II GNIDARIA— ALTERNATION OF GENERATIONS 115 



each of which has long had its supporters. According to one theory, 

 the whole Siphonophoran body represents a single Medusa person, and 

 all its separate appendages — the nectophores, siphons, tasters, tentacles 

 and gonophores — are nothing but displaced organs of this Medusa, 

 whose number increases by multiplication. According to the other 

 theory, the Siphonophoran body is a free-swimming polymorphic Hydroid 

 stock, and each of the appendages just enumerated, even each tentacle, 

 is to be considered as a more or less modified person, suited to some 

 special function, in consequence of an extreme division of labour, 

 either, therefore, as a metamorphosed Hydroid, or (as e.g. the necto- 

 phores and gonophores) as a metamorphosed Medusa. 



XIV. Life-history of the Cnidaria, Alternation of Generations. 



We shall return later to the special ontogeny of the Cnidaria, the 

 arrangement of the layers of the body, and the development of the 

 organs. Here we shall restrict ourselves to depicting the general 

 course of their life-history. 



Hydra multiply both asexually by gemmation, and sexually by 

 means of fertilised eggs. From the latter, by a gradual course of 

 development, Hydra again arise. 



In very many Hydromedusce an attached Hydroid form arises out of 

 the fertilised egg, out of which, by budding, comes a Hydroid stock, 

 which is at least dimorphic and often polymorphic. Some buds 

 become sterile nutritive persons, others sexual persons. The latter 

 detach themselves from the stock as free -swimming Graspedote Medusae 

 (Fig. 78, m, p. 104) and form the sexual products. From the 

 fertilised egg an attached Hydroid may again be produced. We thus 

 find here, in the cycle of development, two consecutive generations, as 

 it were, intercalated : (1) the dimorphic or polymorphic Hydroid stock 

 which reproduces by gemmation ; and (2) the Medusa which arises by 

 gemmation, detaches itself, swims about, and reproduces itself sexually. 

 Such an alternation of diiferently formed generations which multiply 

 in different ways is called alternation of generations (metagenesis). 

 It follows from our description that this alternation of generations is 

 the result of division of labour between the single persons of a 

 Hydroid stock. Each Medusa is originally equivalent to a nutritive 

 person, and it owes its structure to adaptation to the special function 

 of forming the sexual products and of dispersing them by means of 

 its free locomotion. 



We must not, therefore, consider the Hydroid form as a young 

 stage of the Medusa form. Nutritive polyp and Medusa are sisters. 

 The one sister develops further than the other and reproduces sexu- 

 ally, while the latter remains sterile. 



There are two other methods of development to be derived from 

 the alternation of generations of the Hydro -Medusce. There are 

 Hydroid stocks in which the sexual persons do not detach themselves 



