102 BULLETIN OF THE UNITED STATES FISH COMMISSION, 



II. 



Each distinct species of fish is not unusually the host of one or more distinct 

 species of parasite, or, at least, the usual host of one or more species. This is true 

 especially of those species which belong to rather small natural orders, and which 

 stand stiffly apart from other species. It will not, I think, be found so true of species, 

 so-called, which are based on trivial differences. Therefore, when one is examining an 

 animal for parasites from which no parasites have ever been reported, the probability 

 is that if he finds any they will be new to science, especially if no parasites have been 

 reported from any closely related species. 



I will cite but one example, out of many that have come to my notice, in illustra- 

 tion of this conclusion. Some years ago I collected some entozoa from a specimen of 

 Polyodon spathula from the Ohio River, a species from which, so far as I know, no 

 entozoa had been reported, and which, moreover, is so different from any other exist- 

 ing species as to lead one to expect to find some entozoa or entozoon new to science. 

 I found it harboring a considerable number of young cestodes belonging to the genus 

 Dibothrium, which I shall describe and figure in a forthcoming report. Although the 

 anatomy of the proglottides can be made out but imperfectly from my specimens, the 

 external characters present such striking peculiarities that, even in the absence of 

 many details of structure that are desirable to know in describing a new species, I 

 feel justified in regarding it as a new species. I have found the same species in a lot 

 of entozoa sent to me for identification by the IT. S. National Museum. They are 

 also immature, but there is not the slightest difficulty in establishing the identity of 

 species in the two lots. 



The above general statement is not to be understood to refer to larval forms, nor 

 to mean that each species of eutozoon is confined to a single host. Many of the nem- 

 atodes, for example, and many of the cestodes, while in the encysted state, and a few in 

 the adult, have a range of several hosts. I think it may be safely stated, however, 

 that where two hosts are not nearly related the entozoa which infest one will, in gen- 

 eral, be different from the entozoa which infest the other. On the other hand, hosts 

 which are closely related are likely to be infested with similar entozoa. 



For example, I have found repeatedly, in a number of species of flounders, viz, 

 Pseudopleuronectes americanus, Paralichthys dtntatus, and Limanda ferruginca, an 

 echinorhynchus, often very abundant, which I have referred, somewhat questioningly, 

 to the species Echinorhynchus acus Rud. Its favorite host is P. americanus, but it is far 

 from uncommon in other flounders. Its range, however, does not appear to be limited 

 to the Plcuronectidce, for I have found specimens indistinguishable, except by a too 

 refining process of species-making, from E. acus of the flounder, in the cod, haddock, 

 sea-robin, sculpin and goosefish (Gadus morrhua, Melanogrammiis wglifinus, Prionotus 

 evolans, Cottus emeus, and Lophins piseatorins). 



This parasite is easily recognized by its color. In a large lot of them, and they 

 frequently occur to the number of a hundred or more in the same host, particularly 

 in Pseudopleuronectes americanus, the colors will be found to graduate from deep 

 orange through pale lemon-yellow to cream-white. When placed in the ordinary pre- 

 serving or hardening fluids they are apt to contract strongly, but if placed in fresh 

 water while still alive they become turgid and straight. 



