FISSION 



279 



a region or zone of proliferation (Fig. 148, IV). Moreover, in 

 some species not only is the hinder part of the body converted 

 into a sexual zooid, but the zone of proliferation becomes very 

 active, and produces by gemmation a large number of segments, 



Fio. 148. Diagrams 

 illustrating the 

 various stages in 

 the asexual forma- 

 tion of a chain of 

 zooids. (Modified 

 from Malaquin.) 



I, Heteronereid 

 or Heterosyllid 

 stage. A, Non-sex- 

 ual ; A', sexual 

 region of the body, 

 with modified 

 parapodia. 



II, Syllis. The A 1 

 hinder sexual re- 

 gion, B, is similarly 

 modified, and will 

 separate from the 

 parent zooid, A, 

 and become an 

 independent zooid. 



III, Autolytus. 



The hinder zooid, B, develops a head by budding before separation. 



IV, Autolytus, etc. A zone of budding (2) makes its appearance in 

 front of the head of B, and by its growth will give rise to a series of 

 new segments in the middle of the body. 



V, Myrianida, Autolytus, etc. From this zone of budding a very 

 large number of segments have been formed, which have, further, be- 

 come grouped so as to form three individuals, C, D, E ; B is the hind- B 

 most zooid, which is either formed from the hinder segments of the 

 parent zooid or is produced by budding, like C, D, E. 



which become marked out, by the appearance of heads at intervals, 

 into a number of zooids, in which genital organs will later make 

 their appearance. A chain of as many as sixteen zooids may be 

 formed in Autolytus (Fig. 148, V) the hindermost by conver- 

 sion of the hinder part of the body of the original " stock," the 

 intervening zooids by gemmation. 



One original " stock," or asexual zooid, thus produces several 

 sexual zooids, but these are only of one sex for a given stock. The 

 males differ in several important characters from the females ; so 

 different, indeed, are the two sexes that before their history was 



