380 



THE DEVELOPilENT OF CESTODES. 



Fig. 222. — Longi- 

 tudinal section of an 

 isolated head of Tetra- 

 rhynchus. (x 10.) 



plished by means of the growth and segmentation of the cervical 

 portion adherent to the head, by a process, therefore, which gives the 

 parasite an entirely new stamp, and converts it 

 indeed into a perfect tape-worm. Eudiments of 

 this segmentation are even often to be seen during 

 the stay of the parasite within the intermediate 

 host. Thus at least in a Tetrarhynchus-heaA from 

 the gills of Lepidopus, which in some points recalled 

 T. lingualis, but measured only 4 mm., I observed 

 that the neck, in spite of its perfectly simple ex- 

 ternal appearance, was also characterised by a dis- 

 tinct segmentation resulting from the internal 

 arrangement especially of its vessels and muscles. 

 In thin longitudinal sections I could count about 

 two dozen narrow segments, ■which gradually 

 became somewhat longer posteriorly, and even 

 revealed the first traces of the sexual organs. 



But it is not only the isolated Tetrarhynchus-hea.is which grow into 

 the future tape- worms in this directly simple fashion. The same may 

 be said of the worms formerly described as Scoleces, and of all those 

 Cestodes which are in their larval state destitute of a histologically 

 differentiated caudal bladder, as also of a few Cysticerci like G. Taenia' 

 cucumerincB and C. cyclopis, and yet again of Bothriocephalus, &c. In 

 all these the modifications in the course of the passage into the final 

 state are in all probability restricted to an elongation and segmenta- 

 tion of the " body " which is attached behind the proper " head." But 

 if the supposition be correct, that the former is nothing but the body 

 of the embryo which has formed a " frame for the head," then the 

 embryonic body has in these cases its share in the formation of the 

 jointed worm much in the same way as in the Ligulidae. The growth 

 and jointing take place, indeed, sometimes in the definitive, sometimes 

 in the intermediate host ; but the difference which is here expressed 

 cannot be ranked as a very weighty one, especially since we find 

 numerous examples, especially among the Nematodes, where the 

 parasites do not by any means always reach the same degree of 

 development within their various primary hosts. 



In the other Cestodes, Le., in all those which are in their larval 

 state provided with a differentiated caudal bladder, whether "dropsical" 

 or parenchymatous, the metamorphosis into the tape-worm proper is 

 less direct, for the growth and jointing of the body is always pre- 

 ceded by the loss of the bladder. 



We owe our knowledge of these metamorphoses to the above 

 quoted (p. 332) memoirs of Ktichenmeister and v. Siebold (or Lewald). 

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