OSSEOUS LABYRINTH OF CETACEA
MUNESATO YAMADA* AND FUMIHIKO YOSHIZAKI*
Since as early as the eighteenth century, the cetacean labyrinth has
been referred to by the pioneers of comparative anatomy including
Cuvier, Monro and v. Baer. But it was not until 1789 that Monro andComparetti discovered the disproportionately minute semicircular canals,
apparently one of the most important peculiarities in the cetacean ear.
However, the author who first took out the cetacean labyrinth by meansof cast preparation was probably Rapp (1837), and this monograph was
followed by a fascinating one by Hyrtl (1845) which also dealt with anumber of cetacean species. Gray (1907-08) was another who ingeniously prepared and described the membranous labyrinth of vertebrate
animals also including some cetaceans.
The present investigation was attempted with the initial intention ofaffording fundamental data and knowledge towards a histological research proposed by the senior author (Yamada). His impression is thatthe previous records do not help the intended purpose. This applies
not only to the cetacean labyrinth alone but to general labyrinthine
anatomy including man and the so-called experimental animals. This isbecause the criteria of the measurements, for instance, are not always
clear, and also because something else had to be contrived in order topresent its morphology as such. The present paper was first read in 1957,
and incidentally, it became more than a mere presentation of the cetacean
varieties. This work has become a starting point so that in fact somenovel data and propositions have been developed with the aim to standardize the sectioning and indexing of the human labyrinth (Yamada,1959).
MATERIAL AND METHOD
Material of the present investigation consists of sixteen species from fourteen genera representing the two suborders of Cetacea as follows:
Mystacoceti or
Whalebone whales

1 Balaenoptera (three species); 2 Megaptera;
3 Eubalaena;

Odontoceti or
Toothed whales

8 Globicephala; 9 Grampus; 10 Feresa;

4 Physeter; 5 Kogia; 6 Berardius; 7 Ziphius;
11 Lagenorhynchus; 12 Delphinus; 13 Prodelphinus;
14 Neomeris.

* Department of Anatomy, Okayama University Medical School.
291

292

M. YAMADA AND F. YOSHIZAKI

Labyrinths of each of the above species were cast in a small number of specimens but
in many cases in one example only. The findings on these casts were supplemented insome instances by the direct method, in which direct access was made to the labyrinthineinterior by grinding the bone with dental engine or lathe. Such a small number is obviously far from sufficient in order to discuss the specific characteristics, but altogether they
accumulate a good sum of findings so that the cetacean labyrinth may be considered as awhole.
One each specimen of Delphinus and Lagenorhynchus perioticum came from Prof.
Ogawa's collection, and Megaptera came from the Antarctica by courtesy of Mr. G. Tandai. The authors acknowledge their great indebtedness to the above donators.
Cast specimens were made by use of acrylic resin in a way not very unlike the dentalroutine. After complete maceration and rinsing, the whole or part of the perioticum, depending on the size, is embedded in an investment flask within gypsum plaster. Plastershould be made in separable strata of base, so that the bottom seizes the bone capsule andthe upper negative can be detached. As the resin paste is made to permeate into thelabyrinthine lumen through the openings, such as two each of fenestrae and aquaeducts
and the internal acoustic meatus, these openings should be so placed as to line up andopen in the demarcation of the two plaster bases.
After these preliminary procedures, a proper amount of resin paste is squeezed into theumen by pressure of a screw press. The plaster bases tightly encapsulate the bone andthus prevent the raw resin from leaking out when the flask is pressed. However, thismoulding technique is a matter of considerable difficulty. This is followed then by polymerization as indicated, and the cast is taken out. To remove the petrous tissue of theperioticum, 5% nitric acid is applied. Ordinary alkali solution has no effect, especially inthe case of Odontoceti perioticum. As the semicircular canals are generally so minute,cast of these by alloy either of Wood or Lipowitz hardly survives such decalcification,although alloy is really wonderful material for casting purposes.
However, resin cast is easy to examine, and this advantage may be enhanced when thetransparent resin is used, because it renders, for instance, the lamina spiralis well visible
if they remain encapsulated within the cast specimen. Another advantage of the transparent resin may be well appreciated in the stereographs (Pls. I-III).
Measurement was made exclusively under microscope or according to microscopicaldrawing of low magnification. By such drawing, the method of which is to be briefly described, perfect parallel-ray-projection is attained. A specimen is observed under microscope and is traced along its contour by an intersection of a crossline plate, or preferably
a net micrometre plate, which is incorporated within the micrometre eyepiece. Now thatthe intersection is fixed this is attained by shifting the specimen horizontally with themechanical stage. The contour is thus plotted on graph ,paper as loci of nonius readingsof the mechanical stage at a convenient magnification, for example 10 x or 20 x. The
most important advantage of. this method is that the drawing is rendered absolutely freefrom any optical distorsion or perspective. This is because all the points are read on
one definite axis within the optical system. The use of verniers limits the errors within
the order of one tenth of a millimetre. Besides, no danger of destroying the specimen bytouch is incurred, which is otherwise by no means avoidable.
Measurement of the cochlear canal length was made according to a map, which is composed of a sequence of subdivided drawings. In order that each successive portion to be
drawn may be placed horizontally under microscope, a special holder was designed in theball-and-socket principle. Any portion of the specimen may in this way be evenly focusedby an adequate tilting of the holder. A small amount of modelling compound used for
dental purposes has proved suitable for fixing a specimen onto the holder.
Photography was made in all instances with a Leica, screwed on an arm vertically ad-

