THE STOMACH OF THE BOUTU, INIA GEOFFRENSIS:COMPARISON WITH THOSE OF
OTHER PLATANISTIDSFUSAO YAMASAKI* AND TOSHIRO KAMIYA*** Department of Biology, Sapporo Medical College, Sapporo** Anatomical Laboratory, College ef Medical Technology, Universiry of Tsukuba, Jbaraki
ABSTRACTThe stomach of the Bou tu, Inia geoffrensis, was examined macro- and lightmicroscopically, and was compared with those of the three other species of
Platanistidae as well as with those of some sea dolphins. The stomach of theInia consisted of a forestomach, main stomach, connecting channel, and pyloric stomach. The esophagus was more straightly continuous with the main
stomach than with the forestomach, and the opening to the main stomach waslarger than that to the forestomach. The biftucating manner of the esophagusof two species of Platanistidae, Inia and Platanista, was different from that ofsea dolphins, in which the esophagus leads directly to the forestomach. The
forestomach of the Inia, lined with stratified squamous epithelium, was relatively smaller than that of the Platanista, and those of sea dolphins. The
main stomach of the Inia was a large thick-walled musclar sac, rather resem bl·
ing that of Pontoporia, without any partitions as seen in the Platanisata and
Lipotes. It was lined with a plicated glandular mucous membrane possessing
mucous, parietal, and chief cells. A narrow zone of cardiac glands existed
only adjacent to the esophagus. The main stomach communicated with thepyloric stomach by a crooked, narrow connecting channel, which lay in thecaudo-dexter part of the main stomach. The channel was similar to thatof the Pontoporia in its location and shape. The elongated tubular pyloric
stomach lay in the dorso-dexter part of the connecting channel, and was smaller than the globular pyloric stomach of Platanista and the ]-shaped one of the
Pontoporia. The mucosae of the channel and pyloric stomach were similarand contained pyloric glands. The histological nature of each part of thestomach observed in the three species of Platanistidae and in other sea dolphins
was fundamentally the same. A distinct sphincter could be seen between theend of the pyloric stomach and the duodenal ampulla as in other dolphins.
INTRODUCTION

A number of morphological studies of the stomach in several species of sea dolphinshave been made by several investigators. Those done on the stomach of the fresh
water dolphins, Platanistidae (in which four genera, Pontoporia, Platanista, Lipotes,
Inia are included), however, seem to be very small in number, probably due to thisfamily's special distribution. Burmeister (1867, 1869) and Anderson (1879) reSci. Rep. Whales Res. Inst.,

No. 33, 1981, 69-81

70

YAMASAKI AND KAMIYA

ported upon the anatomy of the Franciscana, Pontoporia blainvillei, and the Susu,
Platanista gangetica, respectively, the structures of the stomachs of which were described briefly. We also made macro- and light microscopical observations of thestomachs of Platanista (Yamasaki and Takahashi, 1971) and Pontoporia (Yamasaki
et al., 1974). Hinton and Pycraft (1922) made a preliminary note on the morphology of the Baiji, Lipotes vexillifer, including a very brief description of the morphology of the stomach. Recently, anatomical reports on the stomach of Lipotes
have appeared from Chen and Chen (1975), and Zhou et al. (1978, 1979ab). Sinceamong the stomachs of Platanistidae the morphology of that of Inia has not yet
been reported upon, the structure of the stomach of this dolphin will be reported
upon here and will be compared with those of the three other species of Platanistidae and some sea dolphins which have been described.
MATERIAL AND METHODS
The specimen of the stomach of the Bou tu (Amazon dolphin), Inia geojfrensis, (bodylength 204 cm, female) used for this study was provided by the Kamogawa Sea
World, Chiba, Japan, after death from unidentified disease. Owing to contraction due to fixation, and deformity caused by transportation to our laboratory, theexternal features of the stomach seemed to vary considerably from the natural state.
The organ was preserved in 10% formalin solution and small pieces were taken forlight microscopy. Paraffin sections from these were stained with hematoxylineosin, Azan, PAS and Alcian blue for histological examinations.
The stomachs of Platanista and Pontoporia already reported upon by us and of
Stenella coerureoalba were reexamined for comparision.
OBSERVATIONS
Although the stomach may vary in size and the appearance of the exterior or theinterior may depend on whether the stomach is in a fixed or unfixed state as wellas on the amount of its contents, an empty Inia stomach fixed was examined and thefollowing features were obtained. The stomach of the Inia consisted of three compartments; fore, main, and pyloric stomachs. A narrow compartment or channel
connected the main stomach with the pyloric stomach. The first part of the duodenum immediately distal to the pyloric stomach was dilated and formed the duodenal ampulla. External grooves, which might have been visible between compartments in good condition, could not be clearly distinguished owing to considerable deformity. The thick-walled esophagus, which had distinct longitudinal folds
with many fine transverse folds in its interior, was more straightly continuous withthe main stomach than with the forestomach, and its thick mucous folds generally
continued toward those of the main stomach.
Forestomach The forestomach was located in the dorsal and slightly left side of themainstomach and was a pear-shaped muscular sac (Fig. 1). The cranial twothirds of its ventro-dexter wall adhered to the dorsal wall of the main stomach.
Sci. Rep. Whales Res. Inst.,
No. 33, 1981

