Digestive Tract of La Plata Dolphin, Pontoporia Blainvillei

Home , La Plata dolphin

Okajimas Fol. anat. jap. 51: 29-52, 1974

Digestive Tract of La Plata Dolphin, Pontoporia blainvillei

I. Oesophagus and Stomach

Fusao Yamasaki and Kyozo Takahashi

Departments of Biology') and Anatomy2), Sapporo Medical College, Sapporo, Japan

Toshiro Kamiya

Department of Anatomy, Faculty of Medicine, University of Tokyo, Tokyo, Japan

With 16 Figures in the Text and in 5 Plates

—Received for Publication, January 19, 1974—

Introduction

Anatomical reports on the digestive tract of the fresh-water dolphin, family Platanistidae, in which four genera are included, seem to be very scarce. The anatomy of the Ganges dolphin, Platanista gangetica, was described in 1879 by Anderson and the structure of the digestive system was briefly reported. Recently we made macro- and light microscopical observations on the digestive tract of Platanista, i. e., oesophagus and stomach (Yamasaki and Takahashi, 1971) and small and large intestines (Takahashi and Yamasaki, 1972), in which we described some characteristic features of the Ganges dolphin's digestive tract compared with those of some toothed whales. Concerning the anatomy of La Plata dolphin, Pontoporia blainvillei, which is one of the genera of Platanistidae, Gervais (1844) described the skull for the first time. Burmeister made a preliminary report (1867) and then he gave a macroscopical description of various organs including the digestive system in 1869. Since then, however, the anatomical investigations of this animal seem quite few in number. * This study was supported by a grant from the Education Ministry of Japan (1972, 1973), and was performed as a part of the systematic investigation of Plata- nistidae which has been on going since 1970. 29 30 F. Yamasaki, K. Takahashi and T. Kamiya

In 1970 Pilleri took an expedition to Uruguay to collect Pontoporia and to observe the behavior of this animal. The results of his research were published in 1971 with no anatomical observations of the digestive tract. Our expedition to the same place as Pilleri's research site in Uru- guay afforded us the opportunity of collecting a large number of Pontoporia and of making ecological observations. As the digestive tract of Pontoporia has some characteristic features when compared with that of the Ganges dolphin (fresh-water dolphin) and that of sea dolphins, the detailed macroscopical and light microscopical structures of the oesophagus and the stomach are described here from the viewpoint of comparative anatomy. The structure of the intestines of Pontoporia will be published in a succeeding paper.

Materials and Methods Ninety-five Pontoporia (body length 84.5-171.0 cm) were collected by the Cetacean Research Expedition University of Tokyo*, at the fishing village of Punta del Diablo, Rocha, Uruguay, from the middle of December 1972 to the beginning of February 1973. These animals were caught by chance in gill nets for sharks which were set about 25 to 30 km off the coast. Although the animals collected were not always so fresh, we observed the digestive tract macroscopically in situ in more than half of the specimens in an unpreserved state while four stomachs with surrounding abdominal viscera were fixed in 10% formalin solution and were sent to our laboratory. Materials for the histological observations were excised from nine relatively fresh bodies and were fixed in formalin solution. Small pieces of the materials were embedded in paraffin or celloidin, then sectioned and stained with hematoxylin-eosin and also with Azan. Photomicrographs were taken from typical sections.

Observations Abdominal viscera as a whole : Figure 3 shows a ventral view of the abdominal viscera of a young Pontoporia (body length 113 cm). Although the stomach of Pontoporia is divided into a main stomach, a connecting channel and a pyloric stomach, the upper part of the main stomach is hidden behind the liver. The pyloric stomach in this figure protrudes forward more than the natural position. The small intestine

