2Q2 MANUAL OF ZOOLOGY. 



plant-eaters, and they may be either marine or inhabitants of 

 fresh water. The following families are included in this sec- 

 tion : Natitidcz, Pyramidellidce, Cerithiada, Melaniada, Turri- 

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

 Neritidce, Turbinida (Top-shells), Haliotidcz (Ear-shells), Fis- 

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

 PatdlidcR (Limpets), Dentalidce (Tooth-shells), and Chitonida. 



ORDER II. OPISTHOBRANCHIATA. This order is denned as 

 follows : 



" Shell rudimentary, or wanting. Branchicz arborescent or 

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

 completely exposed on the back and sides, towards the rear 

 (opisthen) 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 Tornatellidce, Bullidcz (Bubble- 

 shells), Aplysiada (Sea-hares), Pleurobranchidcz and Phyllidiada. 

 In the second section, that of the Nudibranchiata (fig. 104), 

 the animal is destitute of a shell, 

 except in the embryo condition, 

 and the branchiae are always placed 

 externally on the back or sides 

 of the body. This section com- 

 prises the families Doridce, (Sea- 



Fig 104. Nudibranchiata. Doris lemons), Tritotiiada, 



Johnston, one of the Sea-lemons. T . '' .. . _, ' . 7 ~ 



Phylhrhoidcz, and Elysiadce. Spe- 



cimens of the Sea-slugs and Sea-lemons may at any time be 

 found creeping about on sea-weeds, or attached to the under 

 surface of stones at low water. The head is furnished with 

 tentacles, which appear to be rather connected with the sense 

 of smell than to be used as tactile organs ; and behind the 

 tentacles are generally two eyes. The nervous system is 

 extremely well developed, and would lead to the belief that 

 the Nudibranchs are amongst the highest of the Gasteropoda. 

 Locomotion is effected, as in the true Slugs, by creeping about 

 on the flattened foot. 



ORDER III. NUCLEOBRANCHIATA or HETEROPODA. This 

 order is defined by the following characteristics : Animal pro- 

 vided with a shell, or not, free-swimming and pelagic ; loco- 



