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 rudimentary 

 or wanting ; sexes united .... Opistho-bmnchiata. 



Gills enclosed in a vaulted cavity ; 

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



Order I. OPISTHO-BRANCHIATA. 



Shell rudimentary or wanting ; gills disposed 

 in tufts (fasciculated], or branching (arlorescent), 

 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 ; gills covered by the shell or 

 mantle . ... TectibrancMata. 



TEIBE I. NUDIBRANCHIATA. 



No gills Mysiada. 



Gills conspicuous. 



Arranged along the sides. 



Tentacles not retractile . . . Eolidida. 



Tentacles retractile into sheaths Tritoniada. 



Arranged in a circle on the back . Doridida. 



* The Order PULMONIFERA contains no marine species. 

 H 



