing the ratio T. inermis has a little higher 95%, and the upper half of the eye is also compressed. eyes showing the ratio about 87%, but no heavy constriction is observed in the fully developed specimens. The compression in the upper part in T . inermis is heavier than T. raschii and T. spinifera. As it has been discussed by Hansen (1911), and Einarsson (1942), the younger Many examples of the specimens of T, inermis shows more higher than broad eyes. younger juvenile stage after the furcilia possess more typical Thysanoessa eyes also in T. inermis, but T. longipes of the same vertical size shows the narrower upper part of the eye and constriction. The ratio, lateral/vertical length is about 81% in T. inermis and 78% in T. longipes, latter value of which is not so different as those in adults.
The general arrangement of the eyes in euphausiids are shown in Table 3. Thysanoessa euphausiids are divided into two groups. One is having the con¬ T. raschii having rounded stricted eyes group and the other is round eyes group. eyes is living in the shallow waters, and T. spinifera also is mainly living in the shallow waters although it is sometimes found in the deeper waters along the edge of coastal regions (Banner, 1954). Other euphausiids bearing constricted eyes live in broader range in vertical Thysanoessa, Tessarabrachion, Nematoscelis, Nematobrachion, distribution in the sea. and Stylocheiron have the constricted eyes. Those species have rather bigger eyes and the range of the vertical distributions attain deeper waters in the sea except some Antarctic and Arctic species of Thysanoessa. The upper and lower part of the con- stricted eyes are sometimes about same size and sometimes different. In Thysanoessa, the lower part is generally bigger than the upper part. In Stylocheiron and Tes- The lower parts sarabrachion, the lower parts are also larger than the upper parts. The of some species are larger than the upper but smaller in other Nematoscelis. upper parts are generally larger in Nematobrachion euphausiids.
The size of the eye
The male has larger eyes than females in general comparing with the body The series of the relative size indicates that the smaller the body length is, size. the larger the eye is among the gregaria group, which has constricted eyes in adults. And this is not the same in T. spinifera, T. inermis and T. raschii.
Allomorphosis in eyes
The relation between vertical and lateral lengths of eyes in adult specimens There are three or four allometry groups are plotted in chart as shown in Fig. 12. both in males and females group. T. macrura and T. longipes form the same line, the coefficient of relative allomor- phosis of which is about 1.2. But T. gregaria and T. vicina shift the line, although the constriction in the upper part of eyes is positive, and the growth coefficient of the allometry is ranging 1.33-1.57. T. spinifera, T. inermis and T. raschii also form one allometry group, but T. raschii shifts to a little high initial growth index in females as illustrated in Fig. 12. The relative growth between the vertical length of the eye and carapace length is very various as illustrated in Fig. 13. There are many allometry lines in males. Only T. macrura and T. longipes take the same line, but T. gregaria, T. vicina, T. ras- chii, T. inermis and T. spinifera occupy different allometry lines respectively. That T. gregaria, T. vicina, and T. macrura differ considerably in the relative growth indices between carapace length and the vertical length of eyes is possibly due to two reasons. The The relative growth for the carapace in T. longipes is the same as T. macrura. body length of T. longipes is about the same as T. macrura, and they are both living in cold pelagic waters. They are considered to be surface living euphausiids shal- lower than 500 meter and the ecological position in the sea may be related with each other both in northern and southern hemisphere. TABLE 4.  RELATIVE GROWTH COEFFICIENT IN ALLOMORPHOSIS BETWEEN
On the other hand, T. gregaria group, T. gregaria, T. macrura T. vicina arc dif- ferent considerably in males. in Thysanoessa euphausiids, the sexual difference in the body length of males and females is very clear in T. gregaria and other species which have bilobate eyes. The type of eyes of females in those species are that of younger stages of the same species eyes of males especially in T. gregaria and T. vicina. The upper part of bilobate eye of males swells like horse bean but guiter-shaped in females. This character is not so clear in T. macrura, T. longipes and T. long- icaudata. In the younger stages of some Thysanoessa euphausiids, eyes are rather related to each other even in the T. inermis. (The eye of T. inermis in about 5 mm specimen is nearly the same with that of T. longipes in the same body length as il- lustrated in Fig. 14 and former discussions. This shape is that of females in adults in T. gregaria group as illustrated also in Fig. 14. The female type is becoming to the younger type is already pointed out as for many other species as the evolution in speciations.   This sexual difference may be firstly connected to the step of the development of variations in the body parts (Eimer, 1890). Brinton (1962b) considered the evolution and local forms of Stylocheiron affine and S. longicorne in the Pacific areas according to the combinations of the variations in the ratios, length of sixth abdominal segment divided by depth and width of lower section of eye divided by width of upper section. This combination is also checked for Thysanoessa cuphausiids in which possible relationship among species is also sug- The inter specific differences in the ratios in those points nearly coincide gested. with other characters. Among Thysanoessa euphausiids, T. inermis, T. raschii and T. inermis make group apart from other species, although species contained show sexual dimorphism. Ex- cept T. gregaria, other four species form group of high ratio in the upper and lower parts of eye and fifth and sixth abdominal segments. The former group scarcely show the differences in two points, but T. gregaria and T. vicina show the sexual dimorphism in the ratio of abdominal segments.  T. longicaudata and T. longipes show little dif- ference in the ratio of eyes. On the other hand, T. gregaria shows heavy sexual dimorphism in the ratio of eyes.
