46 COMPARATIVE ANATOMY CHAP. 



branched appendages. These are richly supplied with blood-vessels, and serve for 

 respiration. In Peronfa there are besides these also dorsal prominences which carry 

 eyes. 



The dorsal integument projects all round the body above the foot, and thus 

 forms, as in Chiton, a peripheral zone, which is ventrally separated from the foot 

 by a groove. In Oncii'ic/la the edge of this zone, i.e. the lateral edge of the body, 

 is dentate or fringed. 



3. Opisthobranehia. 



The typical outer organisation of the Gastropoda here suffers 

 even more varied and thorough modification than in the Pulinonata. 

 We have, on the one hand, forms with head, foot, visceral dome, shell, 

 mantle and gill ; on the other, forms which possess none of these 

 organs and nevertheless are both Gastropods and Opisthobranehia. In 

 one principal division of this order, the PtilUaia or Tc<:til>rancliia, the 

 mantle fold is retained on the right side of the body, and partially 

 covers a typical Molluscan ctenidium ; in other divisions both mantle 

 and ctenidia are wanting. We do not here apply the term mantle to 

 the fold or edge of the dorsal integument which surrounds the body 

 at the part where the head and foot take their rise ; such an edge is 

 more or less developed in most Opisthobranehia and distinctly marks 

 off the foot and head from the rest of the body or back. The mantle 

 here means only the broader fold which covers the mantle cavity, in 

 which lies a typical molluscan gill. The edge of the mantle never 

 forms a distinct siphon in the Opisthobranehia, though there is an 

 approach to such a structure in the Ringwulida'. 



(a) Tectibranchia. 



(a) Reptantia. In this division we have, on the one hand, forms which still 

 have a distinctly projecting visceral dome, whose integument secretes a coiled shell, 

 into which the whole body can be withdrawn. On the other hand, forms occur in 

 which the flattened visceral dome has spread out over the whole dorsal surface of 

 the foot, the shell being rudimentary and internal. Examples of the former are 

 found in the Cephalaspidce, e.g. the Actaeonida; Tonnitiindce, and some Scnp/innili-i'f"- 

 (Atys, Cyliclincf, Amphisphyra), a few Bullitlcc (Bulla), and the fiiiigicitlida: 



In Scaphander among the Scaphandridce, and Accra among the Bullidce, the body 

 cannot be completely withdrawn into the shell. 



In the Cephalaspidce, to which so far reference has been made, the shell is 

 external. 



In (.Tdstropterun the mantle is rudimentary, and is provided posteriorly with a 

 filiform appendage. It covers a delicate membranous internal shell, into which the 

 body cannot be withdrawn. The same is the case in Pliiliiic and !'<>,/'</ in,jt, where 

 there is also a delicate internal shell covering only a small portion of the viscera ; this 

 shell, in Doridium, is produced in the form of two lobes, the one to the left endino- 

 in a filiform process. 



The visceral dome in the Amtr<jii</<r is small as compared with the size of the 

 animal, but rises distinctly above the rest of the body, and is covered by a thin 

 inconspicuous shell. The mantle and shell often only partially cover the gill. In 

 Aplysia, the shell is internal, i.e. it is entirely overgrown by the mantle ; in Dola- 

 Mla. this enveloping overgrowth is not quite complete, as a circular median dorsal 