STOMACH OF PLATANISTIDS71

The depth of the forestomach, from the biflucation between the fore- and main
stomach to the bottom of the forestomach, was approximately 12 cm and its width
was about 4 cm at the lower widest part. The interior of the forestomach had
firm, mainly longitudinal folds. Several folds were continuous with those in theesophagus but were far thinner. There were also occasional transverse folds. The
wall of the forestomach was far thinner than that of the esophagus, and was about
5 mm in thickness at the dorso-sinister wall, becoming thick, about 7 mm or more,at the bottom and at the wall facing the main stomach. There was no definablesphincter at the opening.
The mucosa of the forestomach was non-glandular, consisting of stratified
squamous epithelium continuing from the esophagus (Fig. 3). Its thickness, however, was about half or less than that of the esophagus, measuring from 0.3 to
0.4 mm in thickness. The superficial cells of the epithelium appeared to havelost their nuclei and undergone cornification. The connective tissue papillae occupied about 80% of the total thickness of the epithelium. The tunica musclaris
was far thinner than that of the esophagus, consisting of inner circular and outerlongitudinal smooth muscle layers, about 0.5 mm thick in all near the bottom of
the forestomach, with the former being roughly twice the thickness of the latter.
No glands were seen within the forestomach.
Main stomach This was a firm, thick-walled muscular sac and was the largest compartment (Fig. 1). It was somewhat pear-shaped and measured about 18 cm indepth, and 11 cm across at the upper widest part. The entrance was approximately 1.5 cm in diameter and no definable sphincter existed. The main stomach was lined with a thick, mucous membrane thrown into numerous thick folds
giving the whole a labyrinthine appearance. In the lower part, however, the folds
were somewhat longitudinal in arrangement. An abrupt change at the entrance
from the stratified squamous epithelium of the esophagus to the glandular epithelium of the main stomach was exhibited by the mucous membrane (Fig. 4 ). Thisepitheliar border obliquely ran from the cranio-dexter to the caudo-sinister reaching about 2 cm lower than the septum between the main and forestomachs. There
was an opening, about 3 mm in diameter, that led into the next compartment, aconnecting channel, at the ventro-dexter wall of the main stomach, about 2 cmfrom the caudal end of the main stomach. The wall, which seemed to be considerably contracted by fixation, was very thick, from 1 to 2.5 cm.
Light microscopically, tubroalveolar glands composed of light cells of one kind
could be seen only along the narrow zone, about 1 mm in breadth, adjacent to theborder between the esophagus and the main stomach (Fig. 4 ). Although thematerial was in poor condition, these glands were clearly distinguishable from theremaining glands of the main stomach and were surely cardiac glands. The glands
covering the remainder of the main stomach were seen to be of the same structure.
They were simple and tubular, with little evidence of branching, and measured
from 2 to 2.5 mm in length (Fig. 5 ). The glandular mucosa, with surface mucous
cells which were stained with PAS, were continuous with the epithelial lining of
the gastric pits, measuring about one eighth of the length of the glands. AlthoughSci. Rep. Whales Res. Inst.,
No. 33, 1981

