THE COELENTERATA 



255 



B 



of equilibrium, that is, statocysts ( ). Muscle fibers, 



in both exumbrella and subumbrella, are present. 



Beneath the radial canals the sinuously folded reproductive organs 

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



each individual producing either eggs or spermatozoa. These reproduc- 

 tive 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 



A, Physalia or Portuguese man-of-war, a 1 . .. . 1 ., , 

 colonial Hydrozoon. (From Hegner, after into a Diradial Or bilateral Sym- 

 Agassiz). 



B, Diagram showing possible modifica- 

 tions of medusoids and hydroids of a hydro- Older writers often SDoke of the 

 zoan colony of the order Siphonophora. c. gas- 



frozooid with branched, grappling tentacle, /; Coelenterates as ZoOphyta (ammal- 



g, dactylozooid with attached tentacle, h ; i, . .. - , . 



generative medusoid ; k, nectophores (swim- plants) On aCCOUnt OI their resem- 



ming bells) ; I, hydrophyllium (covering 11 i i ,1 



niece); m, stem or corm ; , pneumatophore. DlanCC tO plants DOttl in appearance 



The thick black line repre^nts rntoderm, the otn/ J fU^.V mofV,^.^ nf off o^Vir>i <*f 



thinner line ectoderm. (From Hegner after and m tneir method Ot attachment. 



Allman) - Then, too, these animals simulate 



Fig. 158. 