* Director : Professor M. Nishiwaki, Ocean Research Institute of University of Tokyo, Tokyo, Japan. Oesophagus and stomach of Pontoporia 31 is extraordinarily long and occupies the greater part of the lower abdominal cavity. The omentum majus was inserted under the stomach and a prominent falciform ligament is seen cut in this figure. The extent of the omentum majus covering the small intestine is rather small (about 10 cm long and 15 cm wide). The cranial left end of the omentum majus begins at the dorso-sinister part of the main stomach roughly at the level of its mid-part, which is close to the part of the stomach adherent to the diaphragm. The ventral line of origin of the omentum majus comes down the main stomach along the greater curvature and extends to the connecting channel. Then it disappears dorsally behind the pyloric stomach and the duodenal ampulla, and it reaches to the initial part of the duodenum. Although the omentum majus itself is thin and transparent, the sheet-like pancreas can be seen flat and embedded throughout most of the dorsal layer of the omentum*. The omentum minus is very short and extends from the liver to the lesser curvature of the main and the pyloric stomachs and to the initial part of the duodenum proper. The foramen epiploicum, if it exists, leads to the bursa omentalis, passing under the liver at the initial part of the duodenum proper. However, the foramen was not found in five cases out of eight examined as a result of the complete fusion of the lesser omentum with the dorsal abdominal wall. Oesophagus : It is a thick-walled muscular tube and is about 25 cm in length (in the case of 171 cm body length) from the base of the epiglottis to the ventral epithelial transition between the oesophagus and the stomach. The dorso-sinister transitional border extends caudad to the main stomach about 2.5 cm more as compared with that of the ventro-dexter side (Fig. 6). The oesophagus has nearly the same diameter, about 2 cm, along its whole length, but becomes slightly thinner in front of the descending aorta and then it bends slightly to the left, passing the diaphragm obliquely. The oesophagus enters the main stomach directly (Figs. 1 and 6) as this animal lacks the forestomach which is seen in other cetaceans (except beaked whales) as a oesophageal diverticulum. Distinct longitudinal folds can be seen on the internal surface of the oesophagus (Fig. 6) throughout its entire course. The epithelia of the oesophagus and the main stomach are clearly distinguishable in color macroscopically the former appears bluish-white and the latter is pinkish-gray in unfixed specimens as shown in Fig. 6 and this transition is also quite distinct in fixed ones.

* The sheet-like pancreas in the dorsal layer of the omentum in Pontoporia seems to be characteristic among cetaceans. In other cetaceans, the whole of the pancreas is situated retroperitoneally. 32 F. Yamasaki, K. Takahashi and T. Kamiya

The epithelium of the oesophagus is a thick stratified squamous one and its superficial flattened cell layers are more eosinophilic than the rest and has no tendency toward keratinization (Fig. 7). The epithelium measures 0.8 to 1.0 mm in thickness and the well-developed papillae, which are variable in height, occupy 20 to 80% of its total thickness. The papillae are more irregular in shape in Pontoporia than in Platanista. Tunica muscularis mucosae is not conspicuous. The submucosa is thick, and thin nerve fiber bundles and blood vessels are found in it. No glands were seen in the submucosa so far as examined. The muscularis is composed of two layers, inner circular and outer longitudinal, approximately 3 mm thick in all at the upper part and about 2 mm thick at the lower part. The outer longitudinal muscle layer is about twice as thick as the inner circular layer at the upper part, but at the lower part the outer layer decreases the thickness and becomes thinner than the inner one. Between these two layers scattered nerve fiber bundles were observed. The muscles at the upper part of the oesophagus are striated and gradually smooth muscles become intermingled at the inner layer from the mid-part of the oesophagus. The muscles at the lower part are wholly smooth ones. Stomach : The digestive tract between the lower end of the oesophagus and the initial part of the duodenum in Pontoporia can be seen as three compartments demarcated by shallow grooves when viewed from the ventral side (Fig. 4). The smallest compartment between the main stomach and the pyloric one is treated as the ' connecting channel ' due to its tube-like passage (Figs. 1 and 2). The form and the proportionate size of each compartment of stomach are shown in Figs. 4 and 5 (with a scale) by one of the typical unfixed stomachs. However, as this unfixed stomach was photographed on a flat board, it seems to be slightly flatter and larger in outline in comparison with the natural state and each compartment is seen arranged in the same plane differing from in situ. Main stomach : This is the largest compartment, which is directly continuous from the oesophagus without any discernible sphincter, and it is located at the left side and slightly dorsal of the pyloric stomach. It is nearly pear-shaped, a thick-walled firm sac and the diameter of the cranial half of the compartment is larger than that of the caudal one (Fig. 4). In the case of fixed contracted stomachs there often can be seen a considerable constriction at the middle of greater curvature. The cranial one-third of the dorsal surface of the main stomach is adherent to the diaphragm which spans from the caudo-dorsal in a cranio-ventral direction with respect to the body axis. Whereas the greater part of its cranial half of the ventral surface adheres to the visceral surface of the left lobe of the liver. Oesophagus and stomach of Pontoporia 33

Fig. 1. A schematic drawing of the stomach of Pontoporia blainvillei. A bold line with an arrow shows the pathway between the main stomach and the pyloric stomach through the connecting channel. A dotted circle shows the entrance of the connecting channel. 0e-oesophagus; Bl-border between oesophagus and main stomach ; Ms-main stomach ; Cc-connecting • channel ; Ps-pyloric stomach ; Da-duodenal ampulla ; Dp-duodenum proper.

