LIFE HISTORY OF A TROUT PARASITE. 
343 
When the size of the trout is not mentioned it be understood to be about 35 
centimeters long. 
Spent females seemed to be more commonly parasitized to a serious degree than 
others, although my examinations have not been extensive enough for safe general- 
izations ill this particular. 
From this investigation one might conclude that the trout of Yellowstone Lake 
are not so badly iufested with the flesh parasites as they have been in previous years. 
This may be due to the fact that the observations were made earlier in the season 
than those of Dr. Jordan for example, who found the worms more abundant than they 
appear to be now. It is to be noted, however, that larvie in various stages of de- 
velopment are obtained from the same individual. In one case cysts no larger than 
a grain of wheat were associated with others larger than a pea, and others from which 
the larvie had escaped and begun to burrow into the flesh of their host, attaining a 
length of 16 centimeters or more and a breadth of 3 or 4 millimeters. 
It may properly be inferred from this that the source of infection continues through 
several months of the year, thus showing marked contrast with the large ligula of the 
Witch Creek sucker, Gatostonius ardeiis, in which case the nearly uniform size of the 
parasite points to a source of infection supplied, perhaps, by the short sojourn of 
some migrating piscivorous birds. 
With regard to the escajiiug of these parasites through the skin of the host, men- 
tioned by Mr. Carrington, I was unable to find any confirmatory proof. In a few 
instances I found that the parasites had penetrated the muscular tissues and were 
lying immediately beneath the skin; more commonly they were on the peritoneal side 
of the body wall or burrowing in the muscles. 1 saw no evidence in the flsh which I 
examined that parasites had escaped. If it can be demonstrated that these parasites 
really do leave the intermediate host and take to the uncongenial medium of the 
water, it would furnish an instance of self-destruction unusual among cestod parasites. 
I was frequently asked, while in the Park, if these worms are in any way injurious 
to man. I think it can be safely answered that they are not, except as their presence 
might make the fish less acceptable to the palate. Fish, perhaps more than any other 
animals, are required by nature to harbor parasites. There is probably not a food 
fish in the world that does not furnish a home for one or more species of parasites in 
some stage of the latter’s existence. Fortunately flsh parasites, as a rule, do not 
live in man ; at any rate, the various processes of preparation for food to which fish 
flesh is subjected effectually destroy the vitality of the parasites. It may be, if not 
pleasant reading to the fastidious, at least consoling to the timorous, to know that 
forms closely related to the subject of this sketch are in some places actually eaten as 
food and esteemed as delicacies by those who eat them, who, it may be inferred, ask 
no questions either for conscience’ sake or for the sake of knowledge. Ligulm, para- 
sites of the European tench and of other related fish, are used as food in Italy, w^here 
they are sold in the markets under the name maccaroiii piatti, and eaten usually under 
the mistaken notion that it is the roe of the fish. It is also eaten in Lyons bj^ many, 
where it goes by the appropriate and truthful name of the ver blanc.* 
* Donnadieu, Contribution a I’histoire de la Ligule. Extrait Journal de Auatoinie et de la Pbysio- 
logie, P. 1. 