7
2

YAMAS
八KI 八N DK A
恥1
1
Y八

pos
t
‑
mortem de
g
ene
r
at
i
ono
ft
h
eglandc
e
l
lsp
r
e
ve
nt
e
d an e
x
a
cth
is
t
o
l
og
i
c
a
lex
‑
aminat
i
o
n,c
h
i
e
fandp
a
r
i
e
t
a
lc
e
lsc
o
u
ld bes
e
e
nal
ong t
h
ec
r
i
p
t
s
. S
t
a
i
n
i
n
gwi
t
h
c
le
n
c
eo
fmucousne
c
kc
e
ls
. P
a
r
ie
t
a
lc
e
l
l
s
PASand A
l
c
i
an bl
u
er
e
v
e
a
l
e
dno 巴vi
werenume
r
o
u
s
,be
i
ngu
n
i
f
o
r
m
l
yd
is
t
r
i
b
ut
e
dc
lownt
ot
hebas
eo
ft
hegl
a
n
d
. Ch
ie
f
c
e
ll
swerenots
ob
a
s
o
p
h
i
li
ci
ndy
e
i
n
gi
nou
rs
p
e
c
imen. Theapproxi
mater
a
t
i
oof

Sc
i
.Re
p.T
V
h
a
l
e
sRe
s.
I
n
s
!
.,
No.3
3
,1
98
1

STOMACH OF PLATANISTIDS73

the parietal cells to the chief cells was 1 : 3 or less. Musclaris mucosa did not form
a conspicuous compact layer. Several relatively well developed smooth muscle
layers, from 0.5 to 1 cm in thickness mainly arranged circularly, could be seen in
the loose connective tissue of the very thick tela submucosa. There were many
relatively thick blood vessels and few thin nerve fiber bundles in the submucosa.
The muscular coat consisted of two complete layers (inner circular and outer longitudinal). The former was about twice as thick as the latter, with both together
measuring about 1.5 mm.
Connecting channel The channel was situated on the right near the caudal end of
the main stomach and, on the whole, was roughly in a sagittal plane (Fig. 1).
The course of the channel was rather dorsad in direction for approximately 1 cm,
then dextro-caudad for about 2 cm. Thereafter it curved upwards for about 2 cm,
reaching almost to the dorsal side of the main stomach. It then curved to the rightfor a short distance, after which it entered the pyloric stomach. The diameter ofthe channel was about 8 mm throughout its whole course, with there being no
sphincteric structures along the channel, though its entrance become slightly narrower. The inner surface of the channel exhibited some low longitudinal folds.
The caudo-ventral wall of the channel of the mid-part was thin, about 5 mm thick,with the remainder being far thicker.
The mucosa of the connecting channel was composed of tubular glands of amucous type (Fig. 6). The thickness of the mucosa was from 0.5 to 1 mm. The
pits extended about halfway the length of the glands. The glands were not tightly
packed and were separated by a relatively well developed lamina propria. Muscularis mucosa was inconspicuous. The submucosa was about half the thicknessof that of the main stomach and was relatively vascularized. The muscularis consisted of two layers and measured approximately 1 mm or more in all at the ventral
wall. The muscle on the dorso-sinister side of the channel was continuous withthat surrounding the main stomach.
Pyloric stomach The pyloric stomach was an elongated tubular compartment, with
its cranio-ventral part closely abuting the dorso-dexter wall of the main stomach(Fig. 2). It was situated in a cranio-sinister to caudo-dexter position, lying roughly
in a frontal plane. The pyloric stomach was approximately 6 cm in length andabout 1.5 cm or more in diameter. The opening from the connecting channel waspresent on the ventral wall and was about 1.5 cm away from the cranial end of thepyloric stomach. The pyloric sphincter, containing muscle and with an opening.-Fig. I. The inner aspect of the stomach and the initial part of the duodenum oflnia geoffrensis, which is cut off along the lesser and greater curvatures. The esophagus is more straightly continuous to the main stomach than to the forestomach.
Note the differences of thickness in the wall, and the appearance of the mucosa of
the forestomach and of the main stomach. E-esophagus; Fs-forestomach; Msmain stomach; Cc-connecting channel; Ps-pyloric stomach; Da-duodenal ampulla;
Dp-duodenum proper. An arrow indicates the entrance of the connecting channel.
.-Fig. 2. Dorsal view of the stomach and the initial part of the duodenum of the lnia.
A portion of the interior of the anal part of the connecting channel, the pyloric
stomach, and the duodenal ampulla are seen. for abbreviations see Fig. I.