The main stomach of Pontoporia varied in size in different specimens examined, the size depending on whether it was fixed or unfixed, and also on the amount of its contents. The sizes of the unfixed, content-filled stomachs of Pontoporia (body length 117 cm through 144 cm) were about 22 cm to 27 cm in depth and 13 cm to 16 cm in maximum diameter. In the case of the fixed contracted stomach of the largest animal examined (body length 171 cm), the size was 20 cm in depth and 13 cm in maximum diameter. The wall of the main stomach is thicker at the lesser curvature than at the greater one; for example, about 10 mm and 7 mm, respectively, in the fixed largest stomach observed. The inner surface of the main stomach in the fresh specimens 34 F. Yamasaki, K. Takahashi and T. Kamiya appears pinkish-gray and there can be seen numerous low folds, especially distinctly at the cranial part (Fig. 6). In fixed contracted stomachs the folds become like reticular mammalian cerebral con- volutions over all of the inner surface. A number of parasites* were always found, sticking in the mucous membrane, especially numerously at the anal part. At the dextro-ventral wall of the main stomach, about 3 cm from the bottom, there is an opening, 8 to 10 mm in diameter, that leads into the connecting channel (white arrow in Fig. 6). Light microscopically, the gastric glands of the whole main stomach are seen to be composed of the same structure except at the cardiac region. The mucous membrane shows an abrupt change at the transition between the oesophagus and the main stomach from a stratified squamous epithelium to a glandular one (Fig. 7). In Pontoporia, the cardiac glands which are composed of mucous cells of one type, can be seen only along the narrow zone adjacent to the epithelial transition (Figs. 7 and 8). The maximum width of the glandular zone is about 1 mm, but the glands are sometimes scarcely observable in sections. The glands which cover the rest of the inner surface of the main stomach are simple tubular and the glands measure about 3 mm long (Fig. 11). The chief (zymogenic), parietal, and possibly mucous neck cells can be seen along the crypt. The diameters of the former -two kinds of cells are about 7 and 15 p or more, respectively. The mucous neck cells cannot be positively recognized in our specimens ,even in the neck portion of the glands . Therefore the glands seem to be mainly made up of parietal and chief cells (Fig. 14), and the -ratio of the former to the latter is approximately 1 3 or less , although the ratio seems to vary considerably from portion to portion of the stomach. The smooth muscles of the muscularis mucosae are scanty. and irregularly scattered. The submucosa is thick and roughly vascu- larized. The muscularis is also thick, about 1.5 to 2 mm in all. Al- though the muscularization of the stomach was not systematically investigated there can be seen at least two layers at the wall of the greater curvature, and the inner layer is more well-developed than the outer one; the former is two or three times as thick as the latter. The circular smooth muscles at the opening between the main stomach and the connecting channel are not so notable. connecting channel: A narrow compartment can be seen between the main stomach and the pyloric stomach which should be called the connecting channel as was designated in sea dolphins. This

*. Identified' as Corynosonta cetaceum Johnston and Best (1942) by Mr. H. Kamiya, Department of Parasitology, Faculty of Veterinary Medicine, Hokkaido University, Sapporo, Japan. Oesophagus and stomach of Pontoporia 35

