418 Prof. C. Claus on the Morphology and 



As is well known, it was the " proglottis," the segment 

 full of ova and embryos and liberated from the body of the 

 Cestode, which, in accordance with the doctrine of Alterna- 

 tion of Generations, was held to be the sexual individual and 

 furnished the starting-point for a comparison with the closely 

 allied but higher organized Trematodes. The proglottis was 

 regarded as the equivalent of the Trematode, which, arising 

 by means of terminal gemmation at the posterior end of an 

 individual belonging to another generation and functioning as 

 " nurse-form " — the Cestode-head or " scolex," with the loss 

 of mouth, alimentary canal, and organs of adhesion, did 

 actually possess the latter, in common with all the other joints 

 of the chain, while it formed part of the chain of segments, in 

 the shape of the circlet of hooks and the suckers of the scolex. 

 The fact that there are Cestodes which are devoid of any 

 trace of segmentation (e. g. Caryophyllceus), and in whose 

 simple Trematode-like body scolex and proglottis are not 

 differentiated, appeared to agree very well with this view, and 

 was explained as a secondary condition, inasmuch as it was 

 supposed that the forms which are spread over two genera- 

 tions in the case of the ordinary Cestode were, just as in other 

 cases of alternation of generations, as a result of simplified 

 and abbreviated development united together again in one 

 individual (Kud. Leuckart). But the ensemble of the pheno- 

 mena proves that in point of fact exactly the opposite con- 

 dition obtains, and that alternation of generations in the 

 Cestodes must be regarded not as a primary but as a secondary 

 developmental process. 



The starting-point of a comparison with the organization 

 of the Trematodes, from which all authors agree that the 

 Cestodes have been derived, is to be furnished not by the 

 proglottis but by the entire Cestode, and, moreover, not as a 

 chain of segments, but in its simplest form, as represented by 

 the genus Caryophyl/oeus, as an unsegmented worm resembling 

 the genera Amphilina and Amphiptyches, which are to be 

 regarded as connecting-links between the Trematodes and 

 Cestodes. The unisegmental Cestode with a single set of 

 genital organs was the primeval form, from which, by means 

 of further and fuller adaptation to the favourable conditions of 

 nourishment and growth in the interior of the alimentary 

 canal, the segmental Cestodes, with progressive individualiza- 

 tion of the joints repeating themselves by growth in the lon- 

 gitudinal axis, have only secondarily been developed. Next 

 to the Caryophyllidse come the Ligulidai, in whose ribbon- 

 like body the generative organs indeed are metamerically 

 repeated, though there is no corresponding outward segmen- 