ABDOMINAL SEGMENTS
Keel and abdominal spines
The arrangement of keels and abdominal spines are given in Table 5. Among nine species of Thysanoessa euphausiids treated here, three species have
K :   Well developed dorsal keel.   SP:   Well developed dorsal spine. Smalls show the presence of keel or spines, but they are not well developed. Extraordinary ones are in bracket. dorsal keels and four species have abdominal spines.    Dorsal keels are well developed in T. spinifera, T. longipes and T. inspinata. Thysanoessa raschii, T. gregaria,T. parva, T. vicina and T. macrura have no developed keel and no spine. T. inermis has only abdominal spines on fifth and sixth segments or sixth segment only, but some ex¬ Two close- traordinary specimens have also keels and spines on fourth segment. ly related species, T. longipes and T. inspinata have keels on third, fourth, fifth seg¬ ments, and spines on sixth segments.  But well developed abdominal spines are Thysanoessa present on third, fourth and fifth segments of T. longipes alone. longicaudata has no clear keel and spine but it is considered to belong to T. longipes group. In some younger furcilia stage 5, slight keels are described on third and fourth abdominal segments by Einarsson (1945) and some present specimens from the station M also bear keels on fourth and fifth segments. Thysanoessa gregaria and three other gregaria type euphausiids are without any keels and abdominal spines. Thysanoessa spinifera has keels on the first, second, third, fourth and fifth segments and abdominal spines of fourth, fifth and sixth segments.    The presence of clear keels on first and second segments is only found in T. spinifera. Two and one spine forms of Thysanoessa inermis In a previous report, (Nemoto, 1959), I described the possible difference be¬ ween one and two spine forms of Thysanoessa inermis in the Atlantic and the Pacific. According to Hansen (1911), the majority of specimens taken in the Pacific has a dorsal spine which semetimes proportionaly long, sometimes rather short, and some¬ times nearly or completely rudimentary, in the fifth abdominal segment and in rare case wanting.   On the other hand, Einarsson (1945) stated none of the numerous specimens examined by him had shown even the slightest sign of a spine on the fifth segment in the Atlantic. Hansen (1911) also saw some specimens, taken off Cape Cod, with a conspicuous spine, while in another it was rudimentary I found none of the two spine form in the sample of Okhotsk Sea in my previous investigation, (Nemoto, 1959). I have examined many specimens of Thysanoessa inermis in the Okhotsk Sea after then, and the average values 16,7% in females and 11.1% in males are obtained by present examination as the percentage occurrence of two spine Further the occurrences of two and one spine forms of Thysanoessa inermis are form. examined on the materials collected from the Bering Sea to the waters of the Gulf of Alaska in the North Pacific. The occurrences of one and two spine form of Thy- sanoessa inermis are shown in Fig. 16. The two spine form is about 50 to 60 percent of the total in the waters off Kamtchatka, and about 60 to 70 percent in the eastern waters of the Bering Sea. The two spine form occupies about 75 or more percent in the waters of the Gulf of Alaska. This geographical cline is more appreciable when we consider the lower occurrences of two spine form in the Okhotsk Sea. This type of geographical cline, specific cline (Huxley, 1940) is also found in a plaice Platichys stellatus (Pallas) in the North Pacific.   Platichys stellatus has the eyes in the right side of the body. But it has eyes in the left side from the specimens in the western Pacific and Japan. The percentage occurrence of left side eyes is about 70% from the specimens collected from Alaskan Peninsula and about 50% in the California. The occurrence of the two spine form of T. inermis is somewhat or con¬ siderably lower than those reported in the former report, (Nemoto, 1957), and this is attributable to the number of specimens examined and the selection of samples. The younger T. inermis lacks completely abdominal spines of fifth and sixth segments. As a example, small size specimens collected at 53-40N, 165-20W, The average length is 8.2 mm in which both abdominal spines are arc examined. undeveloped. Slight protrusion is observed on sixth abdominal segment at the average body length of 9.4 mm. The well developed abdominal spine is observed in samples of 10.5 mm long. So, it is desirable to treat the adults or well developed specimens to compare the occurrences of two and one spine forms of T. inermis as it is done here.
Three spine form of T. inermis
The three spine form of Thysanoessa inermis is reported in 1962 (Nemoto, 1962). Two new female specimens are found among samples collected at 52-53N and 162- 12E, which also bear the abdominal denticles on the fourth segments. One spec- imen has strong spines on fourth, fifth and sixth abdominal segments with well developed keel on fourth and fifth abdominal segments. The another one is without The body length are keels but has spines on fourth, fifth and sixth segments too. 21 and 18 mm respectively and copulated. Einarsson (1945) stated that " I will here confine myself to pointing out the possible relationship of this species to the Pacific species Thysanoessa spinifera Holmes, which has dorsal spines on the three last abdominal segments and the antennal armature of which bears close resemblance to that of T. inermis ". It is very inter- esting to note that one keel on the fourth abdominal segment is higher in the middle position and resembles rather to that of T. spinifera than to T. longipes. Allomorphosis in length of abdominal segments The sixth abdominal segment has been generally used as a taxonomical char- acter to some extent for the species of euphausiids. It has been used as the combined or direct comparison with fifth or fifth and fourth abdominal segments. Here the length of sixth abdominal segments is plotted against the carapace margin length in logarithmic chart in Fig. 17.11FT. macrura and T. vicina form one allometric tribe and T. longipes, T. inspinata and T. raschii form another tribe. T. longicaudata be- longs to T. macrura tribe in males but it shifts in females slightly. T. spinifera and T. inermis occupy the same allometric tribe in females but they take different lines in males respectively. The growth T. gregaria is one species which forms allometric tribe by itself. constant values are about the same for every groups except male T. spinifera. It is very interesting to see that the T. raschii which differs consideraly from T. longipes group in other external characters form the same allometric tribe with T. longipes in sixth abdominal segment for carapace. As the next step, the length of sixth abdominal segments are plotted against the length of fifth abdominal segments as shown in Fig. 18. Like above case, T. macrura and T. vicina form the same allometric tribe, but T. longipes, T. inspinata and T. longicaudata make another tribe.   Other T. gregaria, T. inermis, T. raschii and T. spinifera draw respective allometric tribe by themselves. There is no difference be- tween males and females of each species. The telson attached to the sixth abdominal segment is considered as one swim- The length of telson against sixth abdominal segment in log- ming apparatus. arithmic chart is given in Fig. 19. T. macrura, T. vicina and T. longicaudata form the same allometric tribe and growth constant. T. longipes and T. inspinata make the same tribe and T. inermis T. gregaria and T. spinifera make respective al- and T. raschii make another one. lometric tribe by themselves.
PREANAL SPINES
The preanal spines of euphausiids sometimes have been considered as a taxonomic character. The illustrations of them in Thysanoessa have been drawn for Thy- (Boden, Johnson & Brinton, 1955) and T. macrura sanoessa longipes and T. gregaria. The latter is also drawn for the sexual dimorphism from (Nemoto & Nasu, 1958). The preanal spines of available species of Thysanoessa group are the young stages. These spines are divided into following three groups. illustrated in Fig. 20. Typical sexual dimorphism present Slight sexual dimorphism present Sexual dimorphism nearly absent T. macrura, T. gregaria, T. vicina T. inermis, T. raschii, T. spinifera T. longipes, T. inspinata, T. longicaudata In a strict sence, the sexual dimorphism is existing also in the third group. As it is illustrated in Fig. 21, the male Thysanoessa longipes has rather acute and thick spine. On the other hand, females have rather long and curved spines. In all species in which the sexual dimorphism is observed, females have the deep denticulations in the concave part of the edge of preanal spines. Males have rather simple spines with a few small denticles along the same part. This group includes T. gregaria, T. macrura and T. vicina possibly including T. parva. Thysanoessa raschii and T. inermis show slight sexual dimorphism, but T. inermis shows more distinct differences. Euphausia val- In other euphausiids, these sexual dimorphism is also observed. lentini in the southern hemisphere has heavy denticulated preanal spines in females and small denticles along the inner margin of the preanal spines in males. Another example is given by Boden, Johnson & Brinton (1955) on Nematobrachion flexipes. They stated the preanal spine is simple in males and bifid in females, and the male preanal spine is also simple in Nematoscelis difficilis (Mclaughlin, 1965).
