26 INVERTEBRATE ANIMALS. 



Order OPISTHOBRANCHIATA. oVicrae, behind; 

 ^pky^Qo., gills. Organs of respiration situated towards 

 the rear of the body. 



Group 75. — Sub-order NUDIBRANCHIATA. miclxis, 

 naked. The Sea-slugs, though destitute of shells, are 

 often highly and beautifully coloured. Estimated 

 number of species : recent, 300 ; British, 26. They are 

 referred to two sections. 

 Section i. — Aiolobranchiata. otioKoc,, changeable. Gills 

 in parallel rows. 



H Coloured models of Nudibranchs from the Mersey ; 

 Tritonia, Eolis, &c. 

 Section ii.— Anthobranchiata. avSog, a flower. Gills 

 in the form of a flower. 



51 Coloured models of Nudibranchs, Doris, &c. 



Upper Compartment. 

 Coloured drawings and specimens in spirits. 



Group 76. — Sub-order TECTIBRANCHIATA. tectus, 

 covered. Gills protected by a shell, or by the mantle. 

 Estimated number of species : recent, 338 ; British, 23; 

 fossil, 100. Trias, &c. 



The animals frequent weedy shores and coral reefs : 

 their bodies often exhibit lively colours, and 

 are frequently much larger than the shells 

 which protect the respiratory organs. 

 H Examples of most of the genera in the families. 

 ActcBonidce = Tornatellidcs, Aplustridce, Cyli- 

 chnidce, Bullidce, PhilinidcB, Lophocercidce, 

 Aplysiidce, Pleurohranchidcs. 