crooked channel lies nearly in the dorso-ventral plane and is placed at the dextro-ventral near the bottom of the main stomach (Figs. 1 and 6). The channel is divided into two parts (oral and anal) by a septum, about 7 mm thick, which lies in a frontal plane and stretches caudad about 3 cm from the adherent part of the main and the pyloric stomachs (Figs. 1 and 2). The caudal end of the septum forms a concaved semicircular Fig. 2. A schematic drawing of the connecting channel of Pontoporia. The pathway of the channel is shown, being arranged on the same plane in this drawing, differing from the tortuous course in a natural state. Ms-main stomach ; Ps-pyloric stomach ; St-septum between oral and anal parts of connecting channel. outline and makes an oval opening with a caudal wall of the connecting channel (Fig. 1). The diameter of the opening between the oral and the anal parts is about 1.5 cm. As shown in Figs. 1 and 4, when viewed ventrally, the anal part of the channel (about 6 by 2.5 cm externally) is directed craniad and is demarcated from the main and the pyloric stomachs by shallow grooves. As the connecting channel lies ventrally it is not recognizable from a dorsal view (Fig. 5). The ventral wall of the connecting channel is thin, about 2 to 3 mm thick, and the rest of the wall becomes far thicker, fusing the walls of the main and the pyloric stomachs. The lumen of the oral part is approximately 3 cm long and 1.5 cm in diameter. This compartment directs caudad from the opening from the main stomach but forms a blind recess craniad about 1 cm in depth (Fig. 2). The anal part is about 5 cm in length and about 1.5 cm in diameter and is slightly dilated ventrad at its mid-portion. At the end of the anal part there can be seen an opening, approximately 8 mm in diameter, which leads to the pyloric stomach, perforating its left wall (Fig. 1). The inner surface of the connecting channel is pinkish-white in fresh materials and is almost smooth, but some low folds are seen at the oral part. The same kind of parasites seen in the main stomach are numerously found in this channel. The mucous membrane of the connecting channel is thinner than that of the main stomach (Figs. 11 and 12) and is about 1.2 to 2.0 mm in thickness. The nature of the epithelium between the main stomach and the connecting channel changes abruptly (Fig. 9), about 2 to 3 mm inside of the opening. This microscopic transition corresponds to the macroscopical color change of the compartments. The mucosa 36 F. Yamasaki, K. Takahashi and T. Kamiya of the connecting channel is composed of tubular glands which can neither be classified as pure mucous or serous but are rather mucous- like (Figs. 10 and 15), except at the transitional zone, for about 2 mm, where parietal cells are intermingled. Lymphatic nodules are found in places in the lamina propria mucosae (Fig. 12) and the muscularis mucosae consists of irregularly scattered smooth muscles. The submucosa is thinner and more packed than that of the main stomach. The muscularis consists of two layers and measures approximately 1.5 mm in thickness in all. In the circumference of the opening to the pyloric stomach developed circular smooth muscles are observed. Pyloric stomach : The pyloric stomach is a J-shaped elongated tubular structure roughly directed from cranio-sinister to caudo-dexter (Figs. 1 and 4). Its left wall closely abuts against the mid-part of the right wall of the main stomach and the cranial anal part of the connecting channel. The opening from the connecting channel can be seen about 3 cm apart from the cranial end of the pyloric stomach. The anal one-third of the pyloric stomach bends cranio-dorsad and then continues to the duodenal ampulla (Figs. 1 and 4). The whole length of the pyloric stomach is about 14 cm. The maximum diameter is approximately 5 cm and it becomes gradually smaller towards the anal part. At the anal end of the pyloric stomach there can be seen an opening, about 2 mm in diameter (Fig. 1 and 6), which contains a sphincter consisting of circular smooth muscles. The opening leads to the duodenal ampulla which is demarcated from the pyloric stomach by a shallow groove. The wall of the pyloric stomach is slightly thinner than that of the connecting channel, about 2 mm. At the crooked corner of the pyloric stomach there often can be seen a sickle-shaped fold projecting into its cavity (Fig. 1). It is not so noted in a relaxed state but becomes quite distinct in fixed specimens, about 1 cm high. The inner surface of the pyloric stomach is almost smooth and appears pinkish-white, the same color as the connecting channel. The same kind of parasites seen in the main stomach and the connecting channel, though reduced in number, were seen on the inner surface of the pyloric stomach, especially its anal part. The microscopical structure of the pyloric mucosa is fundametally similar to that of the connecting channel (Figs. 15 and 16) but seems more mucous depending on the section. The thickness of the mucosa is far thinner, about 1 mm (Fig. 13), and lymphatic nodules are more frequently observed than in the connecting channel. The muscularis is slightly thinner than that of the connecting channel and measures about 1 mm. Oesophagus and stomach of Pontoporia 37