THORACIC LEGS
The general arrangement of Thysanoessa is illustrated in Table 6. Three species of Thysanoessa, have non-elongated second legs and thoracic legs are similar in struc-
ture of setae and spine as the skimming apparatus for foods. Other seven species have elongated second legs, and rather degenerated fifth and sixth thoracic legs. The former group may be connected to genus Euphausia and the latter may resemble to the other groups in the thoracic legs, such as Tessarabrachion, Nematoscelis, Nematobrachion and Stylocheiron species.   Of course, there is not enough data for the feeding mechanism of cuphausiids, it scems that euphausiids which have elongated thoracic legs are carnivorous more or less or polyhagous as Thysanoessa species. Especially Nematobrachion species are considered as typical carnivorous euphausiids mainly feeding on copepods in the deeper layer of the sea. The possible relation between the eyes and feeding apparatus for the living environment is now under examination, and here the allomorphosis in Thysanoessa euphausiids in thoracic legs and morphology of dactylus of first and second legs which have special feeding char- acter are investigated. Allomorphosis in the total length of legs The first legs show three allometric tribes when they are plotted against car¬ apace margin length. T. macrura, T. longipes, T. inspinata, T. vicina, T. longicaudata and T. gregaria are making one allometric tribe, which closely corresponds to the shorter spine group in the dactylus of the first logs. The next group is T. spinifera which form the tribe by itself both in males and T. raschii and T. inermis are considered to form the same allometric tribe females. although T. raschii shows some vague points in females. In second legs, the allometric tribes are given for the length of merus. Al- though T. longipes stands a little apart from the main line of the shorter group of the dactylus in the first leg, they fairly well coincide with the allomorphosis in the first dactylus. In third and fourth legs, the wide band includes all species of Thysanoessa as illustrated in Fig. 24.   In these figures, T. inermis and T. raschii form the oblique line which cross the main line. This is the same tendency in the first leg, and T. inermis and T. raschii form the distinct allometric tribes in fifth and sixth legs by themselves. But T. spinifera shift the tribe in fifth and sixth legs as illustrated in It makes allometric line with T, inermis and T, raschii in sixth and fifth Fig. 24. These are confirmed by the skimming structure with well developed setae in legs. On the other hand, T. macrura T. longipes, T. gregaria and T. vicina have those legs. shorter and poorly developed setae in those legs. The allomorphosis in thoracic leg joints in Thysanoessa euphausiids are also very characteristic. HThe length of dactylus of the first legs are plotted against the They form only one allometric length of merus of the first legs in logarithmic chart. tribe except T. macrura. The structure of dactylus of those species bearing somewhat peculiar characters as discussed. The position of T. vicina may shift from the main group if more data were obtained as it is basing rather small number of data. The length of dactylus of other legs are also plotted against for the length of They are shown in Fig. 26, and 27. In second leg, three allometric tribes merus. T. inermis and T. raschii form the same tribe as the first leg, but are observed. T. spinifera differs considerably from the group. The oceanic group, T. macrura, T. inspinata, T. vicina, T. gregaria and T. longicaudata form one tribe and position of T. longipes also show some shift T. longipes is somewhat vague as shown in Fig. 26. from those species in the relation between length of merus in second leg and carapace margin  length. The length of dactylus is very variable in second legs of Thysa- noessa euphausiids, and its variation in length is closely connected with the length T. inermis and T. raschii has shorter merus and longer dac- of other joints of legs. . T. spinifera has somewhat longer merus and shorter dactylus than the former tylus. It should be noted that T. spinifera differs in this point because it belongs two species. to the T. inermis group in many other characters. In the third legs of Thysanoessa euphausiids, the dactylus length against the length of the merus of the same legs form one allometric tribe except T. macrura which has rather shorter dactylus still in the third leg. From fourth legs to sixth legs, all Thysanoessa euphausiids run the same line in allometric tribe in the relation between length of merus and length of dactylus.  And setae in merus and carpus are developed in T. inermis, T. spinifera and T. raschii. The setae of T. longipes, T. gregaria and other pelagic species have rather rigid spines and number of setae are small. The characteristic position of T. macrura in the allomorphosis in the dactylus of first and third legs may suggest the relation to the related species Tessarabrachion oculatus which has elongated second and third legs and shorter dactylus in those legs. The allomorphosis in the relation between length of carapace margin length and total second legs, Tessarabrachion oculatus stands the same line with T. longipes, T. gregaria, and T. macrura (Nemoto, 1962). In the allomorphosis in the relation between dactylus and merus of third legs, it shifts from the group of T. longipes with T. macrura. The relation between carpus and merus in second legs in Thysanoessa euphausi- ids is as follows. Both in males and females, there are two allometric tribes in Thysanoessa. T. inermis, T. raschii and T. spinifera form one line and other six species form another one. It clearly seems that T. spinifera belongs to the same tribe with T. inermis and T. raschii, although it belongs to the different tribe alone in dactylus. The total leg length of each species are plotted against the length of the first In the second legs, T. spinifera, T. raschii and T. inermis form one allometric leg. tribe and other Thysanoessa species having longer second legs form another group. The third and fourth legs show no difference between above two groups in the total leg length, but T. macrura shifts slightly in fourth leg.   In fifth and sixth legs, the general degeneration is observed in T. macrura group which has elongated second The degeneration is heavy in sixth legs as illustrated in Fig. 29. legs.
Dactylus of the first legs
The shapes of the dactylus of the first legs are very various in each species. Thysanoessa inermis and T. raschii have rather long dactylus and shorter top spines. The spines along the inner margin of the dactylus are ranging 5-8 in inermis and 8-12 in raschii. T. spinifera has also long dactylus and the spines of inner margins are ranging 8-14 and has shorter top spines. T. longipes has somewhat shorter dactylus and longer top spines, but it has many spines in the inner margin of the dactylus.   It should be noted T. longipes often has a half size spine along the inner margin at the most basic position as illustrated in Fig. 30. The fringes in the spines are different in T. spinifera and T. gregaria groups. T. spinifera and T. inermis have fringes of the same length even in the top of the spines Dactylus of first leg of Thysanoessa cuphausiids. E A-T. spinifera: B-T. raschii: C-T. Fig. 30. longipes: D-T. inspinata: E-T. longicaudata: E-T. macrura: G-T. vicina. a- spine of dactylus in T. spinifera: c- spine of T. longipes: g- spine of T. vicina. In T. gregaria and related speciees, the fringe although they curve to the acute point. are uneven in length and the top fringes are comparatively short as illustrated. The fringe of T. macrura has small number of spines along the inner side, but type of the fringes is seemed to be inermis type. The closely related species, T. vicina is gregaria type dactylus which has few spines in the inner margin of the dactylus. But it has short and pecurial fringes differing from those of T. inermis and T. gregaria.
Shape of the dactylus in the second legs
T. inermis and T. raschii have slender dactylus which bears the long spines along the margin of the dactylus. The number of the side spines in T. raschii are ranging These two species are 4 to 6 in one row, and T. inermis has 2 to 4 spines. T. spinifera, however, is considered as the same group also in the allometric tribe. considered to belong to another group. The dactylus is not so slender, rather short and the side row of spines is not observed. Those spines are growing in the distal edge of the top as illustrated. Other Thysanoessa have also shorter dactylus, but the arrangement of spines are the same with T. spinifera. There are two rows of small setae in the spines. These shapes of the dactylus are closely connected with the relative growth, allomorphosis of dactylus against the merus and carpus in the elongated second legs. The shape of dactylus in second legs are discussed on Euphausia superba by Barkley (1940) and pecurial spines (Putzborsten according to Zimmer, 1927) are found in the inner side of the dactylus. These spines are yellowish or red in the living condition, and easily observed in their activity.    But the sweeping or cleaning for their legs or setae is not observed by Barkley (1940). None of this peculiar spines is found in Thysanoessa euphausiids.   In T. gregaria group, the dactylus is shorter and the corresponding series of spines or setae become strong spines in the margin or inner edge of the dactylus. It is very interesting that not only hervivorous Euphausia species but it is also found in the carnivorous euphausiids such as Nematobrachion species, although the second leg of Nematobrachion species is not elongated. re mainly examined which are belonging to dif- respectively comparing with other Euphausia and Nematobrachion species ccording to structures of thoracic legs.