OSSEOUS LABYRINTH OF CETACEA

293

justable, and 135mm objective at a fixed beJlows extension 1:1. For stereo-photography(Pls. I-III), a special semicylindrical holder was designed in order to rotate the specimen.
A couple of photographs taken before and after an adequate angle of rotation make astereo. The specimen was water-immersed.

SIZE AND GENERAL NOTE

Text-fig. 1 illustrates the labyrinths of all the species examined at uniform 1.5 x magnification. It is self-explanatory, and therefore no figures
are given, for a common measuring criterion is rather difficult to set
up. At any rate, the size of labyrinth corresponds roughly, except in
the case of Physeteridae, with the body length of the species concerned.
For instance, that of Balaenoptera musculus, the greatest creature
which ever lived on earth, is the biggest, whereas Kogia possesses thesmallest one. Thus a special remark may be made that the two genera
belonging to Physeteridae possess labyrinths of reduced size in comparison with the body length.
Remarkable cetacean peculiarities are generally seen. The flat typeof cochlea is observed to have a rather small number of turns. The
whorl pattern is unusually robust and loose, and the vestibule andsemicircular canals are disproportionately minute. Closer examinationand comparison reveal however, that a rough division may be madeaccording to the taxonomic position of each whale species. As regards
the whorl pattern of the cochlea, for instance, they may be divided infour categories as: 1) Balaenidae, 2) Balaenopteridae, 3) PhyseteridaeZiphiidae and 4) Delphinidae. The labyrinth of Eubalaena (Balaenidae)
is so different and non-cetacean that for the sake of convenience andemphasis the species will be discussed as a separate matter at the end.
COCHLEAThe number of cochlear turns has been a main topic of interest inlabyrinthine morphology, and it more or less exceeds two in Mystacoceti, with an exception of Eubalaena, in which it is two and a half. In
Odontoceti on the other hand, it is slightly below two. The authors
do not intend to enter into more details because they are not inclined
to ascribe to the number of turns such a significance as has commonlylong been attributed to them. This is because the criteria are again
not easy to set up. The authors are inclined rather to put the matterin the ha;nds of those who are interested in the question, and actually
they can find out as they wish from the diagrams of Text-fig. 1.
As a matter of fact, whorl radius decreases as the cochlear canalgoes from the vestibular extremity up towards the apex. The rate of294

M. YAMADA AND F. YOSHIZAKI

Ic

Text-fig. 1. Cast specimens of cetacean labyrinths (1.5 x ). The modiolus of cochlea
is situated upright in all the drawings so that the number of turns of each may
be estimated according to the reader's criteria. 1-a. Balaenoptera musculus;
1-b. B. physalus; 1-c. B. boreaUs; 2. Megaptera; 3. Eubalaena; 4. Physeter;
5. Kogia; 6. Berardius; 7. Ziphius; 8. Globwephala; 9. Grampus; 10. Feresa;
11. Lagenorhynchus; 12. Delphinus; 13. Prodelphinus; 14, Neomeris.

