CRABS, ETC. 93 
denskiold, is the little A/etridia 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 (Piznotheres).—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- 
‘ 
Fic. 104.—A, Pinnotheres, living in a Holothurian (Piauotheres holo- 
thuria). B, the zoea stage of the young of 4, 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 
Fic. 105.—Crabs that form galls on corals. @, Cryptochirus (male); ?. 
Coralliodytes (female); c, Hapalocarcinus marsupials (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 