Upper lip
The general shape and function of the upper lip in T. gregaria is almost the vith Euphausia superba and T. inermis which have also nearly the same shape of Lower lip he lower lip of T. gregaria and T. inermis are also nearly the same with that ng edge spines in the outer margins of t enticles in Thysanoessa euphausiids but they may not have pecial mechanism in feeding.
Mandible
The general shape of mandible of Thysanoessa seems not to be so different from bhausiids, however, the number of furrows in the mill-like process are not n as Euphausia superba (Barkley, 1940).
First maxilla
The first maxilla has very strong processes along the edge of basis. s about seven tough processes. Euphausia vallentini and th more processes along the basis of the first maxilla. aied with other small spines. The coxa has also processes and many spines. nese processes and spines reduced in carnivorous euphausiids, Nematobrachion Other euphausiids, Euphausia superba, E. vallentini have considerably many ines and processes. It is considered Thysanoessa euphausiids stand the intermediate position between carnivorous and hervivorous cuphausiids from the structure of those maxilla. Second maxilla The second maxilla is long and covers the first maxilla. The general shape of the tween T. inermis and I n the last endopodite which corresponds to Euphausia species. (E. arkley: E. vallentini, Nemoto). But the shape of the last segment of T. ria is rather short and rounded. The carnivorous Nematobrachion boopis has rather short and rounded segment, and setae are long but fragile and sparsely attered. The mechanism of those second maxilla is considered to collect and ve the odds which congregated in front of the mouth from the outer side to the pening of lower and upper lip and then to the mandible. The spines and setae f the coxa and the first ishium of the endopodite are the most important for the action, but the long spines and setae in the last segment of the endopodite are also usefull for the collection and not to miss the food. The carnivorous animals use the long sparse spines bearing special structure which is very effective to seize those food such as copepods and other active animals. INNER STRUCTURE OF STOMACH The inner wall of euphausiids are constructed to crash the foods of them such as diatoms, foraminfera, other zoo-planktons and phyto-planktons. The preliminary figures are given in the previous book on foods of whales and another paper (Nemoto, The food organisms are described by Ponomareva (1955). According to 1965). her list, Thysanoessa generally feeds on zooplanktons especially in T. longipes as well as phytoplanktons. The stomach structure of T. inermis is given in Fig. 33. The general pattern of structure is the same with other euphauiids. K It has, however, peculier bunch of They spines in the middle of the lower margin of the side plate of the stomach wall. may be called ' cluster spine ' of the euphausiids. This type of the cluster spine is characteristic in Thysanoessa euphausiids and related species Tessarabrachion. Every species of Thysanoessa euphausiids have row of cluster spines ranging from 10 to 40 in the middle of the side plates. The number of the cluster spines is small in T. vicina and T. macrura, and other Thysanoessa have 20 or more number of spines. The T. raschii and T. general shape of spines are not different among each species. inermis have about 20-25 cluster spines, T. longipes and T. inspinata have about 30 spines, and T. spinifera has about 40 cluster spines. Thysanoessa euphausiids have other spines to crash in the anterior part of the side plates as illustrated in Fig. 33. Especially T. inermis, T. longipes and T. inspinata and T. vicina have strong spines in the anterior edge parts of the side plates. These spines have some taxonomical characters as given in T. longipes and T. spinifera. In Euphausia genus, these spines congregate and form the circle bunch of spines as illustrated in a former report (Nemoto, 1965, Fig. 8). This type of cluster spines are characteristic in Euphausia, but they lack the row of cluster spines in the low or middle part of the side plate of The cluster spines in circles and isolated spines in Thysanoessa euphau¬ the stomachs. siids are made to crash foods with the spines in upper side plates in the stomach walls. The circle cluster spines are fit for the porpose to crash hard wall cell of diatoms and related organisms. So Thysanoessa euphausiids lacking this circle cluster spines show good adoptation to catch zooplankton or other animals without hard shells. In the central part of the side plates, there are many isolated spines as illustrated in These spincs are also considered to crash their foods, and are most dominant Fig. 32. in the central part of the side plates in the Thysanoessa and Euphausia species. In some species in the deep sea living Thysanopoda such as Thysanopoda monacantha has this type of spine, but other Thysanopoda, T. tricuspidata has one row of spines in the middle of the side plates. And Nematobrachion boopis completely lacks those spines in the side plates. All Thysanoessa euphausiids have central row of spines in the bot¬ tom plates. This row of spines is clear in Thysanoessa longicaudata, and T. inermis, but it is not so strong in other Thysanoessa. This central row of spines is also typical in Thysanopoda monachantha, Nematobrachion boopis, (Nemoto, 1965 Fig. 9.) and Euphausia species.  But Nematoscelis has two branch rows and Stylocheiron longicorne and S. abbreviatum have only slight or no row of these spines. The stomach of Thysanoessa is considered as a polyphagus type from the arrange- ment of cluster spines and isolated spines in the side plates. The side view of the stomachs of Thysanoessa shows no swell of the posterior portion of the stomach just above the pyloric region of the intestine. N. boopis has very swollen stomach in the posterior part (Nemoto, 1965 Fig. 9) which fit to take vast quantity of undigested foods such as copepods and other living zooplanktons at one occasion.
COPULATORY ORGANS
The male copulatory organs of euphausiids are considered one of the best specific characteristics and the female copulatory organs, namely thelycum, are also pointed out as one of the best character for taxonomy by Einarsson (1942). In some closely related cuphausiids, Thysanopoda aequalis and T. subaequalis (Boden & Brinton, 1956), Euphausia nana and Euphausia pacifica (Brinton, 1962a), the copulatory organs do not exhibit so well their specific characters. In a former report, (Nemoto, 1963) I also discussed the possible difference in the relative growth of the length of the process and spincs in the male copulatory organs in closely related T. longipes and formarly spincless form, T. inspinata.  On Euphausia nana and E. pacifica , the microstructure of the male copulatory organs are considered as the taxonomic characters, however, the body length, horizontal distribution are also main discriminating points about two species (Brinton, 1962a). The copulatory organs in T. gregaria group in males has terminal process dis¬ tally expanded, but in the younger stages it is not so expanded as discussed in the previous report on T. macrura (Nemoto, 1958). The young Thysanoessa macrura has also broader base of terminal process but the tip is pointed. The copulatory organs of Thysanoessa euphausiids are divided into four or five groups. T. gregaria, T. macrura, T. vicina and T. parva have rather related male copul¬ atory organs as illustrated in Fig. 35. The edge of the tip in terminal process of T. gregaria and T. vicina are serrated, and no clear serration is observed in T. macrura and T. parva. Both species T. raschii and T. inermis have similar male copulatory organs. have two long menbranous wings or expansions from the tip almost to the bases. (Boden, Johnson & Brinton, 1955). The proximal and lateral processes are well developed but not expanded in the ends. T. spinifera has not the broad terminal It has, however, the terminal spine of broad base and both proximal and spine. The broad base of a terminal spine is identical lateral spines have acute ends. with T . inermis and T . raschii and it is considered as the original type of the copulatory organs of Thysanoessa euphausiids of inermis group. T. longipes and T. inspinata have three main processes which are not expanded But those are slender and the base is not broad as T. spinifera. in the tapered ends. T. inspinata has short and robust processes and main characters are the same The base of terminal process is not so broad as T. spinifera. with those of T. longipes. T. longicaudata has pointed terminal process, it has thin keel on the outer side. The proximal process flattens out considerably above the middle and forms some- The terminal end of the pro- what spoon-shaped plate with an uneven upper edge. This character is bearing a resemblance to T. parva ximal process is not serrated. or T. gregaria, but its base of terminal process is rather narrow and pointed end is quite different from those of T. gregaria group.
