336 



MANUAL OF ZOOLOGY. 



tellidcE, Littorinidcz (Periwinkles), Paludinida (River-snails), 

 Neritida, Turbinidcz (Top-shells), Haliotidce (Ear-shells), Fis- 

 surellida (Key-hole Limpets), Calyptrceidcz (Bonnet Limpets), 

 Patellida (Limpets), Dentalidtz (Tooth-shells), and Chitonidce. 



ORDER II. OPISTHOBRANCHIATA. This order is defined as 

 follows : " Shell rudimentary, or wanting. Branchice arbores- 

 cent or fasciculated, not contained in a special cavity, but 

 more or less completely exposed on the back and sides, 

 towards the rear (ppistheti) of the body. Sexes united " (Milne- 

 Edwards). (See Woodward's " Manual.") 



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

 two sections, the Tcetibranchiata and Nudibranckiata, accord- 

 ing as the branchiae are protected or are uncovered. 



Fig. 137. Scalar ia 

 Grcenlandica, a Ho- 

 lostomatous Univalve. 



Fig. 138. Fusus tantalus^ a Si- 

 phonostomatous shell. Post- 

 Pliocene. 



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 Tornatettida, Bullida ($\fo\At- 

 sheD&^^^ft^(Sea-bares)^^ 



In the second section, that of the Nudibranchiata (fig. 139), 

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

 tion, and the branchiae are always placed externally on the 

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

 Dorida (Sea-lemons), Tritoniadce, ^olida, Phyllirhoidat, and 



