ANIMAL LIFE IN THE SEA 175 



the lightness is due. to the 'presence of air or another 

 gas. In the surface forms the body is often transparent 

 and colourless, so that it is invisible as it floats in the 

 clear water, while in other cases it is blue or violet, so 

 that it wiU equally be invisible when seen from below 

 against the sky. That these peculiarities are adaptive 

 is suggested by the way in which they appear in un- 

 related forms. Thus while the shore annehds are 

 brightly coloured, Tomopteris, a pelagic form, is colour- 

 less, and has an expanded body. The same peculiari- 

 ties appear in Sagitta, a worm of uncertain affinities ; 

 in Phyllirhoe, a curiously modified and shell-less 

 gasteropod ; in Salpa, a pelagic tunicate, and so on. 

 The surface jellyfish are blue or violet or almost colour- 

 less, while in deep water bright-red forms occur. Many 

 of the pelagic forms of the surface are phosphorescent. 

 We see this in the protozoon called Noctiluca, in the 

 beautiful tunicate called Pyrosoma, in some pelagic 

 Crustacea, and so forth. Surface pelagic forms are so 

 numerous that no useful purpose would be served by 

 giving a list, but it may be interesting to note those 

 groups which have no pelagic representatives. While, 

 as already suggested, very many types of Coelentera are 

 pelagic, there are no pelagic alcyonarians (sea-pens, 

 &c.), and there are no pelagic sponges. With one 

 exception, to be considered later, there are no pelagic 

 echinoderms, though their larvae are often pelagic. 

 There are no pelagic bivalve moUuscs, though many 

 different kinds of gasteropods are adapted to life in 

 the open sea, the sea-butterflies (Pteropods) being good 

 examples. Some other less familiar groups are also 

 unrepresented in the open sea. 



On the other hand, there are enormous numbers of 

 pelagic larvae. As young animals have often dehcate 



