48 COMPARATIVE ANATOMY CHAP. 



anterior and ventral, through which the fin-like processes of the foot can be protruded, 

 and two lateral apertures stretching far up, so that the shell appears almost bivalve. 



At these lateral slits, which admit water to the mantle cavity, the mantle bends 

 round on to the outer surface of the shell, covering the greater part of it ; and, at 

 the upper angles of the slits, has two freely projecting processes. 



Pteropoda Gymnosomata. In these, the long outwardly symmetrical body is 

 naked and without a mantle, and the foot, which is much reduced, is found on 

 the ventral side of the most anterior part of the body. 



(b) Ascoglossa and Nudibranchia. 



In mature Ascoglossa and Nudibranchia, with the single exception of the Stcgano- 

 branchia, a shell is always wanting, as also a distinctly demarcated visceral dome. 

 The latter, indeed, spreads out over the whole dorsal surface. The dorsal integu- 

 ment, nevertheless, forms a circular fold (mantle fold) separated from the foot by a 

 groove sometimes deep, sometimes shallow ; but, except in the I'lii/lliifiiifa;, no gills 

 lie in this groove. Where this groove has nearly disappeared, the animals strongly 

 resemble Planaria. 



Phyllidiidse. In these, the mantle fold is distinct, and carries on its lower 

 surface, to the right and left, a row of branchial leaves, herein recalling Patella and 

 Chiton. 



The genus Dermatobranchus, which, judged by its organisation, belongs here, 

 has, however, no gills. 



Dorididse. The dorsal integument (notaeum), which here covers the body like 

 a shield, being generally distinctly demarcated from the foot and the head, contains 

 numerous calcareous particles, which give it a firmer consistency. Anteriorly, there 

 are two feeler-like processes, the rhinophores, which can generally be withdrawn 

 into special sheaths or pits ; these are not to be confounded with the tentacles. 

 The anus lies in the median line, generally behind the middle of the body, and 

 is surrounded by an ornamental circlet of pinnate gills. The notaeum is often 

 covered with prominences, and in some genera the margin carries variously shaped 

 processes. 



Cladohepatica. Here there are no anal gills. The dorsal integument has 

 variously formed and variously arranged appendages ; these may be conical, club- 

 or finger-shaped, lobate or branched ; they are, for the most part, very striking in 

 colour and appearance. Sacs of nematocysts are generally found at their tips, and 

 caeca of the intestinal canal (branches of the digestive gland) penetrate them. These 

 dorsal appendages, which, like the rest of the body, are ciliated, have, at least 

 partly, a respiratory function. In many forms they easily fall off, and are later 

 regenerated (Fig. 18, p. 12). 



Many Cladohepatica have a certain external likeness to Planaria with dorsal 

 papillae (Thysanozoon), but this likeness is still more marked in the following 

 family : 



Ascoglossa. Anal gills and also, as a rule, dorsal appendages are here wanting. 

 The whole body is naked and ciliated. The back is indistinctly demarcated from 

 the head. 



Phyllirhoe. This Nudibranchiate genus, of all Opisthobranchia, shows least of 

 the typical external organisation of the Mollusca. The body here is naked and 

 laterally compressed, with sharp dorsal and ventral edges. It has neither foot 

 .nor gills (Fig. 19, p. 12). 



