590 SPECIAL PHYSIOLOGY. 



base of the mantle, not far from the mouth. The ink-bag is situated close to 

 the lower portion of the intestine, and opens near it. 



In the Pteropods, there are also, sometimes, found a crop and a distinct 

 gizzard ; the intestine presents three or four bends, surrounded by the liver. 



In the Pulmo- and Branchio-gasteropods, the oesophagus is long, and fre- 

 quently expands into a crop ; the stomach itself often consists of two or more 

 cavities ; the first is usually lined with a thick epithelium, and is sometimes 

 provided with hard internal laminae or denticles, constituting a sort of gizzard, 

 which is most developed when the buccal masticating organs are least so ; the 

 second has softer walls, and forms the true digestive stomach. The relative 

 position of these triturating and digestive cavities is the reverse of that met 

 with in birds. The intestine, more or less coiled, larger and more tortuous in 

 the vegetable-feeders, and usually embedded in the liver, receives the hepatic 

 ducts, bends once or twice, turns forwards, and ends near the forepart of the 

 body, usually on the right side, but sometimes on the left, or even on the back. 

 It is lined with a ciliated epithelium. 



In the Lamellibranchiata, the transverse mouth is concealed in the mantle,' 

 the oesophagus is short, the stomach forms a simple dilatation, and the intes- 

 tine is relatively simple, describing a few turns, and ending by a straight por- 

 tion, opening at the hinder part of the mantle ; its convolutions are embedded 

 in the substance of the liver, and its terminal part is sometimes embraced by, 

 or perforates, the heart. As already stated (p. 580), the direction of the prin- 

 cipal bend of the intestine, whether to the dorsal or haemal, or to the ventral 

 or neural surface of the body, is characteristic in each Molluscous Class 

 (Huxley). 



Mottuscoida. In the Ascidioida and Brachiopoda, the alimentary canal is 

 very simple, consisting either of a convoluted, or of a short recurved tube, 

 merely dilated at the stomach, and having its terminal orifice approximated 

 more or less to the often wide and valved mouth. In the Salpida, the outlet 

 of the intestine is at the hinder end of the body. In some Brachiopoda the 

 intestine ends in a blind sac, having no inferior aperture or outlet. 



In the Polyzoa, the mouth, situated in the centre of the circlet of ciliated 

 tentacles, leads into a wide pharynx, and short oesophagus, which terminates 

 in a muscular stomach ; from this, the intestine bends upwards again, and 

 opens near the side of the oesophagus, close to the outer border of the tentac- 

 ular circle. In some species, the stomach is muscular or gizzard-like. These 

 creatures present one of the lowest types of animals possessed of a true alimen- 

 tary canal, distinct from the walls of the body, shut off from the peri-visceral 

 cavity, and having a distinct and permanent inlet and outlet. 



Annulosa. The alimentary canal here also presents marked degrees of com- 

 plexity, from the highly developed apparatus found in certain insects, to the 

 simple straight tube seen in the lowest Worms. The oral aperture or mouth, 

 and the anal aperture or outlet, are always at opposite ends of the body. As 

 a rule, the carnivorous kinds have a short intestine, and the vegetable-feeders 

 a longer and even tortuous intestinal canal. 



In the Insects, the alimentary canal varies with the stage of metamorphosis. 

 In the vermiform larva, it is a straight tube, passing from one end of the body 

 to the other ; sooner or later, a dilatation appears, forming the stomach, which 

 sometimes becomes divided transversely, and the oesophagus may also be fur- 

 ther dilated into an ingluvies or crop. The intestinal canal presents caeca, and 

 therefore a sort of distinction into small and large intestine. In the mandibulate 

 Insects, as in the wasps and beetles, the crop is often glandular ; the gizzard, 

 which, unlike what occurs in Birds, is placed above the digesting stomach, 

 has very muscular walls and a chitinous lining membrane, provided frequently 

 with projections, laminae, hairs, or denticles, but sometimes this part is indi- 

 cated only by being a little more muscular. The true stomach has soft deli- 

 cate walls, usually provided with numerous gastric follicles. Sometimes the 

 stomach has no follicles, but its interior is laminated, or developed into cells, 

 or into a few short caecal tubes ; sometimes it is quite smooth. The intestine 

 is generally narrow, more or less convoluted, and seldom supported by a mes- 

 entery, but rather by the tracheae ; it sometimes presents dilatations or divi- 

 sions, so as to imitate, perhaps in form only, the subdivision into a small and 