Discussion In Pontoporia, as shown in Fig. 1, three compartments can be recognized between the lower end of the oesophagus and the duodenal ampulla, e. g., the main stomach, the connecting channel and the pyloric stomach. Early in 1869 Burmeister has already mentioned that the stomatch of Pontoporia had four compartments. His first stomach corresponds to our main stomach, and what we call the connecting channel was treated as the narrow pyloric part of the first stomach. He divided the succeeding elongated tubular compartment (our pyloric stomach) into two parts : the second and the third stomachs. Burmeister's last fourth stomach corresponds to our duodenal ampulla. The reasons that we excluded Burmeister's fourth stomach from the stomach ' are ; 1) an existence of a very narrow opening with a distinct sphincteric structure between the pyloric stomach and the duodenal ampulla (Fig. 6) that may prevent the backflow of bile into the pyloric stomach, 2) no sphincteric structure in the anal part beyond the opening mentioned above, 3) the histological similarity between the duodenal ampulla and the duodenum proper. The oesophagus of Pontoporia enters the main stomach directly (Fig. 6) because this animal lacks the forestomach which is an oesophageal diverticulum seen in other dolphins. Recently Brownell and Harrison also pointed out that the oesophageal forestomach is absent in Pontoporia, preceding our observation.** It is said that dolphins take fish and cephalopods whole and swallow them without chewing and, Harrison et al. (1970) suggested that the dolphins take as many fish as possible when the opportunity arises and the fish enter the forestomach directly. Slijper (1962) described the mechanical action exerted by the muscle wall of the forestomach in breaking down fish. We agree with their opinions due to the morphological structure and it may be sure that the function of the forestomach in dolphins is the storage and breaking down of foods. However, it has been known that Hyperoodon and Ziphius have no forestomach since the description of Weber (1886), and according to Slijper (1962), these animals, which feed exclusively on food as soft as cuttlefish, could easily dispense with the fore- * Burmeister made a preliminary short report in 1867 on this subject but this description is ununderstandable and there are discrepancies with the full report in 1869. ** Burmeister (1869)and Brownell and Ness (1970) used the term first stomach' for our main (glandular) stomach. As the first stomach' in other cetaceans means oesophageal stomach' there is room for confusion in terminology. This is the reason we have not designated the stomachs with ordinal numbers. 38 F. Yamasaki, K. Takahashi and T. Kamiya

