510 GASTROPODA. [Opisthobranchia. 



SUBCLASS II. EUTHYNEURA, Spengel. 

 (Platymalakia, von Ihering. Androyyna, Morch.) 



These are hermaphrodite Gastropoda, whose radula is generally 

 composed of uniform teeth on each side of the median tooth. The 

 head in most cases bears 2 pairs of tentacles ; in Athoracophorus and 

 a few other genera, however, there is only a single pair. The Euthy- 

 neura are specially characterized by the detorsion of their organiza- 

 tion when adult ; this detorsion is particularly well manifested in the 

 visceral commissure, which is no longer twisted, except in some archaic 

 forms of Tectibranchs and Pulmonates, and shows a tendency to the 

 concentration of all its elements round the oesophagus. To such a 

 degree is this concentration carried that, with the exception of the 

 Bullomorpha and of Tethys, the whole central nervous system is ag- 

 gregated on the dorsal or ventral side in the cephalic region. 



The subclass includes the two orders Opisthobranchia and Pul- 

 monata. 



ORDER 1. OPISTHOBRANCHIA, Milne-Edwards. 



Marine Euthyneura with aquatic respiration ; the more archaic 

 forms have a relatively large foot and small visceral hump, from 

 the base of which projects, on the right side, a short mantle-skirt. 

 The anus is placed in such forms far back beyond the mantle-skirt. 

 In front of the anus, and only partly covered by the mantle-skirt, 

 is the ctenidium, with its free end turned backwards. The heart, 

 with the ventricle generally anterior, lies in front of, instead of to the 

 side of, the attachment of the ctenidium. A shell is possessed in the 

 adult state by but few Opisthobranchia. but all pass through a veliger 

 larval stage with a nautiloid shell. Many Opisthobranchia have by 

 a process of atrophy lost the typical ctenidium and the mantle-skirt, 

 and have developed other organs in their place. The free margin 

 of the mantle-skirt is frequently reflected over the shell when a shell 

 exists ; and broad lateral outgrowths of the foot (epipodia), winch 

 may be thrown over the shell or naked dorsal surface of the body. 



The order comprises two suborders, Tectibranchia and Nudi- 

 branchia. 



SUBORDER 1. TECTIBRANCHIA, Cuvier. 



Opisthobranchs provided in the adult state with a mantle and a 

 shell, with the exceptions of Runcina, Pleurobra-nchcea, the Cyrn- 

 buliidcr, and some Aplysiomorpha. There is a ctenidium, except in 

 some Thecosomata and Gfymnosomata, and an osphradium. 



The suborder includes three tribes the BuUomwpha, the Aplysio- 

 morpha, and the Plcurobranchomorpha. 



Tribe 1. BULLOMORPHA. 



In these Tectibranchs the shell is usually well developed (it is 

 wanting in^ Runcina and the Cymbuliidce), and may be external or 



