xii BACKBONELESS ANIMALS 227 



left by the tide ; fringing the rocks are various zoophytes, 

 or, if we twist the name backwards, plant-like animals ; 

 besides these, and hardly visible in the clear water, are 

 minute translucent bells some of which are the liberated 

 reproductive individuals of a zoophyte colony ; and 

 there are yet other exquisitely delicate, slightly iridescent 

 globes the Ctenophores, which have been brought in 

 from the open sea, and move by rows of comb-like 

 structures formed of fused cilia. But we must search an 

 inland pool to find one of the very simplest members of 

 this class the freshwater Hydra which hangs from the 

 floating duckweed and other plants. 



This Hydra is a tubular animal often about a quarter of 

 an inch in length. One end of the tube is fixed, the other 

 bears the mouth surrounded by a crow r n of mobile ten- 

 tacles. 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 polyps like itself, and these 

 are eventually set free. 



If we suppose the budding of Hydra continued a 

 hundredfold, till a branched colony of connected indi- 

 viduals is formed, we have an idea of a hydroid or zoophyte 

 colony. For a zoophyte is a colony of many hydra- 

 like polyps, 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 marked 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. (See fig. 21.) 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 repro- 

 ductive 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 



