LIMACINAD^. 85 



1839) ; to Escliriclit^s " Anatomisclie Untersiicli- 

 ungen liber die Clione borealis" (Copenli., 1838) ; 

 and to Rang and Soiileyet's " Histoire Naturelle 

 des Pteropodes" (Paris, 1852) ; we are indebted 

 for nearly all our knowledge of tlie structure and 

 affinities of these little animals. 



Pteeopoda. 



Molluscous animals provided witli a head, and 

 witli two muscular fins, by which they swim at 

 large in the sea ; sometimes partly enclosed in a 

 glassy shell. 



Animal furnished with a shell : 



Shell symmetrical, two-valved . . Hyaleadrp.. 

 Shell spiral, with an operculum. . Limacinada. 



Animal destitute of a shell . . . Clionidce. 



Family I. Hyalead^. 



Animal furnished with an external shell ; head 

 indistinct ; foot and tentacles rudimentary ; shell 

 symmetrical, globose, formed of two plates. 



Eyalea (Lamk.). Glass-shell. Shell slit at the 

 sides and front, three-pointed behind. 



H. trispinosa. Fig. 124 ; n.s. 



Family II. Limacixad^. 



Animal with a spiral, snail-like, but sinistral 

 shell ; head indistinct ; fins attached to the sides 

 of the mouth, united ventrally by a lobe. 



Spinalis (Souleyet). Shell minute, furnished 

 with a thin, glassy, crescent-shaped operculum, 



