238 



SYSTEMATIC ZOOLOGY 



butterflies. In some of them shell, mantle, and gills are well 

 developed, in others they are entirely absent. All the members 

 of the order are hermaphroditic, but a penis is present and cross 

 fertilization takes place. The sea slugs, known technically as 

 the Nudibranchia (Lat. nudus, naked, and branchia, gill,), naked- 



clc n. 



all 



sh 



r 



±f-- 



pr-ob 



Fig. 243. Pterotrachea scutata. alt, alimentary canal; cten, gills; eye, eye \ fi, foot; mo, 

 mouth ; prob, proboscis ; repr, reproductive gland ; sh, shell ; su, sucker. (After Parker 

 and Haswell.) 



gilled, generally have an elongated body with no shell or mantle, 

 and the foot provided with a broad sole for creeping (Fig. 244); 

 but some pelagic genera have no foot, some have a mantle contain- 

 ing calcareous spicules, and some have a delicate shell. Many 

 have adaptive gills in place of the true gills ; these are fingerlike 



S? 



FlG. 244. . l.olis papulosa ; natural size about 

 thirteen centimeters. (From Brehm's Thier- 

 leben.) 



or leaflike processes very variously arranged on the dorsal side 

 of the body ; their coloring is often extremely beautiful. 



In size they vary greatly from a few millimeters to some thirty 

 centimeters in one tropical genus. One genus lives as a parasite 

 in one of the Holothuroidea ; the adult has lost all its Molluscan 

 characteristics and resembles a worm, but the larvae are of the 



