OPISTHOBRANCHIATA : HABITS, CLASSIFICATION 429 



their environment (p. 71 f.) ; others again (SipJwnaria, Gadinia), 

 stick limpet-like to rocks between tide marks. As a rule, they 

 occur only in clean salt water, but Emlletonia has been found in 

 the Victoria Docks at Eotherhithe, as well as in parts of the 

 Baltic, where the water has only 7 parts of salt in 1000, while 

 Limapontia occurs in nearly fresh water at Bornholm and 

 Gothland. 



Their food varies greatly. As a rule, they are frugivorous, 

 but many cases of carnivorous habit occur. Scaphander has 

 been seen to swallow Dentalium six at a time, and in six hours 

 the shells of all were reduced to tiny fragments. Glaucus devours 

 the soft portions of the pelagic Porpita and Velella ; Idalia 

 elcgans eats its way into the test of Ascidians, and completely 

 buries itself in the body of its prey.^ 



The Opisthobranchiata may be classified as follows : — 



1. Tectibranchiata 



Opisthobrancliiata <' 2. Ascoglossa 



3. Nudibranchiata 



4. Pteropoda . 



{Bulloidea 

 Aflysioidea 

 Pleurobranchoidea 

 Siphonarioidea 



f Cladohepatdca 



\Holo}u'patica 

 f Thecosomata 

 yGymnosomata 



Sub-order I. Tectibranchiata. — Eight ctenidium usually 

 present, more or less concealed by the mantle fold, visceral 

 ganglia united by a very long commissure, shell variable in 

 form, more or less enveloped in folds of the mantle and foot, 

 often becoming rudimentary. 



Section I. Bulloidea. — Shell more or less spiral, internal or 

 external, epipodia more or less developed, a broad cephalic 

 disc, distinct from the dorsal region, usually no tentacles, eyes 

 sessile. 



Fam. 1. Actaeonidae. — Shell spiral, solid, entirely covering 

 the animal ; spire generally prominent, operculum corneous, 

 visceral loop streptoneurous, no epipodia, radula multiseriate, 



teeth numerous, very small. Carboniferous . Genera : 



Actaeon (Fig. 286 A.) ; Voharia (Tertiary), Fortisia (Eocene) Actae- 



1 J. Power, Ann. Mag. N. H. (2) xx. p. 334 ; P.Z.S. 1836 p. 113 ; Arch. Zool. 

 Exp. Gin. (3) i. 1893, p. 105. 