Sci. Rep. Whales Res. Inst.,
No. 33, 1981

7
4

YAMASAI
くI八ND K八l
'
v
f
l
YA

納
5．
S
c
i
.Re
/
J.Wha
l
e
sRe
s
.I
n
s
t
.
,
No.3
3
,1981

STOMACH OF PLATANISTIDS

75

about 2 mm in diameter, lay at the distal end of the pyloric stomach. The opening led into the duodenal ampulla which was about 2.5 cm in diameter (Figs 1 and
2). The wall of the pyloric stomach was approximately 5 mm in thickness. Its
inner surface exhibited several folds arranged longitudinally which might be obliterated in the living state.
The histological structure of the pyloric mucosa was fundamentally similar tothat of the connecting channel (Fig. 7). The mucosa was about 0.6 mm thick.
The lamina propria contained some lymphatic nodules in places. The appearanceof the musclaris mucosa, submucosa, and muscularis also resembled those of theconnecting channel.
COMPARATIVE CONCLUSIONS AND DISCUSSIONAs shown in Figure 8, there are considerable interspecific morphological differencesin the stomach of Platanistidae. One notable difference is the presence or absenceof the forestomach, which is an esophageal diverticulum existing in most cetaceans
and which is sometimes referred to as the first or esophageal stomach. That is, theforestomach exists in Platanista* (Anderson, 1879; Yamasaki and Takahashi, 1971)
and Inia**, but not in Pontoporia (Burmeister, 1867, 1869; Brownell and Ness, 1970;
Yamasaki et al., 1974) and Lipotes (Hinton and Pycraft, 1922; Chen and Chen,
1975; Zhou et al., 1978, 1979ab).
The esophagus of the Inia seems to be more straightly continuous with themain stomach than with the forestomach, and the opening of the forestomach israther smaller than that of the main stomach. In Platanista the esophagus leads tothe forestomach and main stomach with roughly the same diameter through thebiflucation which Anderson (1879) described as the" common opening". In thecase of sea dolphins (Gihr and Pilleri, 1969; Harrison et al., 1970; Smith, 1972;
our observation) the esophagus leads to the forestomach directly. The opening of+-Fig. 3. A photomicrograph of the forestomach wall. Stratified squamous epithelium and the tunica musclaris are thin compared with those of the esophagus. No
glands are seen within the forestomach. H-E stain. X 25+-Fig. 4. A photomicrograph of the epithelial transition between the esophagus andthe main stomach. Cardiac glands are found at a narrow zone between theesophageal mucosa and the fundic glands. Ee-esophageal epithelium; Cg-cardiac
glands; Fg-fundic glands. H-E stain. x 40+-Figs 5, 6 and 7. Same magnification photomicrographs of the mucosa of the main
stomach (Fig. 5), the connecting channel (Fig. 6), and the pyloric stomach (Fig.
7). Note the differences in the glandular nature and thickness of each mucosa.
H-E stain. x40* Anderson (1879) and Yamasaki and Takahashi (1971) referred to the forestomach as the" first cavity"
and the" first compartment", respectively.
** We mentioned the presence of the forestomach of the lnia in the description of the digestive tract ofPontoporia (Yamasaki et al., 1974).
Sci. Rep. Whales Res. Inst.,
No. 33, 1981

76

YAMASAKI AND KAMIYA

the forestomach from the esophagus, which usually projects rather like a nozzle into
the forestomach, is far larger than that of the main stomach. These specific fea-

Fig. 8. Diagramatic representation of four stomachs of Platanistidae: a-Platanista,b-Pontoporia, c-Inia, and d-Lipotes (modified after Zhou et al., 1979b). Note therelationships of the position, proportion, and communication of each compartment. Fs-forestomach; Ms-main stomach; Mso and Msa-oral and anal parts of
the Platanista stomach; Msl, Ms2 and Ms3-first, second and third parts of the
Lipotes main stomach; Cc-connecting channel; Ps-pyloric stomach; Psb and Pstbulbous and tubular parts of the Lipotes pyloric stomach.

tures in lnia and Platanista suggest that swallowed food may enter the forestomach
and main stomach at the same time or the main stomach first, while in sea dolphinsswallowed food may enter the forestomach directly.
The forestomach of the lnia observed was a pear-shaped muscular sac, andwas considerably small compared with the main stomach. It was also relatively
Sci. Rep. Whales Res. Inst.,

