PARASITIC ROUNDWORMS 507 



and as there is no trace of an alimentary canal at any stage of 

 development. 



In collecting parasites one may find adult Nematoda and 

 Acanthocephala side by side in the same intestine but the latter 

 rarely occur outside the alimentary canal and nematodes often do. 

 The Gordiacea are parasitic in larval stages normally in the body 

 cavity of Insecta and are found only infrequently in other hosts. 

 They are most commonly found as adults in general aquatic col- 

 lecting and are well known even to the casual observer of life in 

 ponds and ditches under the popular designation of "Hair Snakes." 



The technic of handling the roundworms is not simple. Para- 

 sitic nematodes are collected in the manner already described for 

 parasites in general (p. 368), but owing to the very resistant cutic- 

 ula and delicate structure of these worms great care is necessary 

 to avoid injuring specimens seriously. Those which are loose can 

 be picked up with a fine camel's hair brush. This instrument is 

 most convenient in the handling of small species. Many species are 

 so firmly attached to the intestinal wall that it is difficult to remove 

 them without injury. Gentle manipulation if prolonged will usu- 

 ally loosen the hold, but the body is easily lacerated by grasping 

 it with forceps other than very lightly or the mouth parts are often 

 torn by pulling the worm too hard. Encysted forms should be 

 freed from the cyst under a dissecting lens with fine, sharp needles. 

 A very good needle is made of a glass rod drawn out to a point. 

 Most nematodes are very sensitive to changes in osmotic pressure 

 and are badly disfigured by rapid changes. Living specimens 

 should not be put into distilled water or normal salt solution. 

 Tap water is fairly good and for nematodes from fresh-water fish 

 a 0.3 per cent salt solution is best, but material should not be 

 left in such a fluid longer than absolutely necessary. 



The resistant cuticula prevents the entrance of cold killing 

 solutions so thoroughly that these worms live even hours in fluids 

 that kill other parasites promptly. Hot fluids coagulate the body 

 proteins and preserve specimens well extended. No successful 

 methods of narcotization have yet been worked out. The killing 

 fluid recommended by Looss is all in all most useful; it is made 

 by adding to alcohol (70 to 85 per cent) from 5 to 10 per cent 



