54 2 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



most clearly shown in the case of the Annelida, that 

 the ancestors of the species which now exhibit alterna- 

 tions of generations originally reproduced themselves 

 at the same time both sexually and by budding, though 

 probably the two modes of reproduction did not take 

 place at the same season. Gradually a differentiation 

 became established, by which sexual reproduction be- 

 came confined to certain 

 individuals, which in 

 most instances did not 

 also reproduce asexu- 

 ally. After the two 

 modes of reproduction 

 became confined to se- 

 parate individuals, the 

 dissimilarity in habits 

 of life necessitated by 

 their diverse functions 

 caused a difference in 

 their organisation ; and 

 thus a complete alter- 

 nation of generations 

 became established. 

 The above is no merely 

 speculative history, 

 since all gradations be- 

 tween complete alternations of generations and simple 

 budding combined with sexual reproduction can be 

 traced in actually existing forms." 



When alternation of generations is fully expressed 

 among the Hydrozoa we find that the sessile hydri- 

 form colony gives rise to buds which gradually break 

 away from their colony and become free-swimming 

 (Fig. 226). Differing in some details from the 

 structure of the Medusa already noted, these forms are 

 still more interesting in that between them and the 

 ordinary hydroid polyp we find a series of stages which 



Fig. 226. Figure of Syneoryne with a 

 number of Budding Medusae on it 

 at Different Stages (a to e) of De- 

 velopment. (After Desor.) 