OSSEOUS LABYRINTH OF CETACEA

295

such decrease is different from one species to another, namely it isgradual in Mystacoceti and more abrupt in Odontoceti. In Mystacoceti,
the apical whorl covers a part of the more basal portion in a way
similar to many terrestrial mammals. In its apical aspect, however,Mystacoceti is characteristic. Here a considerable space is unoccupied,embraced by an arc of apical whorl in a form not unlike a large comma, so that whole of the canalicular system within the upper modiolus
is exposed as cast. In Odontoceti, on the other hand, neither of thesepeculiarities occurs. There is no portion of the cochlear canal coveredby the other. The portion at the apex coils so tightly that the whorl
radius progressively decreases, and in the result there is no space asin the one observed in Mystacoceti. However, space is seen towards
the widely expanded base, particularly within the first quadrant of thecochlea. In short, the cochlear pattern of Cetacea is, with the exception of Eubalaena, loose in two ways. That of Balaenopteridae is looseon the apex, and that of Odontoceti in the basal portion or in the
beginning.
The rate of ascent of the cochlear canal is also different according
to species. Delphinidae is among others very unique particularly in thefirst quadrant of Neomeris as is shown in Plate-fig. 10. In addition to
the peculiarity as observed in its apical aspect (Text-fig. 1, 14) that
this portion of the cochlea considerably swerves out towards the bottom,
left of the reader, it does undulate also up over the round window anddown further, and up again. The detour over the round window isseen in many mammalian cochleae, although it is very conspicuous in
Cetacea and especially in Delphinidae. It should be noted that the
portion that swerves abruptly inclines from above the round window.
The canal also inclines even sideways. It seems natural therefore thatthe cochlear canal is conspicuously elongated within the first quadrant.
In order to put the matter in a more objective light, the cochlear
canal was sectioned by an optical method up into quadrants, and thelength of each was measured. The result (Text-fig. 2) reflects clearly,
as was expected in advance, the nature of whorl pattern. For instance,
the first quadrant of Odontoceti cochlea well exceeds 30% of the total
length. Graph of Physeter is a regular one, and well represents its
regular vorticose pattern. That of Balaenopteridae does not differ
greatly from the more common mammalian pattern belonging to thehigher type of cochlea. In other words, there are three categories ofquadrant graphs, as Balaenopteridae, Physeter and Delphinidae. Thoseof Kogia and Ziphiidae do not differ from Delphinidae, and may be included herein.
For a reference purpose, the human cochlea was sectioned in a similar

M. YAMADA AND F. YOSHIZAKI

296

way (Text-fig. 3). It is noticed, in this higher type of cochlea, thatoccasional irregularities occur in the graphs. The authors are of theopinion, however, that this is due rather to the deviation involved in
the measuring procedure than to the actual differences among individuals.
It is apparently because the section of cochlea in quadrants involves
some arbitrary factors, especially pertaining to the uprightness of the301--------lf - - - - - - - l 30

1------'20

JO%

Text-fig. 2. Graphs of quadrant-length of the cochlear canal.
1. Balaenoptera (muse.); 2. Physeter; 3. Kogia; 4. Globicephala.

20

Text-fig. 3. Graphs of quadrant-length of the human cochlear canal.
Occasional irregularity may not necessarily be the result of individual
variation, but probably measuring artefact which is not avoided in
the case of higher type of cochlea.

modiolus. In other words, if the modiolus is slightly tilted, the lengthof some or all quadrants as measured may deviate more or less.
Because of these experiences the authors propose not to put too muchweight on the number of turns, especially in a way that according to
the sequence of turns a particular locus of the cochlea is mentioned.

