Class 2. Gastropoda. Order 1. Prosobravchiata. 303 



smaller, and which possess no shell at all. As larvse all have shell and operctilum. 

 The Heteropoda are actively predaceous, and swim with the ventral side turned 

 upwards ; they occm- in all waiin seas ; various f onus, for instance, are found in 

 the Mediterranean. 



Pig. 248. One of the Heteropoda (CarinaWa). /foot with a posterior sucker (su), 

 g Igill projecting from the mantle cavity, m mouth, s shell, h keel on the shell. — After 

 Souleyet. 



Order 2. OpisthobrancMata. 



Some Opisthobranchs are provided vrith a visceral liump ; a shell, 

 which is usually spiral; a mantle-cavity enclosing a gill; sometimes, 

 also, like the Prosobranchiata, witb. an operculum ; often, however, 

 the shell is somewhat aborted, and most of the members of the 

 group (Nudibranchiata) have completely lost it, and with it 

 the visceral hump and mantle-cavity ; the viscera have sunk into the 

 lower portion of the body ; the nephridial and genital apertures, and 

 frequently the anus also, are situated on the right side above the foot. 

 In the Nudibranchs, as a rule, the ordinary gill is absent, and is then 

 generally replaced by special outgrowths of the skin, varying in form. 

 It is particularly interesting that the nudibranch larva is furnished 

 with shell and operculum ; both being later thrown off. All Opistho- 

 branchiata, of which the naked forms usually display gorgeous 

 colours, are marine, occurring in cold as well as in warm seas. 



1. Of shell-hearing Opisthobranchs may be mentioned : the Bubble 

 (Bulla), with bidging shell, the apex of which lies in a depression ; common in 

 warmer seas, allied genera in Northern seas. In some allied foi-ms the shell 

 is surrounded by folds of skin ; these fuse with one another, and may com- 

 pletely enclose the shell, which is then always thin (internal shell) ; such, for 

 instance, is the case in the genus Aplysia (Sea-hare), which inhabits the Medi- 

 terranean and other warm seas, s^d is algo niet with on the south coast of 

 flngland, 



