ELYSIA. 255 



lower papilla very much smaller than the upper. Foot twice as 

 broad as the body, so that by rolling the margins upward the bran- 

 chijB are partly covered, broad lanceolate, anterior angles a little 

 dilated, and tip acuminated ; some arborescent vessels appear above. 

 Length, about half an inch ; breadth, one third as much. 



Found in great numbers, in brackish water, at Cambridge, April, 

 1848 (^Agassiz} ; very common in sheltered muddy bays, feeding on 

 filamentous chlorosperms. Grand Manan QSlimpsoii). 



Only one species of this curious genus (tI. modesta^ Loven) has 

 been described, which differs from ours more especially in the bran- 

 chi«, which are much longer and grow longer towards the tail, 

 more uniform and slender, more numerous, having three or four in 

 each range, and seven or eight ranges. The coloration is much 

 paler. The European species seems to have similar habits, being 

 found in " shallow pools of salt or brackish water, on a muddy bot- 

 tom," sometimes crawling entirely out of water. As remarked by 

 Loven, it has the branchias of jEoHs, the vent of Doris, and the head 

 and foot of Akera. 



Family ELYSIID^E. 



Body limaciform, clothed with cilia. Tongue narrow ; teeth in 

 a single, central series. Tentacles subulate or linear, folded ; eyes 

 sessile, near the bases of the tentacles. Gills in the form of plaits 

 or vessels, radiating on the surface of the back. Vent central, dor- 

 sal, on the hinder part of the back. 



In this family the respiratory function appears to be performed 

 by the entire surface of the body, special organs for that purpose 

 being almost obsolete. 



Genus ELYSIA, Risso. 1812. 



Body with the lateral ridges dilated into wing-like natatory ap- 

 pendages. Head distinct, with two conspicuous auriform tentacles. 



Elysia chlorotica. 



Plate XVII. Figs. 251 -255. 



Animal emerald green, dotted with white and red spots ; slender, tapering be- 

 hind, with broad, lateral expansions, folded and overlapping each other on the 

 back when the animal is in motion ; tentacles two, lanceolate, folded beneath ; 



