THE CESTOIDEA 145 



of these forms is not known in all its details, there can be no doubt but 

 that the pore and contractile excretory bladder are identical at both stages 

 of development. The permanency of the strobila in some worms, taken 

 in connection with the absence of external segmentation in Ligula and 

 Triaenophorus, 1 and the easy transition from the latter to the uusegmented 

 Monozoa these and sundry other facts seem to point most strongly to 

 the individuality of the adult tapeworm. 



It is not difficult to see the advantage to the species gained by this 

 segmentation of the body as the genital organs ripen, for the continued 

 peristalsis of the intestine, and the passage of food, must tend to rupture 

 the attached worm. The scolex therefore regenerates this broken portion, 

 as in various Chaetopods ; and this power has been increased and per- 

 petuated, so that the process of regeneration has become premature and 

 resolved into one of " strobilation," or early production of segments, each 

 of which ripens in turn and ultimately separates (Lang). This process 

 is even more precocious in Ligula, where the repetition of the genital 

 organs occurs in the intermediate host. 



If the tapeworm be regarded as a metamerically segmented animal, 

 the neck, as the point of formation of new segments, should be, in order 

 to be comparable with the corresponding point in Annelids, posterior, i.e. 

 antepenultimate, and the scolex or head must be the last segment or 

 telson. This, indeed, is the view taken by Perrier and Moniez, who 

 regard the " caudal vesicle " of T. solium and " tail " of Caryophyllaeus as 

 the anterior end of the worm ; though it is frequently assumed that the 

 "tail" was originally an organ of locomotion, which function it has 

 entirely lost, and has become in many cases part of the " bladder," and 

 aids in protecting the enclosed scolex. 



A curious theory was in early times held by several naturalists (Linnaeus, 

 Dubois, Blumenbach) that the strobila is built up by the mutual attach- 

 ment in series of numerous free-living proglottids, or Vermes cucurbitini. 



The probable phylogeny of the class may be indicated by the following 



tree : 



Tetracotylea. 



Tetrarhyncha. 

 Merozoa. 



Tetrai 



J> 



hyllidea. 



. Dibothridiata. Tetrabothridiafa. 



Trematoda. 



1 In the young Triaenophorus the proglottids are distinct ; obliteration of them 

 occurs as the worm matures. 



10 



