744 ZOOLOGY sect. 



a completely aquatic mode of respiration accompanied by the 

 development of secondary gills — vascular processes of the Avail 

 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 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 

 botryoidal : the latter varies in relative extent and in the arrange- 

 ment 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. 705). 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 caeca take the place of 

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

 there is a peculiar ad-rectal gland, situated at the side of the rectum 

 and secreting a colourless fluid, which in Murex and Purpura turns 

 purple on exposure to the air, and was anciently used as a dye — 

 the " Tyrian purple." 



The heart is, as in other Molluscs, enclosed in a special cavity — 

 the pericardium — a specialised part of the ccelome, communicating 

 with the cavity of the nephridia. It consists usually, as in 

 Triton, of two chambers — auricle and ventricle ; but in some, 

 e.g., Haliotis, there are two auricles and a ventricle. In the 



