256 



General Biology 



librium, that is, statocysts ( ). Muscle fibers are 



present in both exumbrella and subumbrella. 



Beneath the radial canals the sinuously folded reproductive organs, 

 or gonads, are suspended. Gonionemus is dioecious ( ), 



each individual producing either eggs or spermatozoa. These repro- 

 ductive cells break out directly into the water, where fertilization takes 

 place. A ciliated planula develops from the egg as in Obelia. This soon 

 becomes fixed to some object and a mouth appears at the unattached 

 end. Then four tentacles grow out around the mouth, and the hydra- 

 like larva is able to feed. Other similar hydra-like larvae bud from its 

 walls. How the medusae arise from these larvae is not known, but 

 probably there is a direct change from the hydroid form to the medusa. 



POLYMORPHISM 



Whenever there is a division of labor among the different members 

 of the same colony so that each does a particular work, such colony is 

 said to be polymorphic ( ) if there are more than 



two kinds of specialized individuals ; dimorphic, if only two different 

 specializations have taken place. 



The "Portuguese man-of-war" 

 (Fig. 158) is an excellent example 

 of the former, in that it is a bladder- 

 like structure to which many tenta- 

 cles are attached. It floats upon the 

 water. Some of these tentacles 

 are nutritive, others are tactile 

 ( ), some contain 



batteries of nematocysts, others are 

 male reproductive zooids, and still 

 others give rise to egg-producing 

 medusae. 



The Coelenterata (together with 

 the Echinoderma) were formerly 

 called Radiata on account of their 

 radial form. It is now known that 

 in the higher groups of coelenterates, 

 this radial form may be transformed 

 into a b i r a d i a 1 or bilateral sym- 

 metry. 



Older writers often spoke of the 

 coelenterates as Zoophyta (animal- 

 plants) on account of their resem- 

 blance to plants both in appearance 

 and in their method of attachment. 

 Then, too, these animals simulate 



Fig. 158. 



A, Physalia or Portuguese man-of-war, a 

 colonial Hydrozoon. (From Hegner, after 

 Agassiz.) 



B, Diagram showing possible modifica- 

 tions of medusoids and hydroids of a hydro- 

 zoan colony of the order Siphonophora. c. gas- 

 trozooid with branched, grappling tentacle, f; 

 g, dactylozooid with attached tentacle, h; i, 

 generative medusoid; k, nectophores (swim- 

 ming bells) ; /, hydrophyllium (covering 

 piece); m, stem of corm; n, pneumatophore. 

 The thick black line represents entoderm, the 

 thinner line ectoderm. (From Hegner after 

 Allman.) 



