724 ZOOLOGY SECT. 



and the foot. In the Pulmonata, and in some members of other 

 groups, ctenidia are absent, and the mantle-cavity, completely 

 enclosed except for a small rounded opening, has the function of a 

 pulmonary sac or lung (Fig. 644), its roof being richly supplied 

 with blood-vessels : in the aquatic forms its function is apparently 

 as much hydrostatic as respiratory. In one family of Pulmonata, 

 the pulmonary chamber gives off a number of branching air-tubes 

 or trachece. In some of the Pulmonata there is a return to a 

 completely aquatic mode of respiration accompanied by the develop- 

 ment of secondary gills vascular processes of the wall of the 

 mantle-cavity. 



Near the base of each ctenidium is an elevation the 

 osphradium corresponding to the body of that name in other 

 Mollusca and having a similar function. 



Digestive Organs. In many Streptoneura there is a long 

 introvert, capable of being everted and retracted, at the 

 extremity of which the mouth is placed. A single curved horny 

 jaw lies on the roof of the buccal cavity in the Pulmonata ; in 

 most Streptoneura (as in Triton) the place of this is taken by 

 two lateral pieces. 



A characteristic feature of the alimentary canal of the Gastro- 

 poda, which, however, they share with some Amphineura and with 

 the Cephalopoda, is the possession of an odontophore and radula, 

 a typical example of which has been described in that of Triton. 

 In the different groups differences are observable in the odonto- 

 phore as regards the proportions of the parts, and the size, form, 

 and arrangement of the teeth. The structure and relations of the 

 alimentary canal are similar to those already described in Triton, 

 and salivary glands and " liver " (hepato-pancreas) are always 

 present. The former may be tubular, but are usually botryoidal : 

 the latter varies in relative extent and in the arrangement of its 

 lobes in different forms. 



In some Opisthobranchia the stomach contains a series of teeth 

 which are sometimes sharp and chitinous, sometimes plate-like and 

 calcified. Frequently a special development of the cuticular lining 

 of the stomach forms a hard rod the crystalline style, lodged in 

 a caecum and comparable to the body of the same name in the 

 Pelecypoda (p. 687). A pyloric caecum is frequently appended to 

 the stomach. The intestine is long and thrown into folds in the 

 vegetable-feeding forms, short and straight in the carnivorous. 

 In some cases, e.g. Haliotis, it traverses the ventricle, in others 

 the pericardium ; in others it passes through the nephridium. In 

 Eolis (Nudibranchia) the stomach gives off a number of glandular 

 caeca which penetrate into the interior of the secondary branchiae 

 or cerata on the dorsal surface ; these cseca take the place of the 

 " liver " of other Gastropoda. In some of the Pectinibranchia 

 there is a peculiar adrectal gland, situated at the side of the rectum 



