314 



MANUAL OF ZOOLOGY. 



tellidm, LittorinidiB (Periwinkles), Paludinidce (River-snails) 

 Neritidm, Turbinidm (Top-shells), Haliotidm (Ear-shells), Fis- 

 surellidce (Key-hole Limpets), Calyptrceidce (Bonnet Limpets), 

 Patellida (Limpets), Dentalidce (Tooth-shells), and Chitonidee. 



Fig. 



-Gasteropoda, a Holostomatous shell {Turri'tella camwimis)l 

 b Siphonostomatous shell (Buccinum undaium}. 



Order II. Opisthobranchiata. — This order is'defined as 

 follows : — 



"Shell rudimentary, or wanting. Branchim arborescent or 

 fasciculated, not contained in a special cavity, but more or less 

 completely exposed on the back and sides, towards the rear 

 iopisfhen) of the body. Sexes united." — M.-Edwards. (See 

 Woodward's ' Manual.') 



The Opisthobranchiata, or " Sea-slugs," may be divided into 

 two sections, the Tectibranchiata and Nudibranchiata, accord- 

 ing as the branchiae are protected or are uncovered. 



The first section, that of the Tectibranchiata, is distinguished 

 by the fact that the animal is usually provided with a shell, 

 both in the larval and adult state, and that the branchiae are 

 protected by the shell or by the mantle. Under this family 

 are included the families of the Tomatellidce, Bullidce (Bubble 

 shells), Aplysiadce (Sea-hares), Pleurobranchidce and PhyllidiadcR. 



In the second section, that of the Nudibranchiata (fig. in), 

 the animal is destitute of a shell, except in the embryo condi- 

 tion, and the branchiae are always placed extern a.lly on the 

 back or sides of the body. This section comprises the families 

 DoridcE (Sea-lemons), iyitotiiadiB, /Eolidce, Phylilrhoidce, and 



