Diseases of Fishes 351 



the chubs and suckers appeared to avoid the cold water. It 

 may be that what the latter really avoided was the special 

 preserve of the trout, since large chubs and suckers are found 

 in abundance in the lake, which is quite cold, a temperature of 

 40 F. having been taken by us at a depth of 124 feet. 



" Since the eggs of this parasite, after the analogy of closely 

 related forms, in all probability are discharged into the water 

 from the final host and hatch out readily in warm water, where 

 they may live for a longer or shorter time as free-swimming 

 planula-like forms, it will be observed that the sluggish current 

 and high temperature of the water in which these parasitized 

 fish occur give rise to conditions which are highly favorable to 

 infection. 



' ' It may be of passing interest to state here what I have 

 recorded elsewhere, that ligulae, probably specifically identical 

 with L, catostomi, form an article of food in Italy, where they 

 are sold in the markets under the name maccaroni piatti; also 

 in southern France, where they are less euphemistically but 

 more truthfully called the ver blanc. So far as my information 

 goes, this diet of worms is strictly European. 



" It is not necessary to prove cases of direct injury resulting 

 from the presence of parasites in order to make out a case 

 against them. In the sharp competition which nature forces 

 on fishes in the ordinary struggle for existence, any factor 

 which imparts an increment either of strength or of weakness 

 may be a very potent one, and in a long term of years may 

 determine the relative abundance or rarity of the individuals 

 of a species. In most cases the interrelations between parasite 

 and host have become so adjusted that the evil wrought by the 

 parasite on its host is small. Parasitic forms, like free forms, 

 are simply developing along the lines of their being, but unlike 

 most free forms they do not contribute a fair share to the food 

 of other creatures." 



Thorn-head Worms. The thorn-head worms called Acan- 

 thocephala are found occasionally in large numbers in different 

 kinds of fishes. They penetrate the coats of the intestines, 

 producing much irritation and finally waxy degeneration of 

 the tissues. 



According to Linton, there is probably no practical way of 



