2OO GENERAL PRINCIPLES OF ZOOLOGY. 



division, have sprung from a single sexually-produced 

 mother-animal. 



Community of Functions. The connection of tissue, 

 in the majority of cases, conditions a not inconsiderable 

 degree of community of functions. Stimuli which affect 

 one individual are transmitted by common nerves to the 

 other animals of the colony ; thereby movements in com- 

 mon are rendered possible. In a similar way the food cap- 

 tured and digested by one animal serves for the colony in 

 common. On account of the community of its functions, a 

 colony appears like a unified whole, like an individual of a 

 higher order; the same process which led to the formation 

 of multicellular organisms is repeated. Just as there the 

 elementary organisms, the cells, are united into a single 

 animal, so here the single animals are united into a colony. 



Polymorphism. When a whole is made up of numer- 

 ous equivalent parts, the conditions for division of labor are 

 furnished. Instead of the functions of the entire organism 

 being distributed equally to the individual parts, many of 

 the latter become employed solely for this, others again 

 solely for that function, and acquire a corresponding struc- 

 ture. In case of such animal colonies one speaks then of 

 multiformity or polymorphism. Polymorphism appears 

 oftenest in connection with the vegetal functions, leading 

 to a distinction betw r een sexual animals and nutritive ani- 

 mals, as in the case of most Hydrozoa, where often nutri- 

 tion is provided by animals without sexual organs, and re- 

 production is carried on by animals without a mouth. But 

 other functions, movement, sensation, offence and defence, 

 may also become specialized. Sip]ionopliorcs are the classi- 

 cal examples of polymorphism (Fig. 106). Here united 

 into a single body are locomotive animals, the sv.-irnming- 

 bells, serving only for locomotion ; covering parts, 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 referred here to its brothers and sisters; its very 