DISTRIBUTION
Three main distributions in the World Ocean contain following species listed in Table 8 and 9. Two Arctic and boreal species T. inermis and T. raschii, are common in the Atlantic and the Pacific. T. longicaudata is only found in the Atlantic and T. longipes and T. inspinata, T. spinifera are only distributing in the boreal region of the Pacific.   Other two species T. macrura and T. vicina are Antarctic species, and two species T. gregaria and T. parva are distributing both in southern and northern temperate water regions. T. spinifera is distributing only along the American side, the eastern part of the Pacific and T. inspinata and T. longipes are abundant throughout the boreal regions of the Pacific. The latter two species are rather rare in the shallow Arctic Sea. (Johnson, 1956) and the occurrence of T. inspinata in the sea of Japan is somewhat uncertain still in recent studies (Komaki & Matsue, 1958), although Ponomareva considered it also distributed in the Sea of Japan.
OCCURRENCES OF THYSANOESSA EUPHAUSIIDS TABLE 9. IN THE NORTH PACIFIC
T. spinifera shows pecurial distribution in the eastern part of the North Pacific. (Nemoto, 1962 :   Brinton, 1962b).    It has not been recorded from the Arctic Ocean, north Bering Sea, Okhotsk Sea and the Sea of Japan.   It is only distributing in the It is found dominantly shallow neritic waters of the east coast of the North America. in the stomachs of baleen whales in the coastal waters of the east Aleutian Is. (Banner, The most northern record is given in a former report 1954 :  Nemoto,  1959). The (Nemoto, 1962) and the most western record is given by Hansen, (1915). concentration of this species is also observed along the continental shelf of the Gulf The of Alaska from the observation of the stomach contents of baleen whales. southern range of the distribution is given by Banner (1949) and Brinton (1962b) along the California coast of North America. The range of water temperature and salinity is considered slightly different from other neritic T. raschii and T. inermis. T. spinifera occurres in a little wamer temperature and lower salinity and T-S diagram chart for T. spinifera is different from those euphausiids. (Nemoto, 1962). Thysanoessa inermis is distributing in the boreal Arctic water in the Atlantic and the Pacific. In the Atlantic, general distribution is observed between the February ice limit and the surface isotherm for 10°C (Einarsson, 1945). And the Pacific distribution is also limited within the February surface isotherm 10°C (Brinton, 1962b :  Nemoto, 1962) and the main distribution is found within 15°C in summer isotherm (Nemoto, 1962). T. inermis is considered to spawn in the shal- low waters along the continental shelf, and Glover (1952) described T. inermis as the shallow water species. In one former report, I reported that T. inermis occurres in the stomachs of baleen whales as their foods very often in the coastal waters of Kamchatka peninsula and along the continental shelf of the Alaskan peninsula in the Bering Sea (Nemoto, 1962). T. inermis also found in the Okhotsk Sea and Japan Sea. But latter occurrence is restricted in the northern half waters (Komaki & Matsue, 1958 : Ponomareva, 1957). The concentration of T. inermis is also ob- served in the southern edge of the shelf along the Alaskan peninsula and the Gulf of Alaska, and it goes down to the waters off Vancouver (Brinton, 1962b). The possible relation between T. inermis in the Atlantic and the occurrence of it in the western part of the Pacific is expressed in the two and one spine forms of T. inermis. In recent investigations, Johnson (1956) reported it from the Chukchi Sea but only few specimens. The main range of the T. inermis distribution is limited the line of continental shelf of the Bering Sea, and the ice limit in summer in the shallower Atlantic north waters.  Thus the interchanging of Atlantic and Pacific stocks in recent age may not occur to effect the main distribution and of two and one spine forms as shown in percentage occurrence of them. T. raschii is uaully distributing along the shallow continental shelf or the neritic shore waters in the Arctic boreal regions. It plays the role of foods of balleen whales in the off waters of Anadyle Gulf, Bristol Bay and main occurrences are also observed along the Kamtchatka peninsula, Anadyl Gulf, and the shelf of the Gulf of Alaska. Other investigations prove that it also occurres in the Japanese coast of Okhotsk Sea (Nemoto, 1959), north shallow waters of Okhotsk Sea and northern part of the Sea of Japan (Ponomareva, 1955). The main stocks of T. raschii in the Pacific and the Atlantic are living within ice covered waters in winter seasons. It is also more common in the North pole sea than other euphausiids (Johnson, 1956). The interrelation between the Pacific and Atlantic stocks may be closer than other euphausiids, and possible continuity of distribution through the north pole waters is considered. The southern range of T. raschii is restricted to the cold current and summer concentration of this species is found even in the low salinity (Einarsson, 1945 : Nemoto, 1962). This position may be compared with the Antarctic Euphausia cristallorophias along the Antarctic continental shelf. T. longipes is differeing from other neritic Thysanoessa spinifera, T. inermis and T. raschii, considered as pelagic form.  Its main distribution is given by Brinton (1962b) and Nemoto (1962) in the northern part of the North Pacific.  T. longipes is transported to the north American secter of the Arctic sea (Johnson, 1956), which coincides with the flow of the Arctic water current from the southern waters through Bering strait (Johnson, 1956).  Considering from the occurrences in the stomach contents of baleen whales (Nemoto, 1959: 1962) and plankton net investigations (Banner, 1949 :  Brinton, 1962b), T. longipes concentrations are observed in the eastern Bering Sea (Nemoto, 1962 :   Brinton :   1962b) in the off waters of Alaskan continental shelf and mid waters along the Bowers Bank.   The former occurrences are found in company with T. inermis and dominant occurrences are found in a little more off waters in the north waters of Unimak Is. The heavy concentrations are also observed in the southern waters of Aleutian Is. along the latitudinal belt of 50°N (Brinton, 1962b) by plankton investigations. The closely related species T. inspinata (Nemoto, 1963) or the spineless form of T. longipes (Banner, 1949) is found in more southern waters.  It consitutes the smaller part of the food of baleen whales in the southern Bergin Sea (Nemoto, 1959) which is possibly the northern limit of this species. The general distribution of T. inspinata is given by Brinton (1962b) as the unspined form of T. longipes. The southern limit of this species is considered in the western side of the Pacific coincide with the range of cold current Oyashio. Brinton gave the most southern record 34-17N, 156-11E, where the submergence at the southern margin of the range is ob¬ served (Brinton, 1962b). The present data collected by Tansci-maru in KT-64-17 cruise, T. inspinata occurred from 33°-50N and 138-30E as the more western waters in the southern waters of Japan. T. macrura and T. vicina are only distributing in the southern hemisphere. Those two Antarctic species have been found in the circumpoler belts from the subantarctic to the Antarctic. From the external characteristics, they are con¬ sidered to come from gregaria-parva relatives not longipes or inermis , and its distribution suggests also phylogenetic speciations like the relation between T. longipes and T. inspinata. The distribution of T. gregaria is well discussed by Brinton (1962b) with some phylogenetic consideration in view of biantitropical panoceanic distribution of this species in southern and northern hemispheres. The vertical and latitudinal distribution of Thysanoessa euphausiids is given in Fig. 36.  