Worm Parasites 57 



more than their life in the hosts in which they pass their adult 

 existence. There are in most cases two distinct periods in the life 

 of such animals, (i) the adult period, and (2) the larval period. In 

 the first period the parasite lives in some one of the open organs of 

 the host, which has direct communication with the outer world, such 

 as the digestive tract or the lungs. Here the parasite reproduces 

 itself, and from its seat its eggs or young find their way to the out- 

 side, usually with the dejecta of the host. In the larval period the 

 young animal, having thus been carried to the outer world, lives 

 for a while a free, non-parasitic life, and is later transferred either 

 directly to a host similar to the parental host, in which it is destined 

 to pass its adult, parasitic existence, or to a host very different from 

 the parental host, in which it passes into a second larval stage. In 

 the latter case the 3^oung worm leads a more or less inactive life, often 

 enclosed in a cyst, until this host is finally eaten by an animal of 

 the same, or in some cases of a nearly related species, as the parental 

 host. The young worm is thus mechanically transferred to the 

 stomach of this animal and in the course of time makes its way into 

 that particular organ in which nature has intended it shall live and 

 breed. 



It will be seen from this statement how complex is the life history 

 of parasitic worms, and how necessary are intensive studies of them 

 and their several hosts. It is not sufficient to collect the adult worms 

 and to study their relation to their hosts, the larval worms and their 

 hosts must also be studied in order to complete the life-cycle; and 

 until the life-cycle is understood it is often impossible to suggest a 

 means of controlling the ravages of the parasite. It is just for this 

 reason that field studies like those being made on the shores of Oneida 

 Lake are important, Where the conditions are unusually favorable 

 for investigating every phase of the life history of the parasites under 

 consideration. 



The organs of the host animal which are most liable to be infested 

 by parasitic worms are the different portions and appendages of the 

 digestive tract, and all the divisions of this tract from the mouth to 

 the vent may harbor them. It is unusual to find a fish or other 

 aquatic vertebrate that does not have these worms in some or every 

 part of this tract. In most cases, however, the number of worms in 

 a single intestine is not large, but occasionally cases of heavy infesta- 

 tion are found. One large water snake was found on the shores of 

 Oneida Lake the stomach of which was literally packed with large 

 Nematode worms, so that it was difficult to see Where there would 

 be room for the animal's food. Two eels, also, were found with their 

 intestines so distended by the tapeworms they contained that the 

 vitality of the fish must have been seriously lowered. The practical 

 reason why the digestive tract is a usual place of infection of adult 

 parasites is that they are usually brought into the body of the host 

 with its food. 



After the digestive tract, perhaps the most frequently infected 

 organs are the respiratory organs. In fish the gills are a favorable 

 location for numerous species of flukes, which attach themselves to 



