250 



COELENTERATA HYDROZOA 



endoderm (Fig. 125), terminating distally in a conical mound the 

 " hypostome " which is perforated by the mouth and surrounded 

 by a crown of tentacles. This fixed colony, the " hydrosome," 

 feeds and increases in size by gemmation, but does not produce 

 sexual cells. The hydrosome produces at a certain season of the 

 year a number of buds, which develop into small bell-like jelly- 

 fish called the " Medusae," which swim away from the parent stock 

 and produce the sexual cells. The Medusa (Fig. 126) consists of 

 a delicate dome-shaped contractile bell, perforated by radial canals 

 and fringed with tentacles ; and from its centre there depends, 

 like the clapper of a bell, a tubular process, the manubrium, 

 which bears the mouth at its extremity. This free-swimming 

 sexual stage in the life-history of Obelia is called the "medusome." 



It is difficult to determine whether, in the evolution of the 

 Hydrozoa, the hydrosome preceded the medusome or vice versa. 

 By some authors the medusome is regarded as a specially modified 

 sexual individual of the hydrosome colony. By others the 

 medusome is regarded as the typical adult Hydrozoon form, and 

 the zooids of the hydrosome as nutritive individuals arrested in 

 their development to give support to it. Whatever may be the 

 right interpretation of the facts, however, it is found that in 

 some forms the medusome stage is more or less degenerate and 

 the hydrosome is predominant, whereas in others the hydrosome 

 is degenerate or inconspicuous and the medusome is predominant. 

 Finally, in some cases there are no traces, even in development, 

 of a medusome stage, and the life-history is completed in the 

 hydrosome, while in others the hydrosome stages are lost and the 

 life-history is completed in the medusome. 



If a conspicuous hydrosome stage is represented by H, a 

 conspicuous medusome stage by M, an inconspicuous or degenerate 

 hydrosome stage by h, an inconspicuous or degenerate medusome 

 stage by m, and the fertilised ovum by O, the life -histories of 

 the Hydrozoa may be represented by the following formuke : 



The structure of the hydrosome is usually very simple. It 



