166 COMPARATirE AXATOMY 



CHAP. 



off and ejected, the head which remained being, however, able to pro- 

 duce a new trunk by regeneration. This process the tearing off of 

 the trunk with the eggs, the continued attachment of the head, and 

 regeneration must have been of the greatest use to these parasitic 

 forms. By the tearing off of the trunk and its ejection the greater 

 dispersal of the eggs was secured, and the probability of the infection 

 of new hosts or intermediate hosts thus increased. The attached 

 head could keep its ground in the already attained favourable refuge 

 for the parasite, and easily regenerate a new trunk and new genital 

 organs. The strobila consisting of man}* segments, however, offered 

 the immense advantage that many segments could be benefited by the 

 favourable nutritive conditions of parasitism, and could develop the 

 genital organs ; on the other hand, by the periodical tearing off of the 

 trunk of an unsegmented Tcenia, not only a longer time must pass 

 before a new sexually ripe trunk would form, but the favourable con- 

 ditions of nutrition would be much less utilised. 



There are Ta'nicc with only very few segments (To.'nia Echinococcus, 

 with 3 to 4 proglottides); others possess several hundreds. 



In a few Tcenia', such as Ligula and Ti*icenopJiwus, the outer segmenta- 

 tion is more or less indistinct ; internally, however, we find the same 

 repetition of the genital organs as in the typically segmented tape- 

 worms, from which these forms must without doubt be derived. 



In freshwater Tridada, multiplication by fission has been observed. 



Among the Rhaldocoela, in the genera Microstoma and Stenostoma, 

 we find interesting processes of reproduction by axial budding. 

 They can be best investigated in M. linen re. In the posterior 

 end of the body of an individual a double transverse partition 

 wall forms between the intestine and the skin. Immediately behind 

 this the organs characteristic of the head portion of the Microstoma, 

 the pharynx and the brain, with the nerve commissure surrounding 

 the pharynx, form. The two septa subsequently move somewhat apart. 

 An annular constriction of the body takes place between them, and the 

 intestine finally also becomes constricted. Only then does the spon- 

 taneous separation of the two pieces occur. Long before this separa- 

 tion occurs, however, new phenomena have appeared in both pieces. 

 In the first place the posterior piece grows to the same size as the 

 anterior. Then in the posterior part of each a head portion again 

 forms. The posterior parts of each principal piece thus marked 

 off then grow to the size of the two parts lying in front of them. The 

 whole body now consists of 4 pieces of equal size. This process is 

 repeated twice in the same way, till 16 pieces are formed, i.e. till 

 the worm stock consists of 16 individuals, the one most to the front 

 possessing the original pharynx, the original brain, etc. Then follows 

 generally the spontaneous separation of the individuals. 



Reproduction by gemmation occurs further in the young stage of 

 Tcenia called the Finn, and especially in those finns which are known 

 as Ccenurus and Echinococcus. This will be described later on. 



