52 THE SEAS 



fragments, each of which grows into a new individual. 

 The anemones, although some kinds habitually reproduce 

 themselves sexually, often divide (a process which is 

 highly developed in the reef building corals, as described 

 in Chapter VII), or break off small fragments near the base. 

 The little syllid worms break up in a perfectly definite 

 way into fragments of a few segments, each cf which grows 

 a new head at one end and a new tail at the other, and so 

 develops into a full-sized worm. The hydroids have a 

 complicated history. They produce not eggs but little 

 jellyfish, called medusae, which swim about near the surface 

 of the sea and, when fully developed, produce eggs which 

 develop into the hydroids which produce the medusas. 

 This, in common with the production of swimming young 

 by various shore animals, is of great importance in securing 

 the distribution of animals which, because of their slowness 

 of movement or fixed mode of life, would not otherwise be 

 able to spread far from their original home. 



Growth 



Length of life in shore animals varies within the widest 

 limits. Some animals, such as many sponges and sea 

 squirts, are annuals or even go through several generations 

 in a single summer, whilst on the other hand anemones may 

 live to a great age ; there are some in captivity known to 

 be over sixty years old. After the early embryonic or, in 

 the case of animals with swimming young quite unlike 

 their parents, " larval " stages, most animals assume the 

 adult shape changing in size only as time goes by. Growth 

 is usually a stead}'' process, varying in speed according to 

 the time of the year, being generally slower in winter and 

 quicker in summer when it is warmer and there is more food. 

 In the crustaceans, however, growth takes place by a 

 series of jumps. Unlike the molluscs, which increase the 



