THE GASTROPODA 181 



phyllidia, but wholly devoid of " branchiae." Genus Dermatobranchus, 

 van Hasselt. 



TRIBE 4. ELYSIOMORPHA. 



Nudibranchia in which the liver ramifies in the integuments and 

 extends into the dorsal papillae. The genital duct is always triaulic, 

 and the male and female orifices are distant (Fig. 105). The gonad is 

 divided into spheroidal hermaphrodite lobules. There are no mandibles, 

 and the radula is uniserial. There is never more than one pair of tentacles, 

 and these are wanting in Alderia and some species of Limapontia. The 

 otocysts contain each a single otolith. 



FAMILY 1. HERMAEIDAE, Adams. Foot narrow ; dorsal papillae 

 without nematocysts, linear or fusiform, and disposed in several series. 

 Genera Hermaea, Loven ; rhinophores split throughout their length ; 

 dorsal papillae linear ; anus antero-dorsal ; British. Stiliger, Ehrenberg ; 

 rhinophores simple ; dorsal papillae fusiform or ovoid ; anus antero-dorsal. 

 Alderia, Allman ; anus median and posterior ; no tentacles ; dorsal 

 papillae linear ; inhabitants of brackish waters ; British. FAMILY 2. 

 PHYLLOBRANCHIDAE, Bergh. Foot broad ; dorsal papillae without 



Fio. 170. 

 Cenia cocksi, left-side view, magnified a, anus. (After Hancock.) 



nematocysts, flattened and foliaceous. Genera Phyllobranchus, Alder 

 and Hancock ; foot simple ; anus latero-dorsal. Cyerce, Bergh ; ventral 

 part of the foot divided transversely ; anus median. FAMILY 3. PLAKO- 

 BRANCHIDAE, d'Orbigny. Body depressed, without dorsal papillae, but with 

 two very large lateral expansions with dorsal plications ; head flattened ; 

 eyes approximated. Genus Plakobranchus, van Hasselt. FAMILY 4. 

 ELYSIIDAE, Adams. Body elongated, with lateral expansions ; head 

 rounded and eyes separated ; tentacles large ; foot narrow. Genera 

 Elysia, Risso ; British (Fig. 160, D, E). Tridachia, Deshayes. FAMILY 5. 

 LIMAPONTIIDAE, Adams. No lateral expansions of the body and no dorsal 

 papillae ; body planariform ; anus dorsal, median, and posterior. Genera 

 Limapontia, Johnston ; no tentacles ; head and body devoid of crests ; 

 British. Actaeonia, Quatrefages ; head carinated laterally ; British. 

 Cenia, Alder and Hancock ; head with two long tentacles (Fig. 170). 



ORDER 2. Pulmonata, Cuvier. 



Euthyneura with a pallial cavity but no ctenidium. The pallial 

 aperture is diminished by the fusion of the mantle border with the 

 neck, and reduced to a comparatively small contractile orifice at its 

 posterior extremity (Fig. 177, V). The pallial cavity and shell are 



