120 



COELENTERATA. 



detached. In such cases it may acquire the full development of a medusa, but 

 more often its development is not complete, and a degenerate medusa or as it 

 is often called, a gonophore with concealed medusiform structure is produced. 

 These degenerate medusae (hedrioblasts, adelocodonic gonophores, medusoids, as 

 they are variously called) vary considerably in structure from the stage of an 

 almost perfect medusa, to a stage in which they consist of little more than 

 a bud containing the generative cells (sporosacs). 



The Ctenophore. The description of the fundamental form of the 

 Ctenophora will be more conveniently dealt with under the descrip- 

 tion of the class (see below, p. 197). 



Rdc 



Eli 



FIG. 107. Diagrammatic longitudinal sections of a, a hydroid polyp, and 5, c, a medusa. 

 Section b passes through two radial canals, section c between two radial canals. mouth ; 

 T tentacle ; M enteron ; Ek ectoderm ; En endoderm ; Rdc radial canals ; Re circular canal ; 

 vel velum ; Gp vascular or endoderm lamella ; S sub-umbrella ; U umbrella. 



Asexual reproduction by budding and fission is very widely spread. 

 It is found in all groups with the exception of the Ctenophora, and 

 it frequently leads to the formation of colonies, the component 

 members of which are in bodily continuity. In most cases both 

 layers of the body wall participate in this mode of reproduction, 

 but it has recently been asserted that the ectoderm alone participates 

 in the gemmation of the buds of Hydra; and in the so-called 

 sporogony, which is found in some Narcomedusae, a single ovum-like 

 cell of the body has the power of reproducing the whole organism. 

 The latter should perhaps be regarded as a case of parthenogenesis 

 which otherwise has not been observed in the Coelenterata. With 

 this phenomenon of asexual reproduction must be connected the 



