CRABS, ETC. 



93 



denskiold, is the little Metridia armata, that exists in such quantities 

 in the snow on the shores of the Arctic Ocean that persons and ani- 

 mals appear to be walking in fire, the splashes of light presenting a 

 wonderful spectacle. The light is of a bluish-white tint, which in the 

 spectroscope gives a one-colored Labrador-blue spectrum. 



Oyster-Crabs (Pinnotheres). These are the delicate 

 forms commonly found in oysters and various bivalve 

 shells, as well as the water- 

 lung of Holothurians (Fig. 

 104). While the oyster-crabs 

 find protection in other ani- 



FlG. 104. A, Pinnotheres, living in a Holothurian (Pinnotheres holo- 

 thurife). 2?, the zoea stage of the young of A, highly magnified. 



mals, and the hermits steal shells to cover themselves, a 

 number of curious forms shown in Fig. 105 take up their 

 position upon a branch of coral, as Sideropora palmata, and 



FlG. 105. Crabs that form galls on corals, a, Cryptochirus (male) ; >\ 

 Coralliodytes (female) ; c, Hapalocarcinus marsupialis (female >, that 

 carries its young in a sac or marsupium. 



finally produce a gall, or are covered by the coral and live 

 so imprisoned, obtaining their food through a small hole or 



