CECOLOGY 



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parts, the conditions for division of laljor are present. Instead of tlae 

 functions of the entire organism being distributed equally to the in- 

 dividual parts, many of the latter become employed more for this, others 

 again more for that function, and acquire a corresponding structure. 

 In such colonies one speaks of polymorphism. Polymorphism appears 

 oftenest in connection with the vegetative functions, leading to a dis- 

 tinction between sexual animals and nutritive animals, as in the case 

 of most Hydrozoa, where often nutrition is accomplished by animals 



Fig. III. — Praya diphyes (after Gegenbaur). A, 

 group of individuals greatly magnified {Endoxia). 

 polyp; 3, nettle-tlireads; 4, sexual bell. 



the entire animal; S, a single 

 r, covering scale; 2, nutritive 



without sexual organs, and reproduction is carried on by animals without 

 a mouth. But other functions may also become specialized. Siphono- 

 phores are the classical examples of polymorphism (fig. in). Here 

 united into a single body are locomotor animals, the swimming-bells, 

 for locomotion only; covering scales, which serve only to protect the others; 

 nutritive polyps, which alone take in and digest food; sexual animals and 

 tactile polyps, concerned only with reproduction and with sensation. 

 In regard to the other functions each animal is related to its brothers and 



