AOANTHOOEPHALA. 97 



CHAPTER VII. 



AOANTHOOEPHALA. 



AoanthocepkaJa or Thorn-headed worms — General characters and distribution — Classi- 

 fication — Organization of Hchinorhynchus anthuris. Singular mode of egg-formation 

 — Leuckart's researches respecting the development of Echinorhynchus proteus — 

 His discovery of the phenomenon of " alternate generation" in this class of parasites. 



The foTirtli order of helmintlis, in my arrangement, embraces those 

 species of entozoa commonly known as tlie Acanthocepliala, or 

 Thom-lieaded worms. These animals have been variously placed 

 by helminthologists, and I have now departed from the usual plan 

 by associating them with the cestodes to form a third helminthic 

 sub -class entitled Anenterehnintha. The grounds on which I have 

 done this have been already fully explained (page 6). 



The Acanthocephala are characterised by the possession of elon- 

 gated, more or less vermiform bodies, often curved upon them- 

 selves, and marked by tolerably regular transverse folds (Plate 

 VIII., Figs. 2, 10, 12). They are furnished anteriorly with a retractile 

 proboscis, often termed the head, which is armed with a variable 

 number of reciirved hooks disposed in circles at regular intervals. 

 At the point where these rows of hooks cease there is usually a 

 marked constriction or narrower portion of the proboscis which is 

 called the neck, but sometimes this part is even more largely de- 

 veloped than the armed part itself (Fig. 12); As the subclass title 

 indicates, these parasites have neither mouth nor intestine. The 

 sexes are distinct, but during development the species undergo a 

 process of alternate generation. 



The Thorn- headed worms are found infesting all classes of ver- 