stomach. It is very interesting that in Pontoporia the forestomach is absent, though its food habits themselves do not seem to be different from those of sea dolphins. The Pontoporia observed by us had teleost fish remains in their main stomachs, in addition to cephalopods as previous investigators also reported (Burmeister , 1869 B rownell and Ness, 1970 ; Fitch and Brownell, 1971 ; Pilleri , 1971). Therefore, it seems that the existence or absence of the forestomach has no direct relationship with the food habits. The absence of the forestomach is at least not a characteristic in the Platanistidae since Platanista gangetica (Anderson, 1879 ; Yamasaki and Takahashi, 1971) and Inia geoffrensis observed by us have a typical forestomach. In cetaceans which have both oesophageal forestomach and a glandular main stomach, it is generally agreed that the former is larger than the latter. As the main stomach volume of Pontoporia is larger in proportion than those of other cetaceans (except Hyperoodon and Ziphius), this may show compensation for the absence of the forestomach. Thus the main stomach's function in Pontoporia may be both storage and chemical digestion of foods. As is well known the forestomach in dolphins has a well-developed muscle wall and its inner surface is covered with a thick stratified squamous epithelium. These structures seem to be suitable for breaking down foods. As the main stomach of Pontoporia has no such thick muscle wall and the inner surface is glandular, digestion in the main stomach should be chiefly considered from the viewpoint of chemical digestion. In Pontoporia, cardiac glands can be seen only along the very narrow zone adjacent to the oesophageal mucosa (Figs. 7 and 8). Hosokawa and Kamiya (1971) found cardiac glands in blue whales, Balaenoptera musculus, and we also observed this type of gland in Platanista (Yamasaki and Takahashi, 1971). However, Smith (1972) found no cardiac glands in the harbor porpoise, Thocoena phocoena, in this region. As the extent of the glands seems to be very limited at this narrow zone and sometimes scarcely observable, it needs precise examination in all other cetaceans and the biological meaning of the glands needs to be discussed from a comparative anatomical viewpoint. The mucous membrane of the main stomach of Pontoporia, except for the part of the cardiac glands mentioned above, is occupied by glands consisting of possibly three types of cells ; chief, parietal and mucous neck cells, as seen in other mammals. Harrison et al. (1970) could not detect mucous neck cells in the species of Tursiops, Del- phinus and Stenella and Smith (1972) did not mention this type of cells in the harbor porpoise. In Pontoporia we observed possible mucous neck cells but couldn't positively identify them. However, as Hosokawa and Kamiya (1971) observed mucous neck cells in baleen whales (sei, fin and blue whales), it must be clarified by further Oesophagus and stomach of Pontoporia 39 investigation whether mucous neck cells really exist or not in all of the cetacea. The numerical ratio of the parietal cells to the chief cells was roughly 1 : 3 or less in Pontoporia and it is interesting that this ratio is almost the same as those of some sea dolphins (Harrison et at., 1970), baleen whales (Hosokawa and Kamiya, 1971) and also one of the fresh-water dolphins, Platanista gangetica (Yamasaki and Takahashi, 1971), although their living environments are quite different. 'The main stomach of Pontoporia leads to a narrow compartment , the connecting channel, which continues to the pyloric stomach (Fig. 1). Such a channel is generally found in sea dolphins, and, some early anatomists numbered this part as an independent compartment. However, this part in Pontoporia is so narrow compared with the main and pyloric stomachs and looks like the channel seen in sea dolphins that we also used the term connecting channel ' in this animal. Burmeister (1869) described this part as a pylorus and stated that it is so narrow that hardly a piece of wire of average width can go through. But, by our observation it has a diameter of about 1.5 cm along its whole course and the openings on either the oral or anal side have about 0.5 to 1 cm in diameter in the cases observed. The biological meaning of the crooked narrow connecting channel in sea dolphins has been variously considered by some investigators to be for the prevention of sea water passing or regulation of the chyme passage from the main stomach. However, as the position and the form of the connecting channel varies in different species, it may not be possible to unify the function of the channel. But it is probably sure that the channels in various kinds of dolphins, both sea and fresh-water ones, are homologous with each other and act in regulating the flow of chyme from the main stomach. The mucosa of the connecting channel is almost identical with the pyloric stomach in Pontoporia. The connecting channel in sea dolphins (Harrison et at., 1970 ; Simth, 1972) and also the passage ' in Platanista (Yamasaki and Takahashi, 1971) are lined by glands which are similar to that of the pyloric mucosa. Accordingly the narrow part that is called the connecting channel ' or passage ' in dolphins should be considered as a part of the pyloric stomach as was pointed out by Harrison et at. (1970). The pyloric stomach of Pontoporia is a J-shaped elongated tubular structure (Figs. 1 and 4), and differs from that of Platanista which is globular in shape and rather looks like those of sea dolphins. Burmeister (1869) divided this part into two parts, the second and the third stomachs, by a lax sphincter. By our observation, his so- called lax sphincter' can be seen as a sickle-shaped fold projecting into the pyloric cavity from the crooked inside corner (Fig. 1). But, 40 F. Yamasaki, K. Takahashi and T. Kamiya as this fold is not so conspicuous in fresh materials, and, as the histological structure is the same throughout the pyloric mucosa, we did not divide this part into two compartments. The glands of the pyloric stomach in Pontoporia seem to be mucous-like (Figs. 13 and 16) and are not typical mucous glands as are those of human beings. The nature of the pyloric glands in various kinds of cetaceans observed by previous investigators has been said to be mucous but seems slightly different in grade, from typical mucous to sero-mucous. Pontoporia blainvillei is classified as one of the Platanistidae (fresh-water dolphins) as well as Platanista gangetica. But Pontoporia is actually found only on the Atlantic coast of South America and there is no direct evidence of finding them in fresh-water. As mentioned above, the stomach of Pontoporia is rather similar to those of sea dolphins in the form of the connecting channel and the pyloric stomach, except for the absence of the forestomach (oesophageal stomach) which is found in other dolphins including Platanista. What is the reason that Pontoporia has no forestomach in spite of the fact that both Pontoporia and sea dolphins live in sea water and take similar foods ? We must study the stomachs of two other species of fresh-water delphins (Inia and Lipotes) in the future to clarify this problem from a phylogenetical viewpoint.

