72 



ANIMAL BIOLOGY 



pendent free-swimming individuals. At first glance a medusa bears 

 little resemblance to a polyp, though it is essentially a sexual polyp 

 that has been produced on the colony by budding and then liber- 

 ated. Accordingly the complete life history of a typical Hydrozoon 

 involves an alternation of generations consisting of the asexual 



Statocyst 

 — Radial canals 

 ,■ Reproductive organs 



--Tentacles 



Mouth 



Mouth 



Ectoderm / 

 Entoderm / 

 Enteron 



Perisarc' 



Fig. 37. — Life history of a Hydroid, Obelia. A, portion of the asexual 

 colony showing three hydranths and a gonangium; B, free-swimming sexual 

 medusa, oral view; C, medusa, side view; D, ciliated larva. (C and D of closely 

 related species.) (From Hegner, after Allman, and Hargitt.) 



colony and the sexual medusae that swim away and disseminate 

 the species. Hydra is apparently a specialized form in which the 

 medusa generation is suppressed. (Figs. 37, 38.) 



The Hydroid colonies exhibit some interesting examples of 

 physiological division of labor between the component individuals. 

 Thus in Bougainvillea there are two kinds of polyps: the typical 



