254 



ANNELIDA. 



Fig. 124. 



counted by Milne-Edwards in continuity with each other*. The 

 process of division is represented in fig. 124 : the hinder part of the 

 body, including about seventeen seg- 

 ments, is seen to be gradually separated 

 from the anterior or larger portion ; and 

 moreover, at the point of separation, a 

 new head, with eyes and tentacular cirri, 

 is distinctly formed. " In one case," 

 says Miiller, " I found a mother to which 

 three fetuses of different ages adhered in 

 one length. The mother had thirty pedate 

 segments : the youngest daughter, or that 

 nearest the mother, had eleven ; but the 

 head was not yet developed. The most 

 remote had seventeen rings, with both 

 head and eyes, and, moreover, the tail of 

 the mother ; the middle one had seven- 

 teen segments and a head. 



(662.) From various observations, it 

 would seem that similar phenomena 

 present themselves during the develop- 

 ment of other Annelidans ; proving that 

 the bodies of these animals grow by the 

 successive formation of new segments, or 

 zonules, which sprout from those already 

 in existence, in accordance with a funda- 

 mental plan, and become arranged in 

 regular sequence, one behind the other. 



(663.) It is likewise evident that the two extreme portions of the 

 body, namely the oral and anal segments, are first formed, and that it 

 is in the space between these that all the segments of the trunk, how- 

 ever numerous, have their origin, their development being carried on in 

 a single series, which is progressively extended from before, backwards, 

 by the continual addition of new segments, which are so disposed that 

 the relative age of each ring is indicated by the position which it occupies. 

 Each newly-formed zonule is invariably interposed between the last- 

 developed segment and the anal ring ; so that it becomes natural to 

 inquire from which of these two it more immediately derives its origin, 

 a question that at first might appear of difficult solution, but which 

 seems to be set at rest by the following considerations. From the ob- 

 servations of M. de Quatrefagesf, it appears that in some genera, at a 



* The phenomena of fissiparous generation in these Annelidans will be better un- 

 derstood by reference to Mr. Newport's important discoveries relative to the growth 

 of the Myriapoda. Vide post, 732. 



t Ann. des Sci. Nat. 1844. 



Gemmiparous reproduction of 

 Cirrhatula. 



