300 



SKXl'AL DIMOKIMIISM 



The case of the Axolotl seems very similar. In certain 

 localities the transformation of the Axolotl form into the 

 pulmonate form takes place in all individuals, in other 

 localities the larvae of the same species never undergo the 

 metamorphosis, but breed in the branchiate condition, and 

 die in that condition. Artificially such Axolotls can be 

 made to pass through the metamorphosis, and it is quite 

 possible that with further knowledge the conditions which 

 determine whether a young Nereis dumerilii shall reproduce 

 in one of the three states or the other might be artificially 

 controlled. 



In the family Syllidse, which consists of small slender 

 worms, the process of metamorphosis is complicated by 

 another, namely, the tendency of the modified hinder region 

 of the worm to separate and form a distinct sexual zooid, 

 while the anterior asexual portion continues to live and 

 grow. We thus have a combination of multiplication by 

 fission and sexual reproduction. The degrees of complication 

 form a graduated series, and the explanation of the phenomena 

 is best attempted after the salient facts have been described. 



In the typical Polychaeta each parappdium or lateral 

 finlet consists of two parts, a dorsal and ventral, each part 

 having corresponding structures. These structures are a 

 cylindrical process or "cirrus," a bundle of bristles or 

 chsetse, and a more or less prominent projecting lamina 

 bearing the bristles. The relative position of the parts is 

 reversed in the dorsal and ventral portion of the parapoclium, so 

 that the dorsal cirrus is the most dorsal structure, the ventral 

 cirrus the most ventral. In other words, there is a dorso- 

 ventral symmetry in the parapodium. 



In the Syllidas the parapodia are degenerate, or to some 

 extent rudimentary. Each parapodium in the immature 

 form consists of a dorsal cirrus and the ventral portion of 

 the parapodium (the neuropodium), with sometimes a ventral 