OSSEOUS LABYRINTH OF CET ACEA

297

In view of the enormous variety of the cochlear pattern ranging fromMonotremata to Rodentia, the authors should like to remind the readerof the fact that the cochlea is really nothing but a blind canal system.
Its number of turns may be the result of the ultimate length of thecanal itself and the space available to invest the organ. Chronologicalrelationships during development in between various tissue components
may also influence its final vorticose pattern. The authors thus believethat a particular locus in the cochlea should be indexed by its distancefrom the vestibular extremity in percentage of the total length. Thisapplies to all kinds of cochlea alike, and is not a question of cetacean
ear alone. The senior author actually has published recently some dataon its practical application to the human labyrinth (Yamada, 1959).
It is observed finally in the apical aspect that the scala tympani ischaracteristically spacious and exceeds the scala vestibuli in the basal
portion of larger cochleae. This is observed in Mystacoceti, except
Eubalaena, and in Odontoceti representatives such as Physeter, Berardius
and Ziphius. There is a noteworthy contrast between the surfaces of
the two scalae. That is to say, the scala tympani is rough, whereas
the scala vestibuli is smooth. Besides, on the surface of the latter in
Odontoceti, stand out radial markings for cochlearis propria arteries.
LAMINA SPIRALIS SECUNDARIA

A trial to estimate the basilar membrane was made by the senior author
on macerated material of Berardius (Yamada, 1953, Fig. 29). This was
possible because the secondary lamina spiralis of Berardius develops
well in a way that it is situated in the bottom of ligamentum spirale,
and therefore, this supports the whole ligamentum, in the range ofbasal 80% of the cochlear canal, almost to the attachment of the basilar
membrane. Therefore, the spiral slit, where it comes out in macerated
specimen between the two bony laminae, reasonably corresponds withthe actual expansion of the basilar membrane. Later investigation has
revealed that this structure is commonly seen in Odontoceti. The widthof spiral slit, in other words the width of the basilar membrane itself,
increases as it transits towards the apex of the cochlear canal. Width
increase of the basilar membrane is a fact very well known in all
mammalian cochlea, but there is a peculiarity which may be ascribed
to Odontoceti.
Text-fig. 4 shows some examples of such transition as compared witha curve of human basilar membrane after Wrightson and Keith (Fletcher,1939). Although this is presented according to a histological investigation, these may be compared with each other, and it is noticed that in298

M. YAMADA AND F. YOSHIZAKI

Odontoceti nearly half of the canal length is occupied by a very gradualincrease of the width, generally in a range of 0.1-0.2 mm. This is not
the case with the human cochlea, nor with an instance of Balaenoptera
physalus examined. This slow increase and striking development of thesecondary lamina spiralis in Odontoceti may be regarded altogether asmorphological evidence supporting our increasing knowledge of the cet1.0o.s

Physeter

0

11

0

_D
E

0,5

Berardi us

Q)

E

~
0

-"

0

10

20

30

40

20

30

40

~
0
..c

-u

o.s

Globicepholo

~

0

o.s
0

Homo (ofter Wrightson & Keith)

1050

Length of the cochlear canal (mm)Text-fig. 4. Rate of the width increase of the basilar membrane in Odontoceti representatives, as estimated from the slit between the two spiral laminae. Upper
limit of the secondary lamina is also shown (perforated lines), which is 68% forPhyseter, 79-82% for Berardius, and 81% for Globicephala. Note over-all
similarity of the three, each representing its own family. Curve of human basilar
membrane after W rightson and Keith is for reference. Similar data for Mystacoceti is obtained in too small a range because of the far weaker developmentof the secondary lamina. Curve of 15% range of Balaenoptera physalus falls
on that of man. From the macerated specimen no further information is available.

acean hearing capabilities, namely that these whales can hear a wider
range of frequencies and especially high pitches of sound including
ultrasonics (e.g. Kollogg, 1953).
The spiral laminae are delicate but they may be well preserved insome instances as Plate-figs. 4 and 6 show, where they are observed satisfactorily well through the transparent cast material. In many instances

OSSEOUS LABYRINTH OF CET ACEA

299

however, they are broken and the debris are scattered and give to thecast an ugly appearance (Plate-figs. 7-10).
In Mystacoceti, however, the secondary lamina spiralis is definitely
weaker. Reysenbach de Haan (1957) wrote as though there was certain
controversy about Mystacoceti cochlea in this particular respect. According to him, similar development of the secondary lamina is seen
also in some rodents and Chiroptera, and he expected the same to occur
in Mystacoceti also. However, the difference between the two cetacean
suborders in this respect is in actual fact d()finite as stereographs ofBalaenoptera and Megaptera (Plate-figs. 1-2) show it clearly. In one
example of Balaenoptera physalus, the secondary lamina is to be seen
in a much shorter region of no more than 40% of the total length, andthe slit measures 0.19 mm in width at the vestibular extremity. So
far as the basal approximate 15% is concerned, where the slit is expected to coincide with the actual basilar membrane, the width increases
in a similar curve like the one of human membrane after Wrightson
and Keith (Text-fig. 4).
SEMICIRCULAR CANALS

