COELENTERATA. 



121 



power of regenerating lost parts a property which is probably widely 

 diffused in the Coelenterata, and reaches, perhaps, its highest expression 

 in Hydra. The smallest pieces into which the body of Hydra can be 

 cut are able to reproduce the whole polyp. 



In the asexual reproduction budding and fission may achieve 

 results varying from the complete formation of a new individual 

 which separates from the parent, to the mere repetition of a portion 

 of the growing organism, which remains attached to the parent. 

 As an instance of the one extreme we may mention Hydra ; of the 

 other, the Siplionophora ; and between these two extremes we have 

 the colonial Hydroinedusae and Anthozoa. 



In Hydra the whole organism is reproduced; in the Hydromedusan 

 colonies the whole 

 parent except the 

 rooting portion or 

 hydrorliiza ; in the 

 Maeandrine Corals 

 the reproduction is 

 confined to the oral 

 disc and stomodaeum, 

 and perhaps a few 

 tentacles and mesen- 

 teries: finally, in the 

 Siphonophora the 

 manubrium of the 

 parent Medusa (or 

 polyp, according to 

 the view taken) may 

 produce almost any 

 portion of an entire 

 organism from an um- 

 brella to a tentacle. 



Connected Avith this asexual reproduction are two important 

 phenomena which must shortly be referred to here. The one is 

 polymorphism, and the other alternation of generations. In the 

 colonial Coelenterata the individuals associated in the colony are 

 rarely all alike in structure. A certain number have one form, others 

 another form, and with this difference in form is often associated a 

 difference in the part which they respectively play in the life of 

 the colony. As an instance we may take the Tubularian colony 

 called Podocoryne (Fig. 108). Here from a creeping coenosark or 



FIG. 108. Podocoryne carnea (after C. Grobben). P pol^p ; 

 M medusa bud on the blastostyle ; Sk skeleton polyp ; 

 S spiral zooid. 



