CHAPTER VII. 



PHYLUM NBMATHELMINTHES. 



THE Nemathelminthes include three orders the Nematoda, the 

 Nematomorpha, and the Acanthocephala which have little else in 

 common than the round form of body and the parasitic habit. 



The body is unsegmented, rounded, more or less elongated, tubular 

 or filiform, and both ends are, as a rule, tapered off. Appendages 

 are always wanting, as are, with few exceptions, movable bristles. 

 On the other hand, special organs for attack and attachment, such as 

 teeth and hooks, are not unfrequently present on the anterior end of 

 the body : and in some cases small suckers, which serve for attach- 

 ment during copulation, may be developed near the hind end. As 

 a rule the integument possesses a cuticular layer of relatively 

 considerable thickness, and the ectoderm is very generally reduced 

 to a nucleated granular layer, in which cell outlines are absent. 

 These features of the skin are probably correlated with the endo- 

 parasitic habit, for we find them in the Trematoda, and the ectoderm 

 of the Cestoda is, to say the least of it, much modified. In these 

 latter groups the ectoderm is even less conspicuous than in the 

 Nematlielminthes. There is a well-developed muscular layer, which 

 generally consists of longitudinal fibres only, but circular fibres are 

 also present in the Acanthocephala. A body -cavity is always present, 

 but it appears generally to be without an epithelial lining, and in 

 Nematoda, at least, is bounded on one side by the endodermal wall 

 of the alimentary canal. As to the nature of this body-cavity we 

 have little evidence except in the Acanthocephala, in which, from 

 its relation to the generative organs and duct in the female we may 

 infer it to be a coelom. It contains a vascular fluid, and in the 

 other orders it is probably a haemocoele. Blood vessels and 

 special respiratory organs are wanting. A nervous system is always 

 present, but it presents very different features in the three- orders. 

 Of sense organs there are often sensory papillae in the neighbourhood 

 of the mouth and genital openings, and eye spots are often present 



