ANNELIDA 



301 



4. Non-sexual Reproduction ; Alternation of 



Generations. 



The Chsetopoda possess to a high degree the power of restoring parts of 

 the body that have been lost ; not only the less important parts of the 

 body, but also the more important ones, such as the head region, includ- 

 ing mouth and brain, can be formed anew by them. This power of 

 regeneration passes into a kind of non-sexual reproduction (schizogeny), 

 when the body, as in Lumhriculus, separates spontaneously into several 

 pieces, each of which is able to regenerate itself into a perfect worm. 

 Approaching closely to this is the reproduction of one of the marine 

 Annelida, closely allied to the Oligochseta, which is found actively multi- 

 plying throughout the year, without at any time developing genital organs 

 {Ctenodrilus monostylos, according to Count Zeppelin). This worm repro- 

 duces in the most primitive 

 way : a constriction is formed 

 on the body immediately 

 behind a septum, and be- 

 comes deeper and deeper 

 until the worm falls into 

 two parts (Fig. 140 A). Of 

 the two resulting parts the 

 anterior is thus without an 

 anus, the posterior without 

 a head. This primitive 

 kind of division may go so 

 far that parts arise which 

 are destitute of both head 

 and anus, and at times con- 

 sist of only a single segment 

 (Fig. 140 B). Head and 

 terminal parts are formed 

 by the thickening of the 

 integument (hypodermis), 

 which sends inward plug- 

 like ingrowths that unite 

 with the intestine. In this 



Fig. 140. — A, Ctenodrilus monostylos dividing 

 transversely (alter Count Zeppelin). B, a por- 

 tion of the same worm consisting of only a 

 single segment ; c, cirrus ; d, intestine. C, Cteno- 

 drilus pardalis (after v. Kennel); fcn., budding 

 zone, where the worm later separates into the 

 different parts ; d, intestine. 



way the mouth and anus 



arise. The new segments that are to be formed are interpolated between 



the newly formed anus and the preceding segment. 



Less primitive is the condition in another worm belonging to the same 

 genus, Ctenodrilus pardalis, which likewise was found only in a state of 

 non-sexual reproduction (v. Kennel). In this case thickened zones, 

 corresponding to the anterior and posterior ends of the worms about to 

 be produced (Fig. 140 C), are formed— even here in a simple way, it is 

 true — before the division ; that is to say, a so-called budding zone is 