No. 33, 1981

STOMACH OF PLATANISTIDS

77

smaller than that of the Platanista. In observed cases of sea dolphins, it has been
bigger (Jungklaus, 1898; Gihr and Pilleri, 1969; Harrison et al., 1970; our observation), or smaller (Smith, 1972) than the main stomach. The forestomachs of theobserved Inia and Platanista, however, were relatively small in depth and width
when compared with those of sea dolphins.
Harrison et al. (1970) suggested that sea dolphins take as many fish as possible
when the opportunity arises. The forestomach in adult dolphins is therefore considered to act as a place for the storage and breaking down of foods. In unweaned
cetacean calves the forestomach is much smaller than in adults (Slijper, 1962;
Smith, 1972), and in Mysticeti, which feed on small plankton, the forestomach issmaller than the main stomach (Slijper, 1962). In addition, H_yperoodon and Ziphiushave no forestomach (Weber, 1886; Slijper, 1962). It is also said that these animals, which feed exclusively on soft food such as squid, could easily dispense withthe forestomach. However, comparing the stomachs of the four species of Platanistidae with their food habits, provides some indication that there may be no direct
relationship between the presence or absence of the forestomach and food habits,
since dolphins having no forestomach are known to not always feed only on soft
food*. In addition, since the muscular layer of the forestomach of the Inia andPlatanista seems to be comparatively thinner than that of sea dolphins, it could besaid that such a function, storage and breaking down of foods, in the forestomachof the Inia and Platanista is inferior to that in sea dolphins. According to Smith(1972), it is thought by several investigators that despite the non-glandular natureof the forestomach of sea dolphins some chemical activity takes place there involving gastric juice from the main stomach. If this usually occurs in the cetacean
forestomach, it may be assumed that gastric juice may flow far more easily in Inia
and Platnista than in other dolphins.
Hinton and Pycraft (1922) stated that the stomach of Lipotes was very primitive in form, the ventriculus (probably indicating the forestomach) being widely
confluent with the second compartment (=main stomach) in Platanista and otherdolphins. Although further developmental studies may ·be expected to clarify
whether this deficiency in the forestomach of Pontoporia and Lipotes is the result ofregression or confluence with the main stomach or is a case of original lack ofdevelopment, it is of interest to consider the biological meanings that may be attached to this problem of the forestomach of Platanistidae.
The border line between the esophagus and the main stomach is quite clearly
indicated by the abrupt change in lining, from stratified squamous epithelium toa glandular mucosa in all species of Platanistidae, while the line of the Inia runs
almost transversely, or at a right angle to the long axis of the main stomach (Fig.
Sc).
As shown in Figure 8, the main stomach of the Inia is rather a simple sac, with
its shape resembling that of the Pontoporia. On the other hand, the main stomach* See for food habits:

Burmeister, 1869; Brownell and Ness, 1970; Fitch and Brownell, 1971; Pilleri,
1971; Brownell and Herald, 1971; Pilleri, 1972; Chen and Chen, 1975; Zhou et al., 1977; Chen et al., 1980.

Sci. Rep. Whales Res. Inst.,.No. 33, 1981

78

YAMASAKI AND KAMIYA

of Platanista* and Lipotes* * is divided into two and three parts respectively: the oral
and anal compartments in the former (Yamasaki and Takahashi, 1971 ), and thefirst, second, and third compartments in the latter (Zhou et al., 1978; Zhou et al.,l 979ab). The partitioning of the anal part of the main stomach of the Lipotes canbe seen to be more complicated than that of the Platanista. Although it is difficult
to compare the relative sizes of the main stomachs of the species of Platanistidaewith one another, in species lacking a forestomach the main stomach seems to bebigger in proportion than that of species having a forestomach. If one includes
the anal two compartments as part of the main stomach the volume of the main
stomach of the Lipotes may be considered to be rather large. This may be a compensation for the absence of the forestomach for the storage of food.
The cardiac glands, which were found for the first time in the cetacean stomach by Hosokawa and Kamiya in 1971 in a blue whale, exist at the narrow zone
adjacent to the esophageal mucosa in the Inia as well a~ in Platanista (Yamasaki
and Takahashi, 1971) and Pontoporia (Yamasaki et al., 1974). In a Lipotes described by Zhou et al. (1978) the cardiac gland zone was more well developed thanthose in the Platanista and Pontoporia we reported. Although Smith (1972) statedthat no cardiac glands existed in the Phocoena phocoena, the glands may usually befound in other cetacean stomachs when all parts of this region are examined. Allof the mucosa of the main stomach consist of glands of the same type, except for anarrow zone of cardiac glands. Superficial mucous cells, chief cells, and parietal
cells have been detected in gastric epithelium in the Inia, Platanista, and Pontoporia
we observed. Mucous neck cells have not been detected in these dolphins, possibly
due to post-mortem changes. The numerical ratio of parietal cells to chief cells
was approximately 1: 3 or less in all three species observed. This is in good accordance with the results of Harrison et al. (1970) and Smith (1972) in several sea
dolphins, and of Hosokawa and Kamiya (1971) in baleen whales. It is interestingthat this ratio of dolphins living in fresh water is almost the same as that for thoseliving in the sea. Zhou et al. (1978; 1979b) found the mucous glands intermingledwith fundic glands of the main stomach of the Lipotes, and stated that this was acharacteristic feature of this dolphin.
The chief functions of the main stomach and forestomach of cetaceans are,respectively, chemical and mechanical digestion. Sand and small stones are often
found in the cetacean forestomach and may play some part in mechanical digestion
along with the highly muscular wall (Slijper, 1962). In Lipotes, dozens of stones,
3 cm in maximum diameter, were reported in the main stomach by Zhou et al.

