PARASITIC ROUNDWORMS 511 



tinguish adjacent regions by other prominent features. As a rule 

 the anterior end is slightly blunter whereas the posterior end is more 

 pointed. The uniformity of external appearance is very charac- 

 teristic of nematodes. This creates an impression of monotony 

 in structure and renders their classification difficult. The smaller 

 forms are somewhat transparent in Ufe but the larger species are 

 opaque. 



One may also recognize a nematode easily by its peculiar type 

 of movement, which in a liquid medium consists of a more or less 

 rapid and violent coiling and twisting alternately right and left 

 without appreciable progress, but is modified by the presence of 

 solid particles in the fluid into a powerful serpentine movement 

 winding in and out among the debris. This grows in effectiveness 

 as the material becomes more nearly solid and the particles are 

 less readily pushed aside by the twisting of the worm. 



In external features the parasitic species appear somewhat dif- 

 ferent from the free-Uving forms. On the whole they are much 

 larger, thicker and more opaque. Few species are as minute as 

 free forms and only these minute types approach the free species 

 in transparency. The external form is also more monotonous 

 since the delicate hairs and scales that distinguish free species are 

 almost entirely wanting. Eyes, amphids, and setose tactile organs 

 such as already described for free-living types are not present in 

 parasitic species. 



Parasitic nematodes occur in nearly all water-living vertebrates; 

 they are also often found in insects. In crustaceans and worms they 

 are much less frequent and in any other forms their presence is un- 

 usual. While adult forms are found in all hosts, yet the immature 

 stages are more frequent in hosts from the lower groups mentioned 

 and less common in the higher vertebrates. The encysted worms 

 are usually larval forms. The adults frequent commonly the 

 alimentary canal, though some species occur regularly in con- 

 nective tissue and rarer types in other parts of the body. Encysted 

 larvae may be found almost anywhere. 



In structure the parasitic threadworms manifest great similar- 

 ity to the free-living species and in view of the detailed treatment 

 given the latter in the last chapter it will be necessary in the pres- 



