376 THE DEVELOPMENT OF CESTODES. 



and lower animals, especially in fish, which live like bladder-worms 

 in the muscles and in the parenchymatous organs, or occasionally 

 free in the body-cavity (Schistocephalus, Ligula) ; but their mode of 

 development has not been observed, or at least not traced as far as the 

 hook-bearing embryo. The only form which we must except is the 

 peculiar genus Archiyetes peculiar, because while as yet in the 

 Cysticercoid condition, or at least while still possessing the attributes 

 of an embryo, it becomes sexually mature, and that too (the only 

 example in the Cestodes *) in an invertebrate animal (Scenuris and its 

 allies). The development is further very simple in this case, for the 

 six-hooked embryo becomes transformed into the adult animal without 

 interruption or change of abode. During its continuous 

 growth it exchanges its original form for a more club- 

 like one, and by modification of the anterior as well as of 

 the posterior portion, becomes a tail-bearing worm, which 

 at first sight has a striking resemblance to a Cercaria. 

 The embryonic hooks persist at the posterior end of the 

 caudal appendage, and the modification of the embryo, 

 which has at first the usual structure, is effected mainly 

 by the distention of the segment opposite the hooks, 

 while the rest of the body preserves its slender form, 

 and ultimately grows out into a long appendage. This 

 tail-like appendage is thus the direct developmental 

 product of the six-hooked embryo. In spite of its dis- 

 similar form (we have, however, found a similar one 

 among the Tetra-rhynchi) it is homologous with the 

 bladder of the other Cestodes. Like the latter, it plays 

 in the Archigetes the part of a " head-former," for the 

 protuberance out of which the head-bearing body of the 

 worm arises might after all be regarded as a bud-like 

 proliferation, just as is the head -rudiment in the 

 bladder-worms. The fact that this bud does not grow 

 as usual inside the (increased) six-hooked embryo, but is 

 appended exteriorly, cannot constitute a difference, since 

 ,. both external and internal buds occur promiscuously 



FIG. 220. Archi- . . . y . . J 



gete* Sieboldi in the animal kingdom. It is true that in this case 



( x 60). there originates from the bud, not a head merely, which 



afterwards forms the segmented body by new budding, but a structure 



which, from the first, exhibits both head and body ; for the anterior 



1 The significance of this fact, to our conception of the historical development of 

 parasitic life, has already been noted (p. 70). Compare also, regarding this interesting 

 form, Leuckart, Zeitschr. f. wiss. Zool., SuppL- Bd. xxx., p. 593, 1878, and Ratzel, 

 Archivfur Naturgesch., Jahrg. xxxiv., Bd. i., p. 138, 1868. 



