GASTEROPODA. 



327 



and the stomach a simple membranous sac; whilst in j^olis 

 the liver is disintegrated, and its canals so large that the 

 process of digestion must be chiefly carried on in them, and 

 they are regarded as coecal prolongations of the stomach ; the 

 cceca extend into a series of gill-like processes, arranged upon 



Fig. 139. Dendronotus arborescens, 



the back of the animalj which also contain part or the whole of 

 the true liver; the gastric ramifications vary exceedingly in 

 amount of complexity. The Borididce are distinguished by 

 having a short and wide lingual membrane with numerous 

 similar teeth ; the Solids have a narrow ribbon with a single 

 series of larger teeth. In Dendronotus a large central tooth is 

 flanked by a few small denticulated teeth. (Alder and Hancock, 

 H. II., Fig. 8.) 



The only Nudibranch with a solid upper jaw, is ^girua 

 punctilucens (A. and H., PL XVII., Eig. 15). In other instances 

 the two halves are articulated and act as lateral jaws. In 

 ^girue the mouth is also furnished with membranous fringes 

 (A. and H., PI. XYIL, Pig. 14). Ancida cristata has a for- 

 midable spinous collar (PL XYII.j Pig, 7). 



fig. 140. a, Mouth of ^girus punctilucenSo 



b. Horny upper mandible detached, 



c. Prehensile collar of Ancula. 



a, mantle ; x, dental sac ; h, insertion-plate of mandible ; c, passage of mouth. 



The vascular system and circulation of the nudibranchiate 

 molluscs is incomplete. In Doris veins can be traced only in 

 the liver and skin ; the greater part of the blood from the 

 arteries escapes into the visceral sinus and into a network of 