Summary The oesophagus and the stomach of more than half of the collected ninety-five Pontoporia blainvillei (body length 84.5-171 cm) were examined macroscopically and nine fresh specimens were observed light-microscopically. The oesophagus is an ordinary muscular tube which is lined with a thick stratified squamous epithelium and continues to the stomach with a distinct epithelial transition. The stomach of Pontoporia consists of three compartments ; the first one is the glandular main stomach, the second is the connecting channel and the third is the pyloric stomach. The oesophageal forestomach, which is found in other dolphins, is not seen in Pontoporia. The main stomach is continuous from the oesophagus and is a pear-shaped firm sac. A narrow zone of cardiac glands was seen adjacent to the oesophagus and the rest of the glands consist of chief and parietal cells and possibly mucous neck cells. The ratio of the parietal cells to the chief cells is about 1 : 3 or less. An opening which leads to the connecting channel can be seen near the bottom of the main stomach. Oesophagus and stomach of Pontoporia 41

The connecting channel is situated slightly ventrally between the main and the pyloric stomachs. The channel initially directs down- wards and then upwards with a sharp crooking. Its mucosa consists of mucous-like tubular glands. The pyloric stomach is a 3-shaped tubular compartment and is continuous from the connecting channel with an opening near its top, then leads to the duodenal ampulla with a small opening which contains circular smooth muscles. The glandular structure of the pyloric stomach is similar to that of the connecting channel. The structure of the stomach of Pontoporia was compared with that of other cetaceans, and the absence of the oesophageal forestomach in Pontoporia was especially discussed from the viewpoint of the function of the stomach, the living environment and the food habits of the animals.

Acknowledgement

The authors are greatly indebted to Mr. R. L. Brownell Jr., Department of Vertebrate Zoology, National Museum of Natural His- tory, Smithsonian Institution, Washington, D. C., who arranged for us to collect materials and to the staffs of the National Museum of Natural History, Montevideo, who rendered this project possible. We thank the hospitality of Mr. Brownell's family and of the fishermen in Punta del Diablo, Uruguay. Technical assistance of Mr. A. Mori, Department of Anatomy of Sapporo Medical College, is acknowledged.

References Anderson, J.: 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, 1879. Burmeister, H.: Preliminary observations on the anatomy of Pontoporia blainvillii. Proc. Zool. Soc. Lond. pp. 484-489, 1867. : Pontoporia blainvillii Gray. In : DescripciOn de cuatro especies de delfinides de la costa Argentina en el Ocean° Atlantic°. Anal. Mus. PUblico Buenos Aires. 1: 389-445, 1869. Brownell, R. L. Jr. and Ness, R.: Preliminary notes on the biology of the franciscana, Pontoporia blainvillei (Cetacea : Platanistidae). Proceedings for the Sixth An nual Conference on Biological Sonar and Diving Mammals. pp. 23-28, 1970. Fitch, J. E. and Brownell, R. L. Jr. : Food habits of the franciscana, Pontoporia blainvillei (Cetacea : Platanistidae) from South America. Bull. Marine Sci. 21 : 626- 636, 1971. Gervais (1844) : Cited from Burmeister, 1869. Harrison, R. J., Johnson, F. R. and Young, B. A.: The oesophagus and stomach of dolphins (Tursiops, Delphinus, Stenella). J. Zool. Lond. 160 : 377-390, 1970. Hosokawa, H. and Kamiya, T.: Some observations on the cetacean stomachs, with 42 F. Yamasaki, K. Takahashi and T. Kamiya

special considerations on the feeding habits of whales. Sci. Rep. Whales Res. Inst. 23 91-101, 1971. Pilleri, G: On the La Plata dolphin, Pontoporia blainvillei off the Uruguayan coasts. In : Investigation on cetacea. Vol. III. pp. 59-68. Edited by G. Pilleri. Waldau- Berne, 1971. Slijper, E. : Whales. pp. 288-289. Hutchinson, London (English translation), 1962. Smith, G. J. D.: The stomach of the harbor porpoise Phocoena phocoena (L.). Can. J. Zool. 50: 1611-1616, 1972. Takahashi, K. and Yamasaki, F.: Digestive tract of Ganges dolphin, Platanista gangetica. II. Small and large intestines. Okajimas Fol. anat. jap. 48 : 427-452, 1972. Weber, M.: Vergleichung des Magens mit dem anderer Sdugethiere. In z Studien iiber Saugethiere. Ein Beitrag zur Frage iach dem Ursprung der Cetaceen. pp. 54-66. Gustav Fisher. Jena, 1886. Yamasaki, F. and Takahashi, K.: Digestive tract of Ganges dolphin, Platanista gangetica. I. Oesophagus and stomach. Okajimas Fol. anat. jap. 48: 271-293, 1971.

