300 



LOWER INVERTEBRATES. 



In Tethys we have another peculiar form embracing some of the largest of the 

 naked molluscs, T. fimbriata, occasionally reaching a foot in length. Its general 



appearance can be seen fi-om our illustrar 

 tion, which, however, fails to convey any 

 impression of the coloration of the animal. 

 It is nearly transparent, and covered with 

 dots and spots of red of different shades, 

 some so dark as to be almost black. The 

 curious gills on the upper surface were 

 once described as parasites. It is a native 

 of the Mediterranean, and, though often 

 captured, it lives but a short time in 

 aquaria, even in the large ones of the 

 Naples Zoological Station. It is a ra- 

 pacious animal, feeding upon other mol- 

 luscs and small crustaceans. 



In the remaining forms the branchiae 

 are arranged upon the back in a more or 

 less complete circle which suri-ounds the 

 anus. As an examjile of the Poltcekid^ 

 we may mention the beautiful Polycera 

 lessonii of our coast, with a pale, flesh- 

 colored body, flecked with bright green, 

 while the tentacles, gills, and tubercles on 

 the back are variously spotted with white 

 or yellow, and occasionally green. There 

 are several other American forms in this 

 family. 



The Phylliehoid^ is a verj- peculiar family, whose position among the molluscs 

 would not be certain were it not for the fact that it possesses a lingual ribbon. 

 PhyUirhoe hucephalus, the best known species, is a thin, compressed, translucent ani- 



FlG. 340. — Tetliijs .limbriala. 



FXG. 341. — PhyUlrkoe Imcephalus ; 6, brain; A, heart; i, intestine; /, liver; m, mouth; r, renal organs; s, salivary 



gland ; v, vent. 



mal with a rounded, fin-like tail, which swims freely through the water in much the 

 same manner as a fish. The head is furnished with two long tentacles, gills are absent, 

 and the intestine terminates on the right side of the body. Most of the specimens 

 bear a parasitic medusa, Miiestra parasitica. The most interesting fact connected 

 this animal is its phosphorescence. At night, when swimming in the sea or in an 