In both Arctic and Antarctic seas, Thysanoessa euphausiids extend to the deeper waters following to the equator. Thysanoessa raschii, the most northern species is living in shallow water usually upper than 200 metre, and T. inermis upper than 300 metre in the North Pacific. The southern waters species, T. macrura and T. vicina are also living in the layers upper than 300 metre (Hardy & Gunther, 1935). T. longipes is generally considered to live in the layers upper than 500 metre, and deeper waters in the southern waters near 40N latitudes. The closely related species T. inspinata which is living more southern waters (Brinton, 1962) is considered to distribute into deeper waters (Nemoto, 1962). The deep water species T. parva is usually found from 750 to 2000 metre depth in the waters. The vertical zones of euphausiids distribution are illustrated by Brinton (1962b) as the mid Pacific profile in the North Pacific. The Arctic and subarctic species of cuphausiids found in the surface layers often found in the middle layers of temperate waters of lower latitudes. The deep sea living T. parva has far reduced spine and setae in the thoracic legs, although T. gregaria and other species have also reduced setae in thoracic legs especially in second legs. This tendency is also observed in carnivorous euphausiids Nematobrachion species.   But the remarkable point in T. parva is the lack of strong fringes in setae as well as the reduction of setae. As a taxonomic character in T. parva, these fine fringes in setae in legs are described. These fine fringes may be also adoptation to the deep sea living where the small food organisms such as diatoms are comparatively scarce as compared with surface layers. Among Thysanoessa euphausiids, T. inspinata is found by KT-64-17 cruise from the south waters of Japan by deep oblique haul, where T. gregaria possibly distributes from the surface to about 1000 m deep. The Pacific boreal species Tessarabrachion oculatus is also collected by this cruise in the same position. This submargence of boreal species in temperate waters is observed in other groups of plankton such as chactognata Sagitta elegans in the south waters of Honshu (Marumo, 1965). Brinton considered the biantitoropical panoceanic distribution of Thysanoessa gregaria may be explained by Pleistocene cold waters distribution.  The 7°C and 11°C isotherms at 200 m are associated with the limitation of the recent range of T, gregaria, and T, gregaria crossed the equater by this cooling of 2 1/2° to 3°C at 200 m in Pleistocene age. At this circumstance, the North Pacific subarctic species, T. longipes may still occupy the coastal fauna in the region 20-30N where the colder California current runs and it never has the chance of transgressing the equator. From above explanations for the distribution of Thysanoessa euphausiids two hypotheses may be derived. One is the trespass of T. vicina and T. macrura or their ancestor in the Antarctic waters is earlier than the transgression of the equator by T. gregaria, and speciation of T. inspinata from T. longipes or its relatives is possibly later than that age. The second hypothesis is that T. parva is also specialized into southern and northern hemisphere at about the same age of T. gregaria or a little later age, and it also transgressed the equator after the specialization as T. parva, not the age of the original type of T. parva in the older ages.
DISCUSSION
The allometric study and comparative morphology explain fairly well the taxonomic and ecological positions of each species in Thysanoessa euphausiids. The similarity in morphological characters of Thysanoessa euphausiids is listed in Table 10. In general consideration T. inermis and T. raschii resemble each other. T. inermis differs in the sexual dimorphism in the first antenna, presence of spines in the posterior abdominal segments, shape of the rostrum and the absence of the cara- pace denticle in the lower margins.  But other main characteristics are the same with T. raschii. The arrangement of thoracic legs resembles to Euphausia species. T. spinifera shows the first characteristic point in the sexual dimorphism in the The rostrum of the carapace has also specific difference among these first antenna. The most peculiar point is that the eye hole spine along the carapace three species. margin is present in T. spinifera. None of other Thysanoessa has this spine. The shape of dactylus in thoracic legs approximately the same among those T. inermis, T. raschii and T. spinifera.   Only the dactylus of T. spinifera in the second leg differs from other two species. It bears the intermediate characteristics between T. raschii group and T. gregaria and other species which have elongated second legs. Third dactylus does not differ among every species except T. macrura as it is dis¬ cussed in the former part. And no special difference is given in other dactylus in posterior legs. Preanal spines in those Inermis species show slight sexual dimorphism, and other two groups also divide themselves from those species as fairly clear dicisions. The group of T. parva is still unknown, but it may possibly belong to T. gregaria group. The Atlantic T. longicaudata has non-sexual different preanal spines in denticulation and belongs to T. longipes group.  Longipes group species have the same character in the dactylus of the first legs and shape of the rostrums, but T. longicaudata lacks the lateral denticle of the carapace. The second leg of T. spinifera stands peculiar inter- mediate position between T. inermis group and T. gregaria group. And this tendency is also observed in the relation between merus and dactylus in the second leg.    These characters show that the second leg of T. spinifera becomes elongating and the dactylus shortening which is more relating to T. gregaria group. T. gregaria group is considered off shore living species and they are now developing to more carnivorous feeder in selection of the food. The short and bearing tough spines in the dactylus of the elongated legs are also often observed in other carnivorous euphausiids. And a little  clongated second  leg in some younger forms of T. inermis (Hansen, 1911, Einars- son, 1945) is noted already, especially in T. neglecta form in the Atlantic.   According to the Einarsson's consideration, T. neglecta is only a transitory form in the develop- mental history of T. inermis. It has longer second legs and shorter dactylus from the figures by Einarsson, (1945 Fig. 19) and constricted eye. The illustrated specimens stand clearly in the gregaria group from the length of each joint. The larval forms of T. inermis usually have eyes longer vertically, which is almost the same as T. longipes eyes in those stages. Thus T. inermis sometimes shows in its deve¬ lopmental stages the different pattern of allomorphosis in second leg, and it is con¬ sidered as the one explanation for the different group of allomorphosis in Thysanoessa euphausiids is originated from the above different developmental stages. Another neritic species T. raschii has non-elongated second leg, and non-con¬ stricted eye in adults, and it has no clear form of clongated second leg in furcilia stages like neglecta form of T. inermis. But it has lateral denticles in the anterior part of the margin of the carapace, although the denticle is situated far posterior part of the lateral margin of the carapace in the furcilia stages of T. raschii. This would suggests full development in this character if the speciation is attributable to excessive growth. There are three main types are observed in Thysanoessa euphausi- ids in male copulatory organs.   