ANIMAL LIFE IN THE SEA 169 
algae are often covered with the delicate tracery of 
the sea-firs, colonial coelenterates of sedentary habit. 
Among the branches of the ‘ fir’ special cups may be 
seen in which buds form. These buds drop out and 
float away, becoming swimming-bells—sexual forms 
which carry eggs and sperms to a distance, so that the 
fertilized egg may find a home far from the parent 
stock. This, which the biologist calls alternation of 
generations, is of common occurrence among the 
coelenterates. We find, however, that frequently one 
of the two modes of life is emphasized at the expense 
of the other. Those delicate bells which we call Cteno- 
phora, or comb-bearers, have no littoral stage at all, 
but are pelagic throughout their life. On the other 
hand, sea-anemones, sea-pens, and their allies, the 
corals of warm seas, and so forth, are littoral, save in 
their earliest stages, and are fixed save in those very 
early stages. No swimming-bell floats away from 
the coral as it does from the sea-fir colony. Between the 
two extremes all stages exist. For example, many of the 
great jellyfish are shore-born, spending a considerable 
part of their lives attached to rocks. Others are pelagic 
throughout, while still others remain permanently 
attached to the rocks, like larvae which have never 
grown up. Somewhat similar conditions occur in the 
tunicates already mentioned (p. 166). 
The sponges, which all require the presence of a sub- 
stratum, are necessarily either littoral or abyssal. 
A great number occur in shallow water, all these being 
attached to some solid body. But there are no very 
obvious adaptations to littoral life, save perhaps that 
shallow-water sponges seem to be always attached, 
while some abyssal forms lie loosely on the surface of 
the ooze. 
