692 



ZOOLOGY 



SECT. 



picl.ap 



red 



-vent 



itr 



neph 



FIG. 596. Pulmonary cavity and related parts in a slug 

 (Itimax). aort. aorta ; aur. auricle ; neph. iiephridium ; 

 peric. pericardium, laid open ; pul. ap. pulmonary aperture ; 

 pul. . pulmonary vein with its ramifications ; rect. rectum ; 

 ur. ureter; vent, ventricle. (After Pelseneer.) 



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

 ently as much hydrostatic as respiratory. In some of the Pulmon- 

 ata there is a return to a completely aquatic mode of respiration 

 accompanied by the development of secondary gills vascular 



processes of the 

 ^ wall of the mantle- 



cavity. 



Near the base of 

 each ctenidium is 

 an elevation the 

 osphradium cor- 

 responding to the 

 body of that name 

 in other Mollusca 

 and having a similar 

 function. 



Digestive Or- 

 gans. In many 

 Streptoneura there 

 is a long introvert 

 capable of being 

 everted and re- 

 tracted, at the ex- 

 tremity 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 wit! 

 the Cephalopoda, is the possession of an odontophore and radula 

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

 In the different groups differences are observable in the odontc 

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

 and arrangement of the teeth. The arrangement of the alimentai 

 canal is similar to what has already been described in Triton, anc 

 .salivary glands and liver (hepato-pancreas) are always present. 

 The former may be tubular, but are usually .botryoidal. Th< 

 latter varies in its extent and the arrangement of its lobes 

 different forms. 



In some Opisthobranchia the stomach contains a series "of teeth 

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

 calcined. Frequently a special development of a cuticular lining 

 of the stomach forms a hard rod the crystalline style lodged ii 

 a coscum and comparable to the body of the same name in th< 

 Pelecypoda (p. 655). A pyloric coecum is frequently appended t( 

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

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



