2 8o Shell Life 



the median portion, and retain only their power of 

 flapping for the purpose of natation. The series of 

 forms illustrating^ the evolution of the lateral folds of 

 Aplysia is so complete as to leave no doubt about the 

 truth of this view." ^ 



The Crowned Runcina (Ritncina coronata) is even 

 more distinctly slug-like, for it has no shell. It is of 

 a brown colour with a distinct mantle. There are no 

 tentacles, but there are three slightly plumose gills. 

 It frequents tide-pools and shallow water, crawling 

 over mud and brownish weeds, but avoiding those of 

 green hue upon which its colour would be danger- 

 ously conspicuous. It is quite small, full-grown 

 specimens being less than half an inch long. 



The foregoing species represent the sub - order 

 Tectibranchiata, in which the mollusks agree in 

 possessing a gill on the right side more or less con- 

 cealed by a fold of the mantle. The following sub- 

 order though called Nudibranchiata is chiefly charac- 

 terised by the absence of both gill and shell — the 

 latter, however, being present in the embryo. Many 

 species are covered with long fleshy points (cerata), 

 enclosing portions of the liver, and some have the 

 tentacles ringed with projecting sense-organs (rhino- 

 2'>hores). They form two groups, the Holohepatica, in 

 which the liver is entire and internal, and Cladohepa- 

 tica, in which the ramifications of tliat organ are 

 generally branched or twiggy. 



Cerata are frequently present in the Holohepatica, 

 but they serve the function of gills only, and are 

 grouped in the middle line of the back; there are 



^ Journal of the Marine Biological Association, vol. i., n.s., 

 p. 419. 



