202 AN INTRODUCTION TO ZOOLOGY 



whereas in Tcenia they are formed at the anterior end of the chains of 

 proglottids by the unsegmented neck a very fundamental difference. 

 Then, again, the only parts that are repeated in the latter animal are 

 the reproductive organs, whereas in the earthworm the metamerism 

 is impressed on all the various parts of the body. Lastly, as we have 

 seen, the segmentation of the adult Lumbricus is founded upon a 

 very early developed and primitive division of the mesoderm into 

 mesodermal somites, a process not paralleled in Tcenia. For these 

 reasons, among others, we cannot regard the segmented condition 

 in the two animals as homologous or essentially similar, but as only 

 superficially alike. How then did this condition arise in the tape- 

 worms ? The most generally accepted explanation is briefly as 

 follows : We find the most simple and primitive members of the 

 group are unsegmented, and have but a single set of reproductive 

 organs situated towards the hinder end. Such an animal living in 

 the gut might conceivably have had its posterior end broken off 

 by the passage of the food and the peristalsis of the intestine and 

 then proceeded to regrow the lost parts, as these lowly animals can 

 readily do. This would be distinctly advantageous to the species, 

 for it would bring about a greatly increased power of reproduction. 

 The possibility of regeneration we now imagine became moved 

 forwards in the life history of the individual, and the worm, as it 

 were, produced several posterior ends in anticipation of their being 

 broken off, and these had a line of demarcation between them, so 

 that when the break did come it would not be so injurious. If this 

 were done to a slight extent we should have a form like T. echinococcus 

 with its three proglottids, or, if carried further, many proglottids, as 

 in T. solium. This gives a possible mode of origin of the tape- worm 

 condition, and so, if true, segmentation in these animals may be 

 regarded as a special adaptation to the peculiar parasitic mode of 

 life, and not an essential of their plan of organisation, as it is in the 

 Annelids. 



In the case of T. solium the egg produces a single proscolex in 

 the muscles of the pig, and this in its turn grows directly into one 

 adult animal, so that in this case there is obviously only a single 

 generation with a sexual reproduction in the adult condition. 

 T. echinococcus and certain other forms, on the contrary, have not 

 such a simple life history. Their eggs produce proscolices, as in 

 TV solium, but each of these in its young or cysticercoid stage gives 

 rise by a kind of asexual budding to a large number, it may be 

 thousands, of scolices. In these species then we have a true alterna- 

 tion of generations between the asexual proscolex and the sexual 

 tapeworm. It is an additional wa}' of securing a continuance of the 

 species. 



