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 

 fioat 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 dehcate bells which we call Cteno- 

 phora, or comb-bearers, have no littoral stage at aU, 

 but are pelagic throughout their hfe. On the other 

 hand, sea-anemones, sea-pens, and their aUies, the 

 corals of warm seas, and so forth, are littoral, save in 

 their earhest 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-bom, 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 aU require the presence of a sub- 

 stratum, are necessarily either littoral or abyssal. 

 A great number occur in shallow water, aU these being 

 attached to some sohd body. But there are no very 

 obvious adaptations to littoral life, save perhaps that 

 shaUow-water sponges seem to be always attached, 

 while some abyssal forms lie loosely on the surface of 

 the ooze. 



