224 The Study of Animal Life part m 



This Hydra is a tubular animal often about quarter of an inch in 

 length. One end of the tube is fixed, the other bears the mouth 

 surrounded by a crown of mobile tentacles. It is so simple that 

 cut-off fragments if not too minute may grow into complete animals ; 

 when well fed, the Hydra buds out little polypes like itself, and 

 these are eventually set free. 



If we suppose the budding of Hydra continued a hundred- 

 fold, till a branched colony of connected individuals is formed, 

 we have an idea of a hydroid or zoophyte colony. For a zoophyte 

 is a colony of many hydra-like polypes, which are supported by a 

 continuous outer framework and share a common life. Numerous 

 as may be the "persons" on a branched hydroid, all have arisen 

 from one more or less Hydra-like individual. 



Sometimes, however, there is a markei} division of labour in such 

 a colony, as in Hydractinia which has nutritive, reproductive, 

 sensitive, and perhaps also protective "persons," — three or four 

 castes into which the colony is divided. The difference between 

 nutritive and reproductive members is often well marked, and 

 this has a special interest in the case of many zoophytes. For 

 many of these, especially among those known as Tubularians and 

 Campanularians, have reproductive individuals which are set adrift 

 as small swimming-bells or medusoids, somewhat like miniature 

 jellyfish. A fixed plant -like, asexual hydroid colony buds off 

 free- swimming, sexual medusoids, from the fertilised eggs of which 

 embryos develop which grow into hydroids. This is known as 

 alternation of generations, and is a remarkable illustration of 

 activity and passivity combined in one life-cycle. 



But all the miniature jellyfish in the sea are not the liberated 

 reproductive buds of hydroid colonies. Some which are in 

 structure exceedingly like the liberated medusoids never have any 

 connection with a hydroid. Their embryos grow into medusoids 

 like the parents. Quite distinct from these medusoids, though 

 sometimes superficially like them, are the true jellyfishes which are 

 sometimes stranded in great numbers on the beach. These medusae 

 belong to a different series, and some of their features link them 

 rather to the sea-anemones than to the hydroids. 



The sea-anemones and the corals are tubular animals whose 

 mouths are encircled by tentacles, but they are more complicated 

 internally than the polypes of the Hydra or hydroid type. For the 

 latter are simple tubes, while the sea-anemones and their relatives 

 have turned-in lips which make a kind of gullet, and the inside 

 tube thus formed is connected with the outer wall of the body by 

 many radiating partitions some idea of which can be gained by 

 looking at the skeletons or shells of many corals. Related to the 

 sea-anemones but different in some details, are many colonies, of 



