MANUAL OF ZOOLOGY. 



and are mostly carnivorous in their habits. The following 

 families are comprised in this section : Strombidce (Wing- 

 shells), Muricida, Buccinida (Whelks), Conidce (Cones), Volu- 

 tida, and Cyprczidcz (Cowries). 



The Holostomata, of which the common Periwinkle (Litto- 

 rina littorea) is a good example, are either spiral or limpet- 

 shaped, in some few instances tubular, or multivalve; the 

 aperture of the shell being in most cases entire (fig. 200). 

 They are mostly plant-eaters, and they may be either marine 

 or inhabitants of fresh water. The following families are in- 

 cluded in this section : Natitidce, Pyramidellidce, Cerithiadce, 

 Melaniadce, Turritellidtz, Littorinidce (Periwinkles), Paludinidcz 

 (River -snails), Neritidce, Turbinidcz (Top - shells), Haliotidce 

 (Ear-shells), Fissurellida (Key - hole Limpets), Calyptratda 

 (Bonnet Limpets), Patellidce (Limpets), Dentalidcz (Tooth- 

 shells), and Chitonida. 



The Dentalida are often regarded as a separate order of the Gasteropods 

 (viz., Scaphopoda], or, by Huxley, as referable to the Pteropoda. They 

 constitute a lowly-organised group, distinguished by the absence of distinct 

 gills or heart, the imperfect development of the head, and the slender 

 tubular shell, with an aperture at each end. 



The Chitonida and Patellidce are often united into a separate order 

 (Cyclobranchiata), characterised by the generally circular disposition of the 

 branchiae. The former have a multivalve shell, and are stated to have the 

 sexes united. 



By many naturalists, the Prosobranchiate order is divided into sub- 

 orders, in accordance with the structure and form of the "odontophore " 

 or "radula." 



ORDER II. OPISTHOBRANCHIATA. This order is defined as 

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

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

 more or less completely exposed on the back and sides, to- 

 wards the rear (opistheii) of the body. Sexes united " (Milne- 

 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 section 

 are included the families of the Tornatellidce, Bullidce (Bubble- 

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

 diadce. 



In the second section, that of the Nudibranchiata (fig. 219), 

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