Text-fig. 5 shows some examples of semicircular canals presented in aform comparable with those of the human labyrinth. The canal system
is a three-dimensional structure, and it is consequently difficult to illustrate. Text-fig. 5 is, in that sense, a new type of illustration, in
which crus simplex of the lateral or horizontal canal is dislocated at
the base and the whole canal system is arbitrarily expandea in a single
plane. An important advantage of this is probably appreciated thatthe three canals are put into a better comparison both in form anddimension. It is again self-explanatory to a certain extent.
The size differs roughly in accordance with the body length, andNeomeris is the smallest of all. However, dimensions of the ampullae
as well as the span of each canal are not exactly proportionate. In
other words, in smaller canals the height of arch becomes progressively
less and the ampullae relatively greater. It is observed, in histologicalsections and whole preparations of the membranous labyrinth, thatthe osseous canals do not directly reflect the actual calibre of themembranous ductus within them. Thus the membranous ductus isrelatively thicker in smaller canals.
But the most conspicuous respect of the semicircular canal system of
Cetacea is the dimensional relationship of the three. It is noticed generally that the posterior canal is, with the exception of Eubalaena, thesmallest of the three. In Odontoceti, this is more conspicuous and the300

M. YAMADA AND F. YOSHIZAKI

lateral canal is, in good contrast with the posterior, the greatest. The
superior or anterior canal is the intermediate. This is really striking because in many of the land mammals, either anterior or posterior canal is thegreatest, whilst the lateralis always the smallest.
It may be too early tomention from this findingthat the horizontal movement either of the headand neck or the entire
body is of the most important functional signi2
ficance in cetaceans. But
the authors' impression isthat such muscular control
as is called upon by the4vestibular reflex, originating among others in thelateral canal, may be aunique one which thecetaceans have learned in
the long course of evolution to be important in
the life under water.
Such control might beneeded the more in Odontoceti which is generally
supposed to be more alert
and active.

(J Oo

EUBALAENA GLACIALIS; A STRIKING EXCEPTION

Text-fig. 5. Semicircular canals, arbitrarily expanded.
In all the drawings the canals are from left to right:
lateral, anterior and posterior. 1. Balaenoptera (musculus); 2. Kogia; 3. Physeter; 4. Neomens; 5. Ziphius; 6. Eubalaena; 7 Man (for reference).

Finally special mention
should be made of theright whale, Eubalaena
glacialis. The labyrinthof ' Balaena ' has been
described and illustrated
Hyrtl
only once by(1845) as reproduced inOSSEOUS LABYRINTH OF CETACEA

301

Text-fig. 6. His illustration was also reproduced in Bolk's Handbuch
der vergleichenden Anatomie by de Burlet (1934). Gray (1907) was another who mentioned the labyrinth of 'Balaena australis ', but no illustration was given because his specimen of the membranous labyrinth
was far from perfect due to the putrefactive changes. However, hiscomment seems worthy of quotation: " From
what could be seen the labyrinth differedfrom that of the porpoise only in size ".
The present authors themselves, andprobably all those who were interested in
the topic alike, could hardly suspect thegenuineness of these previous findings. Consequently, it has long been a widely acceptedview that there exists such a general uni- Text-fig. 6. Balaenamysticetus,
formity which may be called "cetacean" as
only one illustration of thiswas mentioned previously. It was only resort given by Hyrtl (Naturalsize reproduction after Fig. 7

