The Classification of Animals. 191 



the Heteropods differ from the forms already considered in the 

 subdivision of the foot into three regions, pro-, meso-, and 

 metapodium. They are pelagic creatures, and the foot has in 



Fig. 82. —Pahidlna vivipara. 



Fig. 83 — Liimima peregra. 



consequence become a swimming organ. The shell is unim- 

 portant, and has in some species disappeared. 



3. Opisthobranchia.— In these the gill (single) lies behind 

 the heart. The shell is often absent altogether, and when 

 present is frequently small and enveloped in the mantle. These 

 Molluscs are hermaphrodite, and have complicated reproductive 

 organs with various accessory glands. The common Aplysia 

 represents one section of this group in which the single gill, the 



Fig. 84. — Planorhis linecttus. Enlarged. 



mantle and the shell, though a small one, have been retained. 

 These Dpisthobranchs are termed " Palliata." The non-palUate 

 division includes the Nudibranchs, which are devoid of mantle 

 and apparently of gills morphologically corresponding to those 

 of the Palliata. In Doris, for example, there is a circlet of 

 gills surrounding the anus, which seem to be independent 

 structures. These Mollusca have reacquired a certain amount 

 of bilateral symmetry. Examples of the Nudibranchs are Doris, 

 Eolis, with numerous processes on the back, which may be 



