NUDIBRANCHIATA. 97 



protected by a shell, for tlie most part formed of a 

 single piece, and generally spirally turned ; mouth 

 provided with lips, and a ribbon-like tongue, 

 armed with rows of teeth ; eyes two ; tentacles two 

 or four. Young subject to metamorphosis. 



Gills exposed ; shell nidimentaiy 

 or wanting ; sexes united .... OpistJio-branchiafa. 



Gills enclosed in a vaulted cavity ; 

 shell capacious ; sexes distinct . . Proso-branchiata.^ 



Order L OPISTHO-BRANCHIATA. 



Shell rudimentary or w^anting ; gills disposed 

 in tufts [fasciculated)^ or branching [arhorescent)^ 

 not contained in a spiral cavity, but more or less 

 completely exposed on the back and sides, towards 

 the hinder part of the body ; sexual functions 

 united in the same individual. Feeding principally 

 on zoophytes. 



No shell, except in the infant state ; 

 gills external, on the back or sides . Nudibranchiata. 



Shell usually present throughout 

 life ; giUs covered by the shell or 

 mantle Tectibranchiaia. 



TeIBE I. NUDIBRANCHIATA. 



No gills Elysiad<s. 



Gills conspicuous. 



Arranged along the sides. 



Tentacles not retractile . . . Eolididce. 

 Tentacles retractile into sheaths Tritoniadce. 



Arranged in a circle on the back . Borididce. 



* The Order Pulmonifera contains no marine species. 



H 