One is inermis type which includes T. raschii and T. inermis. T. spinifera has this character, but it bears also longipes type as discussed in the former part. T. gregaria is considered to belong the same group with T. macrura, T. vicina and T. parva. T. longicaudata is standing in the intermediate posi¬ tion between T. longipes and T. gregaria. This is also one of the expression of the phylogenetic position of T . longicaudata that it does not belong to the gregaria group. The position of T. longicaudata is also confirmed by the presence of central spines in the base plate of the stomach structure. This central spine is clear also in some other euphausiids. Thysanopoda, Nyctiphanes, and Nematobrachion species have well developed central spines in the base plate. Only T. longicaudata among Thysanoessa has well developed central spines and T. inermis group has vague spines. T. longipes and T. gregaria have trace of central spines but sometimes they have spines like T. inermis. The type of cluster spines in the anterior part of the side plate of the stomachs are nearly the same in Thysanoessa species, but some difference in related species such as T. longipes and T. inspinata is observed. The bilobate eye of seven Thysanoessa euphausiids in adult may directly be connected to the broader living range in the depth of the sea at present or their ancestors (Brinton, 1962b). The similarity in allometric tribes are illustrated symbolically in the Table 11. As the general aspects of the list, those tribes are also divided into two main groups. In the relative growth of the lateral denticle and carapace length, however, each species demonstrates respective tribe except T. vicina and T. macrura. It is suggested the same allometric tribe of T. macrura and T. vicina in the position of lateral denticle means they closely related each other and position of T. vicina is larval stage of T. macrura in this relation. Because the position of lateral denticle of the carapace is situated in the posterior part and goes up to forward with the body development especially in the furcilia stages. The body size of T. vicina is smaller than T. macrura and lateral denticle is situated a little posterior position. These characters are the larval characteristics. The general distribution of those species show that the adult form, namely the lateral denticle of the carapace situating in the anterior position, of each group is found in the colder waters. The body size of T. inspinata is smaller than T. longipes, and T. vicina is smaller than T. macrura. Thus those specific differences are connected with the body development in those species. The wamer water type may be the larval type, which has the posterior carapace denticles, and having smaller body length. The phenomena like this have been found on American fresh water fish as a consideration of the evolution of those fish (Hubbs, 1940). The legs of euphausiids are generally considered as the feeding and swimming apparatus and legs vary very much among euphausiid species. The general allomorphosis in legs of Thysanoessa euphausiids shows also two main groups.  The total leg length against carapace margin length and total first leg are both the same in third and fourth legs, but it differs in first, second, fifth and sixth legs. As for the relation of total length of the first legs for carapace length, T. inermis, T. raschii and T. spinifera form one allometry group and other six and possibly T. parva form another group. T. spinifera stands itself in the second leg aparting from T. inermis but it forms the same allometric tribes in fifth and sixth legs with T. inermis and T. raschii. The allometric group of second legs are also divided into two groups. These groups coincide with also groups of fifth and sixth legs. From the observations on structure of legs, the reduction in the posterior thoracic legs is considered as the steps to the carnivorous feeding type. Thysaneossa euphausiids have even in the gregaria group, rather long and skimming setae in sixth and fifth legs. But  other carnivorous Nematobrachion species have none functional setae and reduced fifth and sixth legs. The surface feeding and hervivorous euphausiids, such as Euphausia, have functional and long sixth and fifth legs in general. Thus it may be considered some of Thysanoessa euphausiids bear both types of feeding and omnivorous feeding type. Among them, T. inermis, T. raschii are surface feeder type. The general features of setae in thoracic legs as the feeding apparatus are discussed by Barkley (1940). T. inermis, T. raschii and T. spinifera resemble to E. superba in the arrange- ment of those setae in ishium and merus and in the allomorphosis relationship of every leg length. In the allomorphosis in the relation between merus and dactylus of each legs shows also two main allometric tribes as in the relation between merus and carpus in second legs. But T. macrura shows peculiar position in first and third legs which occupys the independent allometric tribe. T. spinifera occupys also the tribe itself in second leg. Although the different allometric tribes are observed in second legs, no dif- ference is found in the allomorphosis from fourth to sixth legs in the relation between merus and dactylus.   Thus the speed in the development in cach joints from small and occanic species to large neritic species is the same for each species.  Only second legs, and first and third legs partly, show the significant allomorphosis dif- ference, which also have special means for ecological and morphological structures. The allomorphosis in the relation between carapace margin length and abdominal segments varies considerably in each species. It is very interesting that T. raschii forms the same tribe with T. longipes and T. inspinata. T. macrura, T. vicina and T. longicaudata form one tribe and T. gregaria forms itself one tribe. In the relation between the length of sixth and fifth abdominal segment, T. gregaria also forms tribe only itself.  T. gregaria is only one species living in the tempeate surface waters, and posterior parts of the abdominal segments are considered as the swimming apparatus of euphausiids. If it is true, T. raschii takes somewhat curious position to form the same allometry line with other two oceanic species. T. vicina and T. macrura form the same tribes not only in the denticle length but also the legs and abdominal segments relationship in general. The former relation in lateral denticles is con¬ sidered as the close interspecific relation and the latter are partly the similarity in ecological characteristics such as feeding and swimming in the sea. The preliminary phylogenetic development in Thysanoessa euphausiids is in con¬ sidered as follows. The original form of Thysanoessa is possibly belonging to the group of Nematoscelis and Tessarabrachion relatives. From the ancestor of Thysanoessa, two branches are considered which com¬ paratively earlier times developed to the different directions. Those are T. longipes and T. gregaria group, and T. inermis and T. spinifera group.  The former stem is developing along the Thysanoessa line, that is oceanic and wider vertical range of distribution. The later stem is developing to the different neritic inhabitance such as T. inermis, T. raschii and T. spinifera. In the Antarctic, Euphausia crystallorophias is living as the real neritic species. Its colonization of Antarctic waters is considered by John (1936) as the series of E. lucens—E. vallentini—E. frigida and this series is divided into two branchs, those are E. superba and E. crystallorophias. And the transgression of the ancestor of E. frigida series from the south as the E. pacifica series is possibly at the same time that the genus Thysanoessa become antitropical (Brinton, 1962b).  At these time, the later stem became developing as neritie and surface living species. T. longipes or its ancestor is considered coming from the stem different T. inermis-ancestors and T. gregaria-ancestors as it bears the male copulatory organs and preanal spines of different forms. Further the distribution of T. longipes and T. inspinata is fixed in only North Pacific. This shows the transgression of equator of these T. longipes group did not occur at the age of transgression of T. gregaria group. They possibly occupy the northern colder waters already at that times, and they shift the postulated distributions of transgression of equator at the cooling age discussed by Brinton (1962b). Thysanoessa inermis and T. raschii are common in the north Atlantic and Pacific, and they had possibly contact through North pole sea still in later ages. This may be endorced by the distribution of one and two spine forms of T. inermis and neritic species T. raschii. Before the differenciation between T. inermis and T. raschii, T. spinifera or its ancestor is fixed as the neritic and shallow water species from the con¬ sideration on the structure of male copulatory organs and the allomorphosis of body parts.  The restricted distribution of T. spinifera along the west coast of North America may contradict this assumption.  But it is generally accepted that it is closed in Aleutian and California current, and the cold North Bering Sea and general water transport from the west to the east in the North Pacific drift may be the bar¬ riers for the extension of T. spinifera to the west Pacifiic (Nemoto, 1962). The position of Atlantic T. longicaudata is very characteristic.   