342 ALTERNATIONS OF GENERATIONS. 



separated from the muscular wall outside, which is also formed 

 of splanchnic mesoblast. 



The anus is a late formation. 



Alternations of generations. 



Amongst Chsetopoda a considerable number of forms exhibit 

 the phenomenon of alternations of generations, which in the 

 same general way as in the case of the Ccelenterata, is second- 

 arily caused by budding or fission. 



The process of fission essentially consists in the division of a 

 parent form into two zooids by the formation of a zone of fission 

 between two old rings, which becomes differentiated (i) into an 

 anal zone in front which forms the anal region of the anterior 

 zooid, and (2) into a cephalic zone behind which forms the head 

 and some of the succeeding segments of the posterior zooid. 

 The anal zone is capable, by growth and successive segmenta- 

 tion, of giving rise to an indefinite number of fresh segments. 



In Protula Dysteri, as shewn by Huxley, there is a simple 

 fission into two in the way described. Sexual reproduction does 

 not take place at the same time as reproduction by fission, 

 but both zooids produced are quite similar and multiply 

 sexually. 



In the freshwater forms Nais and Chaetogaster a more or 

 less similar phenomenon takes place. By a continual process of 

 growth in the anal zones, and the formation of fresh zones of 

 fission whenever four or five segments are added in front of an 

 anal zone, complicated chains of adhering zooids are produced, 

 each with a small number of segments. As long as the process 

 of fission continues sexual products are not developed, but even- 

 tually the chains break up, the individuals derived from them 

 cease to go on budding, and, after developing a considerably 

 greater number of segments than in the asexual state, reproduce 

 themselves sexually. The forms developed from the ovum then 

 repeat again the phenomenon of budding, etc., and so the cycle 

 is continued 1 . 



The phenomena so far can hardly be described as cases of 



1 Reproduction by budding and formation of the sexual products to some extent 

 overlap. 





