166 



GENERAL PRINCIPLES OF ZOOLOGY. 



equally to the individual parts, many of the latter become employed 

 solely for this, others again solely for that function, and acquire a 

 corresponding structure. In case of such animal colonies one 

 speaks then of multiformity or polymorphism. Polymorphism 

 appears oftenest in connexion with the vegetative functions, lead- 

 ing to a distinction between sexual animals and nutritive animals, 

 as in the case of most Hydrozoa, where often nutrition is accom- 

 plished by animals without sexual organs, and reproduction is 

 carried on by animals without a mouth. But other functions, 

 movement, sensation, offence and defence, may also become 

 specialized. Siphonophores are the classical examples of poly- 

 morphism (fig. 110). Here united into a single body are locomotor 



-FlO. 110. Praya diphyes. (After Gegenbaur) A, the entire animal; J3, a single 

 group of individuals greatly magnified (Eudoxia). 1, covering scale; 2, nutritive 

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



animals, the swimming-bells, serving only for locomotion; cover- 

 ing scales, which serve only to protect the others; nutritive polyps, 

 which alone take in and digest food; sexual animals and tactile 

 polyps, which are concerned only in sexual reproduction and with 

 sensation. In regard to the other functions each animal is related 