It is considered it comes from the way between T. longipes stem to the ancestors of T. gregaria or directly from ancestor of T. longipes. T. longicaudata completely lacks the carapace denticle but it is sometimes absent in many species which usually have carapace denticles and some species have two forms which have denticles and none ( Nematoscelis difficilis, Mclaughlin, 1965).  It is apparent that the external characters such as male copulat¬ ory organs and carapace denticles and preanal spines, the T. gregaria ancestor has four other present Thysanoessa species, T. macrura T. vicina, T. parva and T. gregaria at least. I would thank here Emeritus Professor Yoshiyuki Matsue of the Ocean Research Institute, University of Tokyo for his kind guidance and helpful criticism of this paper. Dr. Ryuzo Marumo, Professor of the Ocean Rseearch Institute, University of Tokyo, kindly gave me guidance as well as fruitful criticism and kind coopera¬ tions through the collection of some deep sea materials on board of R. S. Tansei- maru in the cooperative research cruise KT-17 in 1964.   Drs. Yutaka, Kaharada, Masataka Kitou and Kenzo Furuhashi of the Meteological Agency of Japan kindly gave me samples of euphausiids collected in Japan Deep Sea Investigation (JEDS cruise) and the sea of Japan, which are used in one part of this study, and I thank also their helpful discussions. The Atlantic specimens of Thysanoessa longicaudata have been sent by courtesy of Dr. L. T. Jones of the Oceanographic Laboratory at Edinburgh.  I would thank Dr. Sigeru Motoda, Professor of the Hokkaido University and Mr. R. S. Glover, Director of the Oceanographic Laboratory of the Edinburgh for their kind arrangement for the sending of these T. longicaudata specimens. Thysanoessa euphausiids are investigated under the comparative morphology and allomorphosis studies, to get general considerations on morphological and taxonomic¬ al characters as well as ecological peculiarities. The main results are summarized as follows. 1. Modified and simple key for the taxonomy of Thysanoessa genus is given. Three main groups, namely longer, intermediate and shorter forms are found on the first antenna. T. inermis, T. spinifera and T. raschii form the longer group, and they run also the same allometric tribe in allomorphosis in the relation between carapace length and length of the flagellum.  T. vicina, T. macrura, T. inspinata, T. longipes and T. gregaria form the shorter group and also the same allometric tribe. T. longicaudata forms different tribe apart from those two groups. 3. The shape of the rostrums, lateral denticles are examined. The position of the lateral denticle have very important taxonomical characters especially in closely related species under the evolution of the neoteny-like speciation. 4. Shape of eyes of Thysanoessa are very various. Neritic and shallow water species have round and none constricted small eyes which show no sexual dimor¬ phism. The allometric tribes in eyes are very various among each species and sex, which is possibly due to the reflection of the complex ecological characteristics. One 5. Arrangement of dorsal keels and abdominal spines are examined. and two spine forms of Thysanoessa inermis show the geographical cline in the North Pacific in occurrences of each form.   One spine form occurrence is high in western Pacific especially in the sea of Okhotsk, in which it is ranging about 80 to 90 percent like the Atlantic specimens. But it decreases following to the east and only about 25 percent in the waters of the Gulf of Alaska. Three spine form of T. inermis is again described from the North Pacific. 6. T. gregaria stands in peculiar position among Thysanoessa species in the allomorphosis in telson, fifth, and sixth abdominal segments. It is possibly due to the wamer water habitat in the transition zone apart from other Thysanoessa. T. macrura and T. vicina have close relations also in those points, which shows the ecological and phylogenetic similarities are very close also in these body parts. 7. In T. spinifera and T. longicaudata, the allomorphosis in each sex is different in the relation of abdominal segments and other body parts. The dalay in the development of the carapace in males may bring this variation, and these sexual differences among allometric tribes are considered as one of speciation of the direc¬ tion from the female type to the male type. 8. Preanal spines of Thysanoessa euphausiids are divided into three groups, according to the sexual dimorphism. It is present in T. macrura, T. gregaria, T. vicina and possibly in T. parva. Only slight dimorphism is observed in T. inermis and T. raschii, and nearly absent in T. longipes, T. inspinata and T. longicaudata. These sexual dimorphisms are already found still in younger adlescents. 9. Allomorphosis and structures of thoracic legs especially dactylus of them are examined. In the relation between carapace length and second thoracic legs, three allometric tribes are observed. T. inermis and T. raschii run the same line, and T. longipes, T. inspinata, T. gregaria, T. macrura, T. vicina and T. longicaudata form another tribe. T. spinifera forms one tribe itself. These tribes are nearly the same with the first legs, and T. longipes group is corresponding to the shorter dactylus group. There is no difference in the allomorphosis in third and fourth legs, but T. inermis, T. raschii and T. spinifera form another tribe apart from T. gregaria group which has shorter and poorly developed setae in those legs. The characteristic positions of the dactylus in the first and third legs of T. macrura are observed. 10. The structures of the dactylus of first and second legs are investigated. The number of the spines along the inner margin of the dactylus of the first leg varies in each species, and corresponding to the shorter and longer groups of the dactylus. T. spinifera and T. raschii, the longer group, have 8 to 14 spines and T. inermis has 5 to 8 spines. The shorter group has less than 9 and T. gregaria, T. macrura and T. vicina have only 1 to 3 inner spines. The dactylus of second legs is long in T. inermis and T. raschii, and T. spinifera has rather shorter dactylus. Other Thysanoessa euphausiids have shorter dactylus and elongated merus and carpus, which may be the simpton of carnivorous habit. The first and second maxilla of Thysanoessa species show the carnivorous and hervivorous characters, which stand the intermediate position between the carnivorous euphausiid, Nematobrachion and the mainly hervivorous euphausiids Euphausia and other species. 12. The inner structure of the stomachs of Thysanoessa euphausiids has special row of spines which is named as ' Cluster spines '.  This row of cluster spines is found The isolated along the base part of the side plate of inner wall of the stomach. tough spines are also found along the anterior part of the side plate, which also very effective for the crashing their foods such as diatomes and other hard shell organisms. But Thysanoessa euphausiids have no circle ' cluster spines ' in the anterior part of the side plate. 13. Male copulatory organs of Thysanoessa prove also T. macrura T. vicina and possibly T. paria are belonging to the relative of T. gregaria.  T. longicaudata stands peculiar position, and T. inspinata shows close similarity to T. longipes. 14. The similarity and difference in allomorphosis and morphological study are compared to get characteristics and phylogenetic considerations. T. longibes, From the morphological observations, three main types are found. T. inspinata and T. longicaudata form one group, T. raschii, T. inermis and T. spinifera second group, and T. gregaria, T. vicina and T. macrura (possibly T. parva is included in this group) third group. The most typical difference among those three groups is the sexual dimorphism in preanal spines. The characteristics expressed in preanal spine and male copulatory organs of T. longicaudata suggest that it comes from the T. longipes ancestors rather than T. gregaria group. 15. General distribution of each species shows the possible correlation between the evolution and present distributions, suggesting that T. vicina and T. macrura are fixed in the Antarctic before the transgression of the equator by T. gregaria, and the speciation of T. inspinata from T. longipes is possibly later than that age in the North Pacific. 16. The preliminary consideration of the phylogeny of Thysanoessa is stated. 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