124 NEMATIIELMINTHES 



succeeding parts. They are not provided with any appendages 

 or limbs, but sometimes bear a few bristles or hooks, and in rarer 

 cases suckers. The body, which is elongated and, as a rule, 

 thread-like and tapering at each end, is enclosed in a thick 

 cuticle or hardened secretion of the underlying cells. In no 

 Nemathelminth is there any closed vascular system, nor are 

 special respiratory organs developed. 



In many respects the most remarkable peculiarity of these 

 animals is that, with the possible exception of the excretory 

 organs of the Acanthocephala, there is a complete absence of cilia 

 throughout the whole group. In this respect they resemble the 

 Arthropoda. The universal presence of these small nickering 

 processes of cells from man down to the simplest unicellular 

 organisms makes the absence of these structures most remarkable. 

 In many animals they are the sole organs of locomotion, arid in 

 almost all they perform very important functions, both in bring- 

 ing food and oxygen to the body, and in removing waste matter 

 from it. At present there seems to be no adequate explanation 

 for their absence in the two large groups mentioned above. 



Nemathelminthes are, with hardly an exception, dioecious 

 that is to say, their male and female reproductive organs are in 

 different individuals. Their young do not differ markedly from 

 the adults, except in the absence of sexual organs, but the im- 

 mature stages are usually termed larvae, and not infrequently 

 either inhabit a different host from the adult, or are free when 

 the adults are parasitic, or vice 



Sub-Order I. Nematoda. 



Anatomy. The Nematode worms, or thread-worms, form by 

 far the largest and most important division of the group Nema- 

 thelminthes. The number of species is great, and although the 

 conditions under which they live are of the most varied kind, 

 there is, as a rule, little corresponding difference in structure, 

 and hence the determination of the species is attended with no 

 small difficulty. 



With few exceptions the shape of the body is filiform (Figs. 

 66 and 71), the two ends being more or less pointed, and the 

 posterior end of the male, which is generally a smaller animal than 

 the female, is usually slightly recurved. The worms are, as a rule, 