* Anderson (1879) and Yamasaki and Takahashi (1971) referred to the main stomach as the "second
cavity" and the "second compartment'', respectively. Anderson did not consider the second cavity as
two separate chambers.
** Hinton and Pycraft (1922) observed that the stomach of the Lipotes was less completely segmented
proximally than in most other genera and found that towards the pylorus several small compartments wereshut off as usual. Chen and Chen (1975) indicated that the stomach consisted of four chambers, twolarge proximal compartments separated by a rudimentary septum, and two anal ones communicating bysmall openings with each other and with the second compartment.
Sci. Rep. Whales Res. Inst.,

No. 33, 1981

STOMACH OF PLATANISTIDS

79(1977), suggesting the possibility of the employment of stones in mechanical digestion.
The connecting channel of the Jnia observed was situated in the ventro-dexterregion near the caudal end of the main stomach and was rather similar to that of
the Pontoporia in its location and shape (Figs 8b and 8c ). In Platanista the channel*
is located at the cranio-dexter region of the anal part of the main stomach (Fig.
8a). The entrance of the channel of the lnia and Pontoporia is located near thecaudal end of the main stomach, while in Platanista it is found at the cranial wall of
the anal compartment of the main stomach. According to Zhou et al. (1978;
1979b) no connecting channel exists in Lipotes (Fig. 8d). In the case of sea dolphins (Stenella, Delphinus, Tursiops observed by Harraison e_t al., 1970), the connecting channel is located near the cranial end of the main stomach. However, in
Tursiops shown by Slijper (1962), in P. phocoena observed by Smith (1972), and in
Stenella in our observation the connecting channel was located consistently towards
the caudal end of the main stomach. Its entrance was placed nearly at the midpart of the ventro-dexter wall of the main stomach in our specimen. Although
Gihr and Pilleri (1969) did not mention the connecting channel in Stenella andDelphinus, they observed in their communication two different alternatives between
the second and third stomachs (which may correspond to the main stomach andthe connecting channel, respectively) in Odontoceti; the opening of the third stomach may be situated in the middle of the second stomach or in a more proximal
position. They stated that the junction in the Stenella styx was placed well to theproximal end, so that this led to the conclusion that the stomach of this dolphin was
highly differentiated. If this conclusion is acceptable, the Platanistidae stomach
may be of an undifferentiated type. The connecting channel of Platanistidae islongest in Pontoporia (about 8 cm), shortest in Platanista (2 cm), and intermediate inlnia (4 cm). Platanista's seems to be noticeably shorter than those in sea dolphins.
Although the entrance and exit of the channel of the observed Platanistidae becameslightly narrow, no marked sphincteric structures or constrictions existed along its
course. The channel, except for that in the Lipotes, however, at least may as awhole perform a valvular or sphincteric function. In Platanistidae there may bea tendency for a long channel to exist in species having a stomach with a simple
main stomach (e.g. Pontoporia and lnia), and for a short channel (Platanista) or theabsence (Lipotes) of a channel in species with complicated partitions of the main
stomach. Septal structures seen in the anal part of the main stomach in Platanista
and Lipotes may have some sphincteric functions. Harrison et al. (1970) have madeseveral suggestions on the biological significance of the connecting channel of sea
dolphins. Even taking these suggestions into consideration, it is difficult to explain the relationship between the developmental degree and the functions of eachkind of channel of Platanistidae. The histological nature of the connecting channel is identical with the pyloric stomach, and is fundamentaly the same in three
species of Platanistidae and also in sea dolphins described by former authors (Har-* This was called the "passage" by Anderson (1879) and Yamasaki and Takahashi (1971).
Sci. Rep. Whales Res. Inst.,
No. 33, 1981

