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 will 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 annelids 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 Phyllirh6e, 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 molluscs, 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 delicate 
