26 INVERTEBRATE ANIMALS. 



MoUuscs. The latter when very young have ciliated 

 wings on the sides of the head, by which they swim 

 freely about ; the body being contained in a minute 

 spiral shell, having an operculum.— H. & A. Adams. 



Sub-class BRANCH lO-GASTEROPODA, 



^payxioi, gills. Organs of respiration constructed to 

 breathe air suspended in water. 



Group 74.— Order HETEKOPODA. 'hspoi, diverse; Trovg, 

 the foot. 



This and the succeeding order are retained at the commencement of the 

 Gasteeopoda, because, although they may be regarded as more 

 highly organized, than the Peosobeanchiata, they are less character- 

 istic members of the Beanchiate Gasteeopods. 



Estimated number of species : recent, 70 ; British, 3. 

 IT Examples of Oceanic Shells — Janthina, Atlanta, 

 &c., including Carinaria ritrea. 



Upper Compartment. 

 Coloured drawings of Firola, &c. Preparations in 

 liquid of Heteropodous animals ; Janthina 

 with its raft, from which the eggs of the 

 animal are suspended. 



Ord§r OPISTHOBPANCHIATA. oV.crSs, behind; 

 jSpaypj^ta, gills. Organs of respiration situated towards 

 the rear of the body. 



Group 75.— Sub-order NUDIBRANCHIATA. niidm, 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. 



