158 AM INTRODUCTION TO ZOOLOGY. 



are present in addition to a supporting skeleton. The 

 latter may be horny (sea-fan, Gorgonia), composed of 

 horny and calcareous joints alternately {Isis), or all 

 calcareous (red coral, Gor allium rubrum). These are 

 the so-called Sclerodermic corals. 



In the Htdeozoa, as already stated, there is fre- 

 quently an " alternation of generations." The polyps 

 produced by division adopt that principle of the 

 " division of labour " (which has been spoken of on 

 p. 24 as the secret of the differentiation of cells and 

 tissues), so that some of them only fulfil digestive 

 functions, while others develop reproductive functions 

 and produce eggs ; while in some cases others again 

 degenerate, lose even the digestive organs, and 

 are useful merely to protect the others. These are 

 spoken of as " nutritive zooids " ^ or " persons," and 

 "reproductive zooids"^ or " persons," and ''protec- 

 tive zooids " or " persons." The reproductive persons, 

 under these circumstances, frequently become free, 

 and float about as independent animals. In this 

 stage they form -what are commonly known as Me- 

 dusae, or "jelly-fishes"; and they are exactly like 

 certain other forms of jelly-fish, which are complete 

 and single animals : animals, that is to say, which do 

 not form a colony, but produce from one egg one 

 zooid only, which is complete in structure, possessing 

 the reproductive in addition to the other organs 

 and functions of animal life. In some forms all the 

 polyps are much modified with a view to their respec- 



' Trophosomes. ' Gonosomes. 



