244 ANNELIDA. 



(638.) Besides the ordinary mode of propagation by ova, it has long- 

 been asserted that some of the Annelida, at least, are reproduced by spon- 

 taneous division. Bonnet, Miiller, and Duges all agree that this is 

 the case with certain species of Nais ; and in Nais Jiliformis the process 

 of separation has been witnessed from its commencement to its termi- 

 nation. The division was said to occur near the middle of the body of 

 the animal, the posterior half remaining motionless upon the mud of the 

 bottom of the vessel, whilst the anterior portion buried itself as usual : 

 after some days the truncated extremity of the hinder part was observed 

 to become swollen, transparent, and vascular, and ultimately to assume 

 the complete structure of the mouth of the perfect animal; it then 

 buried itself in the mud, and no doubt there completed its develop- 

 ment. 



The following is what Dr. "Williams conceives to be the interpretation 

 of the above facts : 



(639.) Every Nais, as relates to its reproductive apparatus, is identi- 

 cally constituted, and this worm is consequently androgynous. Every 

 individual, towards the latter end of the summer, dies by the bisection 

 of its body. It is not true, as reported by Duges, and before him by 

 Spallanzani, that the fragments into which the body of each worm be- 

 comes resolved are again reconstructed into a perfect whole. Although 

 the sexual system exhibits a tendency to segmental repetition, there 

 devolves upon the large anterior portion, described by Duges, a special 

 function which the rest cannot perform ; and, on the contrary, a duty 

 falls on the posterior segmental units of the system which the anterior 

 cannot discharge. It is consequently evident that neither of the moieties 

 into which the body is resolved during the crisis of the reproductive 

 season can be organically perfect. Such fragmentary organism is want- 

 ing in elements paramountly essential to individuality. 



(640.) These Annelids are annuals : the term of existence is completed 

 when the organic cycle is once accomplished. They are born during the 

 latter months of one summer, and survive the winter, attain to the 

 maturity of growth, reproduce the species, and die by the spontaneous 

 subdivision of the body into fragments on the arrival of the same season 

 of the succeeding year. This brief round comprehends the history of 

 each individual. Since these worms are monoecious, each shares the 

 common fate. Each contributes by its own death to the multiplication 

 of the species : the species being multiplied, the ends of its own exist- 

 ence are accomplished. 



(641.) For some time before the fissure of the body occurs, the process 

 of the maturation of the ova is proceeding. Arrived at the matured 

 phase, they escape from the ovarian system into the free space of the 

 peritoneal cavity, wherein they sojourn until the next phase of their 

 growth has been attained. It is during the period marked by the 

 presence of true ova in the chamber of the peritoneum, floating in the 