80

YAMASAKI AND KAMIYA

rison et al., 1970; Smith, 1972). The channels of all dolphins should be considered
as a part of the pyloric stomach.
Although the pyloric stomach of the lnia, which in tubular in shape (Fig. 8c),
is situated at the caudo-dexter part of the main stomach is a similar manner to thatof the Pontoporia (Fig. 8b ), it is considerably smaller than that of the Pontoporia.
While those of the Platanista* and of Lipotes, (Figs 8a and 8d), are globular in shape,that of the latter is rather small, with its anal part becoming a narrow tube (Zhouet al., 1978; Zhou et al., l 979b ). The pyloric stomach of sea dolphins (Harrison etal., 1970; Smith, 1972; our observation) is an elongated tubular structure and far
bigger than those of Platanistidae. A distinct sphincteric structure exists at the
distal end of the pyloric stomach in all species of Platanistidae as well as in otherdolphins. In Platanistidae this structure is obviously the one which can be considered as a sphincter of the stomach, which may regulate the neutralization of
stomach contents by the backflow of duodenal contents.
When comparing the stomachs of four species of Platanistidae with one another, considerable interspecific differences can be found as mentioned above. The
digestive tract distal to the stomach of each species of this family also varies considerably from species to species: for example at the macroscopic level, the whole
length of the intestine is 55, 50, 15, and 10 times longer in proportion to bodylength in Pontoporia (Yamasaki et al., 1975), Inia (unpublished), Lipotes (Chen andChen, 1975), and Platanista (Takahashi and Yamasaki, 1972), respectively. Platanista's intestine is very unique among Platanistidae; there being a marked caecum
as observed in some baleen whales. The structure of the stomach ought to beclosely related to that of the whole digestive tract and closely concerned with food
habits. It is assumed that the modifications of the digestive tract seen in eachspecies of Platanistidae do not always show a characteristic feature that results from
adapting to living only in fresh water, since Pontoporia is actually found only incoastal sea water. Although sufficient consideration could not be taken concerning
the biological significance of the modifications of the stomach in this study, further
morphological observations of the stomach of all species of Platanistidae will be
useful to the study of the morphology and function of general cetacean stomachs.
ACKNOWLEDGEMENTS

We wish to thank Dr. T. Tobayama of the Kamogawa Sea World, Chiba, for kindly supplying the lnia stomach for this study.
REFERENCES
J., 1879. Anatomical and zoological researches: comprising on account of the zoological results of the two
expeditions to Western Yunnan 1868 and 1875; and a monograph of the two cetacean genera, Platanista and Orcella.
2 vols. B. Quaritch, London.

ANDERSON,* This was called the "third gastric cavity" and the "third compartment" by Anderson (1879) andYamasaki and Takahashi (1971), respectively.
Sci. Rep. Whales Res. Inst.,
No. 33, 1981

