Linton. — Seasonal Distribution of Parasites 49 



fishes which are taken there throughout the year or at 

 least during a considerable part of the year. 



NEMATODA. 



The parasitic helminths that are most commonly en- 

 countered belong to the order Nematoda, commonly called 

 round, or thread worms. It may be added that the mem- 

 bers of this order come the nearest to the popular concep- 

 tion of the word "worm." In this paper I shall consider 

 only those Nematodes that belong to the family Ascaridse, 

 which includes the most commonly occurring round 

 worms of fishes. Those who have had much to do with 

 the preparation of fish for the market and have been at 

 all observant can scarcely have escaped noting the not 

 unusual presence of worms of this order enclosed in mem- 

 branous coverings and distributed on the viscera, some- 

 times, as is often the case in the whiting, for example, 

 forming a tangled mass on the viscera generally, or, at- 

 tached to the mesentery. In the butterfish they some- 

 times occur in considerable numbers on the pyloric cseca. 

 Careful search will reveal the fact that scattering speci- 

 mens may be found in the mesentery or on the viscera of 

 a large proportion of the food fishes. In most cases they 

 will be found to be coiled in a flat spiral. They are 

 quiescent, although when liberated from the cyst which 

 the tissues of their host have built around them, may be- 

 come somewhat active. They will measure, as a rule, 

 from 10 to 20 millimeters in length. Whatever the pre- 

 ceding life-history of these worms may have been their 

 situation represents now, as a rule, the final stage of act- 

 ivity in the particular fish in which they are encysted. 

 They are invariably immature and must await the, to 

 them, happy fate of being eaten by a suitable host before 

 they can become sexually mature. This final reproductive 

 stage must be looked for therefore in the alimentary 

 canals of fishes, or of fish-eating animals. Many fishes 

 such as the cod, haddock, pollock, sword-fish, etc., harbor, 

 at the same time, adult nematodes in the alimentary can- 



