PARASITIC ROUND WORMS 519 



form of the adult. Ten to fourteen days after introduction into a 

 fish the young worms have become fully matured and pair. 



In most cases the larval Camallanus is introduced directly into 

 the final host from the first intermediate host, but in others en- 

 cystment in a second intermediate host becomes an enforced pre- 

 liminary to the attainment of the final host. This takes place 

 when the intermediate host is eaten by some species other than 

 the final host; the larva is set free by digestion but immediately 

 encysts again, usually in the intestinal wall. Such erratic encysted 

 larvae occur in a wide variety of unusual hosts (Seurat). 



Too many complications enter into individual cases to be dis- 

 cussed in detail here. It is necessary to mention briefly, however, 

 one type of life history of a different character. Among the 

 Filariidae, the adult is parasitic in the connective tissues or body 

 cavity of the host and is viviparous. The embryos are produced 

 in enormous numbers and invade the blood stream from which 

 they are drawn out by biting insects such as the mosquito. After 

 a period of development in the mosquito they escape into the final 

 host when the insect is biting again, and now are ready to develop 

 into the adult parasite. In this case no part of the life-history is 

 spent in the outer world and the only link which connects the life- 

 history to aquatic biology is the intermediate host which may be, 

 like the mosquito, a typical aquatic organism in early life. 



The nematode life histories which have been partly worked out 

 are mostly those of the parasites of man and the domestic animals. 

 Almost nothing is known of the development of parasites from 

 characteristic aquatic hosts and the field offers enticing oppor- 

 tunities to the student. 



Concerning other phases in the biology of parasitic nematodes 

 little or nothing has been ascertained. Observations are too 

 scanty to furnish data on their length of life, on seasonal variation, 

 or on factors that influence their frequency. Their distribution 

 evidently cannot transcend that of the hosts and in many cases 

 falls far short of conforming to that, but the conditions which 

 affect such variations are beyond safe conjecture. 



No satisfactory outline for the classification of parasitic nema- 

 todes has yet been worked out and the very imperfect knowledge 



