692 



ZOOLOGY 



SECT. 



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 



7 wall of the mantle- 



pul.v 



cavity. 



Near the base of 

 each ctenidium is 

 an elevation the 

 osphradium cor- 

 responding to the 

 body of that name 

 in other Mollusca 



ctur 



picl.ap 



reel 



vent 



rtr 



and having a similar 



neph 



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

 (Limaxi. nort. aorta ; ('/. auricle ; ntph. nephridium ; 

 jii.i'ic. pericardium, laid upon ; /ic/. <(/<. pulmonary aperture ; 

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

 ur. ureter ; i-t/it. ventricle. (After Pelseneer.) 



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 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 arrangement of the alimentary 

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

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

 The former may be tubular, but are usually botrvoidal. The 



t/ */ i/ 



latter varies in its extent and 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 

 calcined. Frequently a special development of a cuticular lining 

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

 a co3cum and comparable to the body of the same name in the 

 Pelecypoda (p. 655). A pyloric ccecum 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. 