Explanation of Figures

Plate I Fig. 3. Ventral view of the abdominal viscera of Pontoporia as a whole. Omentum majus is inserted under the stomach. The main and the pyloric stomachs are more hidden under the liver in situ than in this photograph. Note that the small intestine is thin and extraordinarily long. Di-diaphragm; Lir and Lil-right and left lobes of liver ; Ms-main stomach ; Ps-pyloric stomach ; Si-small intestine. Specimen No. RLB 824 (body length 113 cm). 43

Plate I

Fig .3

F. Yamasaki, K. Takahashi and T. Kamiya 44 F. Yamasaki, K. Takahashi and T. Kamiya

Plate II

Figs. 4 and 5. Ventral and dorsal views of an unfixed stomach of Pontoporia which contains food. The Omentum majus and minus are removed. The connecting channel cannot be seen from a dorsal view. 0e-oesophagus; Ms-main stomach ; Cc-connecting channel ; Ps-pyloric stomach ; Da-duodenal ampulla ; Dp-duodenum proper. Specimen No. RLB 856 (body length 144cm). 45

Plate II

Fig .4

Fig.5

F. Yamasaki, K. Takahashi and T. Kamiya 46 F. Yamasaki, K. Takahashi and T. Kamiya

Plate III

Fig. 6. Inner aspect of the dorsal wall of an unfixed stomach and the initial part of the duodenum of Pontoporia, which is cut off along the lesser and the greater curvatures. Note the remarkable folds in the main stomach and the smooth surface in the connecting channel and the pyloric stomach. A white arrow shows the entrance of the connecting channel. At the duodenal ampulla there can be seen small opening (black arrow) from the pyloric stomach. Oe-oesophagus ; Ms-main stomach ; Cc-connecting channel ; Ps-pyloric stomach ; Da- duodenal ampulla ; Dp-duodenum proper. Specimen No. RLB 845 (body length 126.5 cm). Fig. 7. A photomicrograph of the epithelial transition between the oesophagus and the main stomach. A narrow zone of cardiac glands is seen between the mucosae of the oesophagus and the fundus glands. Oee-oesophageal epithelium ; Cd-cardiac gland ; Fd-fundus gland. H-E stain. x 35 Fig. 8. A higher magnification photomicrograph of the cardiac glands in another specimen. Note the mucous type of the cardiac glands and the neighboring fundus glands. Oee-oesophageal epithelium ; Cd-cardiac gland ; Fd-fundus gland. H-E stain. x 95 47

Plate III

Figh

F YAmasaki. K Takahashi and T. Kamiya 48 F. Yamasaki, K. Takahashi and T. Kamiya

Plate IV

Fig. 9. The mucosal border between the main stomach (left half) and the connecting channel (right half) which is roughly indicated by an arrow. Fd-fundus gland ; Ccd-glands of connecting channel. H-E stain. x 45 Fig. 10. A higher magnification view of a part of Fig. 9. The difference in glandular nature between the main stomach and the connecting channel can be clearly seen. An arrow shows the border between both types of glands. Fd- fundus gland ; Ccd-glands of connecting channel. H-E stain. x 120 Figs. 11, 12 and 13. Same magnification photomicrographs of the mucosae of the main stomach (Fig. 11), the connecting channel (Fig. 12) and the pyloric stomach (Fig. 13). Note the difference of the thickness of each mucosa. H-E stain. x 40 49

Plate IV

F Yamacaki K Takahashi and T. Kamiva 50 F. Yamasaki, K. Takahashi and T. Kamiya

Plate V

Fig. 14. A higher magnification photomicrograph of the fundus gland of the main stomach. Note the numerous large parietal cells and the small chief cells. Mucous neck cells are not positively recognizable. H-E stain. x 225 Figs. 15 and 16. Higher magnification photomicrographs of the glandular bases of the connecting channel (Fig. 15) and the pyloric stomach (Fig. 16). Both of the glands are similar and mucous-like. H-E stain. x 225 51

Plate V

F. Yamasaki, K. Takahashi and T. Kamiya