STOMACH OF PLATANISTIDS81

BROWNELL, R. L. jR. and R. NEss, 1970. Preliminary notes on the biology of the Franciscana, Pontoporia
blainvillei (Cetacea: Platanistidae). Proc. for Sixth Anual Conj. on Biol. Sonar and Diving Mammals, pp.
23-28.
BROWNELL, R. L. jR. and E. S. HERALD, 1972. Lipotes vexillifer. Mammalian sp., 10: 1-4.
BURMEISTER, H., 1867. Preliminary observation on the anatomy of Pontoporia bleinvillii. Proc. -?:,ool. Soc. Land.,pp. 484-489.
BURMEISTER, H., 1869. Pontoporia blainvillii Gray. In: Descripci6n de cuatro especies de de!finides de la costa
Argentina en el Oceano Atldntico. Anal. Mus. Publico Buenos Aires, I: 384-445.
CHEN, I. and W. CHEN, 1975. Notes on some morphological and anatomical features of the white-flag dolphin, Lipotes vexillifer. Acta Hydrobiol. Sinica, 5: 360-368. (in Chinese with English summary).
CHEN, P., P. Lm, R. Lm, K. LIN and G. PILLER!, 1980. The distribution, ecology, behavior and protectionof the dolphins in the middle reach of Chang Jiang River (Wuhan-Yueyang). Oceanologia et Limnologia
Sinica, 11: 73-84. (in Chinese with English summary).
FITCH, J.E. and R. L. BROWNELL, jR., 1971. Food habits of the Franciscana, Pontoporia blainvillei (Cetacea:
Platanistidae) from South America. Bull. Marine Sci., 21: 626-636.
GrnR, M. and G. PILLER!, 1969. On the anatomy and biometry of Stene/la sryx Gray and Delphinus delphis L.
(Cetacea, Delphinidae) of the Western Mediterranean. In: Investigation on cetacea (Berne), 1 : 15-65. ed.
by G. Pilleri, Berne.
HARRISON, R.J., F. R.joHNSON and B. A. YouNo, 1970. The oesophagus and stomach of dolphins (Trsiops,
Delphinus, Stenella). J . .(ool. Lond., 160: 377-390.
HINTON, M.A.C. and W. P. PYcRAFT, 1922. Preliminary note on the affinities of the genus Lipotes. Annals
and Magagine of Natural History Lond., (Ser. IX) 10: 232-234.
HosoKAWA, H. and T. KAMIYA, 1971. Some observations on the cetacean stomachs, with special considerations on the feeding habits of whales. Sci. Rep. whales Res. Inst., 23: 91-101.
juNGKLAUS, F., 1898. Der Magen der Cetaceen. Jena .(.F. Naturriss., 32: 1-94. (cited from Gihr andPilleri, 1969).
PILLER!, G., 1971. On the La Plata dolphin, Pontoporia blainvillei off the Uruguayan coasts. In: Investigation
on cetacea, (Berne), 3: 59-68.
PILLER!, G., 1972. Poppiana argentinianus Rathbuen, 1906 (Crustacea, Decapoda) in the Stomach contentsof Inia geoffrensis from the Rio Ibare, Bolivia. In: Investigation on cetacea (Berne) 4: 104.
SLIJPER, E.J., 1962. Whales. Hutchinson, London.
SMITH, G.J.D., 1972. The stomach of the harbor porpoise Phocoena plwcoena (L). Can. J . .(ool., 50: 16111616.
TAKAHASHI, K. and F. YAMASAKI, 1972. Digestive tract of Ganges dolphin, Platanista gangetica. II. small
and large intestines. Okajimas Fol. anat.jap., 48: 427-452.
WEBER, M., 1886. Vergleichung des Magens mit dem anderer Siiugethiere. In: Studien uber Siiugethiere. Ein
Beitrag zur Frage nach dem Ursprung der Cetaceen, pp. 54-66. Gustav Fisher. Jena.
YAMASAKI, F. and K. TAKAHASHI, 1971. Digestive tract of Ganges dolphin, Platanista gangetica. I. oesophagus and stomach. Okqjimas Fol. anat. jap., 48: 271-293.
YAMASAKI, F., K. TAKAHASHI and T. KAMIYA, 1974. Digestive tract of La Plata dolphin, Pontoporia blainvillei.
I. oesophagus and stomach. Okajimas Fol. anat.jap., 51: 29-52.
YAMASAKI, F., K. TAKAHASHI and T. KAMIYA, 1975. Digestive tract of La Plata dolphin, Pontoporia blainvillei.
II. small and large intestines. Okajimas Fol. anat.jap., 52: 1-26.
ZHou, K., W. QIAN and Y. L1, 1977. Studies on the distribution of Baiji, Lipotes verxillifer Miller. Acta.(ool. Sinica, 23: 72-79. (in Chinese with English summary).
ZHou. K., W. QIAN and Y. L1, 1978. Recent advances in the study of the Baiji, Lipotes vexillifer. Acta Nanjing Normal Coll., I: 8-13. (in Chinese with English summary).
ZHOU. K., W. QIAN and Y. LI, 1979a. The osteology and the systematic position of the Baiji, Lipotes vexiliijer. Acta .(ool. Sinica, 25: 58-74. (in Chinese with English summary).
ZHOU. K., Y. L1 and W. QIAN, 1979b. The stomach of the Baiji, Lipotes vexillifer. Acta .(ool. Sinica, 25: 95100. (in Chinese with English summary).

Sci. Rep. Whales Res. Inst.,
No. 33, 1981