613 



ELYSIAD.^. 



This family constitutes the order Pellibrancliiata of 

 Alder and Hancock, a most natural group, distinguished 

 by important anatomical characters, and a peculiar external 

 habit. It consists of naked sea-slugs, whose structure has 

 close relationship with that of the NudihrancMate orders, 

 but differs materially, insomuch as the respiratory function 

 is effected by the whole surface of the body, which is 

 entirely clothed with vibratile cilia. The usual aspect of 

 these little creatures reminds us of a land slug. In some 

 tentacula are conspicuously developed, in others they are 

 obsolete. For an excellent and detailed account of their 

 anatomy we must refer our readers to a paper by the 

 authors above cited, contained in the first volume of the 

 second series of the " Annals of Natural History." 



ELY SI A, Risso. ACT.EON, Oken. 



Animal depressed, oval, with lateral aliform expansions 

 produced beyond the posterior extremity of the body, 

 where they unite with one another along the mesial line. 

 Head distinct, bearing two conspicuous auriform tentacula, 

 behind which are two rather distant eyes. Vent placed 

 centrally on the hinder part of the back. Genital orifice 

 at the right side. Foot narrow. 



