ORDER OPISTHOBRANCHIATA. 



Marine Euthyneura with aquatic respiration; the ventricle 

 of the heart is generally anterior, and the pallial cavity, when 

 present, is widely open. There is a marked tendency to a re- 

 duction of the shell, which may become internal or disappear. 

 In the naked forms spicules are sometimes developed. 



Suborder TECTIBRANCHIATA. 



Hermaphroditic opisthobranchiate Mollusca provided in the 

 adult state with a mantle and shell, with certain exceptions ; with 

 one branchial plume and osphradium, with certain exceptions. 



Tribe I. Bulloidea. 



Shell usually well developed, sometimes wanting, external or 

 internal. Operculum seldom present. Pallial cavity well devel- 

 oped and containing the usually plicate ctenidium. Head usually 

 without tentacles, and with dorsal surface forming a shield, 

 usually separate from the neck, and with more or less scalloped 

 margins. Edges of foot, continuous with its ventral face and 

 often modified into fins. Stomach usually with chitinous or cal- 

 cified masticatory plates. Visceral commissure usually rather 

 long. The monaulic genital duct usually connected with the penis 

 by a ciliated groove. 



Tribe H. Aplysoidea. 



Shell much reduced, more or less internal, or lost altogether 

 in the adult state. Head with two pairs of tentacles. Margins of 

 the parapodia separate from the ventral surface of the foot, and 

 generally modified into natatory lobes. Visceral commissure 

 usually very much shortened, except in Tethys. Genital duct 

 monaulic, the hermaphroditic duct connected with the penis by 

 a ciliated groove. 