cently, however, when two specimens ofin Taf. IX).
Obviously the
Eubalaena were captured in the Japanese
species is not genuinely identiwaters for a scientific survey (e.g. Omura,fied.
1957) and when the authors were blessed with the opportunity of examining these animals that the labyrinth of right whale was first
known, against the common belief, as an extraordinary exception.
Cast specimen is illustrated stereographically (Plate-fig. 3), and one
may at first sight notice this. The cochlea is of rather pointed hightype in two and a half turns which coil tightly. The semicircular
canal system is not diminished. Moreover the conspicuous relationship,
noticed in the size of the semicircular canals generally, is not apparent
in the right whale. Any significant difference is not noticed in the dimension of the three canals. In actual fact it is rather terrestrial, andstrikingly "non-cetacean". It is obvious therefore that the right whaleof neither Hyrtl nor Gray was genuinely identified.
The authors are pot in the position to raise a speculation as to thereason of diminished vestibular organ of the cetaceans, but it seems
worth while to pay attention to a recent paleo-biological statement by
Edinger (1955). According to her, the cerebellum develops in earlycetaceans better than the cerebrum, vestibular nerve being dominant tothe acoustic. Such interrelationship apparently has been reversed during the course of evolution as the function demands. In view of thefact that the unveiled findings on the right whale have something incommon with Edinger's statement, it may well be mentioned that theright whale is, at any rate, probably a species which still retains to a302

M. YAMADA AND F. YOSHIZAKI

marked degree such primitive status as the early fore-runners had once
possessed. This whale is, in that sense, a "Living Fossil" in the
cetacean evolution. The authors naturally look forward to the possibilityof examining a fossil labyrinth. Gray whale (Rhachianectes) may be
another species that should be investigated.
One more mention may be made in this connexion of a recent communication by Aoki (1957). A high school boy, who is diagnosed as
cerebellar ataxia, is incidentally an excellent swimmer but strongly
ataxic when he tries to stand and walk. It is suggested in- the resume
that the cerebellar innervation of muscular tonus is under-developed andthat the tonus itself becomes unnecessary to such an extent, when thesubject is in the water, as up to the amount of floating force which
reduces his body weight considerably. Although nothing is known
about the vestibular system of the patient, neither in the central nervous system nor in the labyrinth itself, this may eventually be a representation of the actual transition that happened in the cetacean
history.
The posture is, in Cetacea, no longer balanced on the limbs as in theland animals. The authors have become rather inclined therefore, tobelieve that the noticeable alteration of the balancing in Cetacea might
be responsible for the diminution of the semicircular canals. No doubt,
however, much evidence should be extensively collected, probably also
in the central nervous system, before anything more definite can besaid. Previous views on this problem will be discussed when such opportunity arises.
SUMMARY

With the aim of obtaining fundamental data for subsequent histologicalinvestigation, labyrinths of 16 species of Cetacea were investigated,
mainly on cast specimens. New findings and remarks are summarised
as follows.
1) As regards the dimension of the labyrinth, Physeteridae representatives (Physeter and Kogia) are relatively small in comparison withthe body length. Otherwise the dimension diverses roughly according
to the specific body length.
2) Vorticose pattern of the cochlea is different according to thetaxonomy. In Mystacoceti the vortex is open on the apex, and in
Odontoceti, especially in Delphinidae, the cochlear canal of the first
quadrant region swerves in a unique way away from the modiolus. In
the consequence of such detour, the first quadrant is strikingly long.
The length exceeds 30% of the total length.

OSSEOUS LABYRINTH OF CET ACEA

3033) Since the cochlear pattern differs considerably according to species,
a proposition is made to adopt widely an " index method " in order toindicate a particular locus of the cochlea. In place of the more popularmethod, in which the sequence of turns is mentioned, the proposed
method puts the special weight on the distance of each locus from thevestibular extremity in percentage of the total length. This is not so
difficult a matter as might be expected, and it has been found to bepractical. This applies not only to Cetacea, but to all mammals including man and experimental animals.
4) A unique peculiarity of Odontoceti is that the secondary lamina
spiralis develops up to as far as 68-82% of the cochlear canal. Thereis evidence that the slit between this and the primary lamina spiralis
coincides, within this range, with the actual basilar membrane. In thefirst 50% range, the basilar membrane increases its width at an extremely slow rate, generally between 0.1-0.2 mm. This is probably related
to the high frequency hearing capability of those animals. In Mystacoceti, the secondary lamina spiralis is far less noticeable. In one exampleof Balaenoptera physalus, the slit between the two laminae is estimated to coincide only in the first 15% range with the basilar membrane. Width increase in this range is as in human membrane.
5) As regards the semicircular canals, which as unanimously agreed
are extraordinarily diminished, the size of each is in a reverse relationship in contrast to many, if not all, terrestrial mammals. The lateralis the greatest and the posterior the smallest, especially in Odontoceti.
6) Eubalaena presents a serious exception, in which none of the
known cetacean peculiarities are to be found. Cochlea is of high type,closely coiled in two and a half turns. Semicircular canals are wellpreserved, and the general impression is strikingly " non-cetacean".
This is an absolutely new finding. In view of paleontological transition
between the equilibria! and acoustic nervous components (Edinger, 1955),
Eubalaena may well be called as a " Living Fossil " in the cetacean
descent.
The authors are grateful to Professor H. Outi for his encouragement
and helpful criticism. Miss D. E. L. Brown kindly read the script andmade corrections.

