SELF-DIVISION AND REGENERATION 143 



parts develop new organs more or less completely before they sepa- 

 rate. In the trematodes self-division does not take place. The 

 division of the body of the tapeworm into proglottids may represent 

 a process of self-division, but the proglottids do not regenerate after 

 separation. 



The nemertians break up readily into pieces, if roughly treated 

 or if the conditions of life are unfavorable, but this can scarcely be 

 spoken of as a process of voluntary self-division. Regeneration 

 takes place in some species, but imperfectly or not at all in others. 



In the group of annelids we find many cases of self-division, 

 especially in marine polychaetes and in fresh-water oligochaetes. One 

 of the most interesting forms, belonging to the first group, is the palolo 

 worm in which the swimming headless form, that is set free by divi- 

 sion, serves to distribute the sexual products. Subsequently it appears 

 that the piece dies without regenerating a new head. If we examine 

 more in detail some of the cases of self-division in annelids, we find 

 the following interesting facts. In nereis the posterior region of the 

 body undergoes great changes of structure, the new worm being 

 known under a different name, viz. heteronereis. In this part of 

 the worm, eggs (or sperm) are produced, but it does not separate 

 from the anterior end as a distinct individual. In the family of 

 scyllids the changes that take place in the posterior or sexual end of 

 the body are often accompanied by non-sexual modes of fission. 

 In some species the changes that take place are like those in 

 nereis, and no separation occurs ; in other species the sexual region 

 becomes separated from the anterior or non-sexual regions. In 

 scyllis a new head develops, after separation, on the sexual or pos- 

 terior piece. A new tail is also regenerated by the non-sexual or 

 anterior piece, and as many new segments are formed as are lost. 

 The new posterior region may again produce sexual cells, and again 

 separate. In autolytus a new head develops on the posterior piece 

 before it separates. A region of proliferation is also found at the 

 posterior end of the anterior part. In some species new individuals 

 develop in this zone of proliferation, and a chain of as many as six- 

 teen worms may be present before the one first formed drops off. A 

 still more complicated process is found in myriana. The region just 

 in front of the anus elongates, and gives rise to a large number of 

 segments. These form a new individual with the head at the ante- 

 rior end. Then another series cf segments is proliferated at the 

 posterior end of the old, or anterior worm, and just in front of the 

 first-formed individual. This region also makes a new individual. 

 The process continuing, a chain of individuals is produced, with the 

 oldest individual at the posterior end and the youngest at the ante- 

 rior end of the series. Each individual grows larger, and produces 