REFERENCES
AOKI, S. (1958).

One case of equilibria! ataxia (Japanese Resume). Jibi Rinsyo, 51(1): 9.
Die Innere Ohrsphiire. Bolks Handb. d. vergl. Anat., 2(2):BURLET, H. M. DE (1934).

1293-1380.
EDINGER, T. (1955).

Hearing and smell in cetacean history.

Monatschr. f. Psych. u.

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M. YAMADA AND F. YOSHIZAKI

Neurol., 129: 37-58.
FLETCHER, H. (1939). Speech and Hearing. New York (Nostrand), p. 117, Fig. 64.
GRAY, A. A. (1908). The labyrinth of animals. London (Churchill), vol. 2: 22-34 (Cetacea
and Sirenia).
HYRTL, J. (1845). Vergleichend·anatomische Untersuchungen iiber das innere Gehororgan
des Menschen und der Siiugethiere. Prag (Ehrlich): 91-130 (Labyrinth), Taf. IX.
KELLOGG, W. N. (1953). Ultrasonic hearing in the porpoise, Tursiops truncatus. J. comp.
and physiol. Psych., 46: 446-450.
MONRO and COMPARETTI (1789). Observationes anatomicae de aure interna. Patavii: 156(cited from Rapp).
OMURA, H. (1958). North Pacific Right Whale. Sci. Repts. Whales Res. Inst. (Tokyo),
13: 1-52.
RAPP, W. (1837). Die Cetaceen, zoologisch·anatomisch dargestellt. Stuttgart and Tiibingen(Cotta): 98-104 (Gehororgan).
REYSENBACH DE HAAN, F. W. (1957). Hearing in whales. Acta Oto·Laryngol., Suppl. 134:
1-114.
YAMADA, M. (1953). Contribution to the anatomy of the organ of hearing of whales. Sci.
Repts. Whales Res. Inst. (Tokyo), 8: 1-79.
- - - (1959). Basic data for the standard sectioning of the human ear labyrinth (Japa·
nese Text). Kagak:u, 29(7): 363-364.
YAMADA, M. and YOSHIZAKI, F. (1957). On the osseous labyrinth of cetacea (JapaneseResume). Acta. Anat. Jap. (Proc. Soc. Anat. Jap.), 32: 44.

EXPLANATION OF PLATESSpecimens are generally so placed that as good as possible comprehension of the structure
may be attained, hence there is no uniformity as to the direction like that of Text-fig. 1.
Specimens are water-immersed. As regards the method of photography see ' Material andMethod'.
PLATE I
Fig. 1.
Fig. 2.
Fig. 3.

Balaenoptera musculus (left). 2. 0 x
Megaptera nodosa (right). 2. 6 x
Eubalaena glacialis (right). 2.6 x

Fig. 4.
Fig. 5.
Fig. 6.

Physeter catodon (right). 2.6x
Kogia breviceps (right). 4.6x
Berardius bairdii (right). 3.0 x

PLATE II

PLATE III
Fig. 7. Globicephala melaena (right). 3.2x
Fig. 8. Delphinus delphis (right). 3. 8 x
Fig. 9. Neomeris phocaenoides (right). 4.3 x
Fig. 10. Neomeris phocaenoides. 4. 7 x The same specimen as Fig. 9 is seen from the side.
Note the strong detour and undulation of the cochlear canal in the first quadrant region.

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