293] PROTEOCEPHALIDAE—LA RUE 293 



host species overlap greatly in their distribution. Several of the species 

 occur in the same waters. Some live only in the depths of deep lakes 

 while others live only on the surface ; others live in brooks and streams, 

 still others as Perca fluviatilis are ubiquitous. One host, Trutta trutta, 

 lives in salt water and enters fresh water to spawn. Thus tho a species 

 may have a wide distribution its habitat may be narrowly restricted. 

 In the restricted habitat the food of the species is likely to be more or 

 less restricted. Surface fish do not get the same kinds of food as do 

 those that live on the bottom. 



A survey of the table shows that all the hosts of P. longicollis de- 

 pend on animal food tho some of them may take plant food on occasion. 

 Certain species are much restricted as to food, so much restricted, in- 

 deed, that it may be said with some degree of certainty that some of the 

 plankton forms must be carriers of this infection. Coregonus wart- 

 manni and C. wartmanni nobilis are referred to as hosts which eat 

 plankton exclusively. It is also apparent that if the records be true 

 some other invertebrates must be carriers of the infection for several 

 species are found which do not feed on plankton. Some molluscs may 

 play a part as carriers. Still other fish of the list eat fish almost ex- 

 clusively hence it must be that fish can serve as intermediate hosts 

 for this parasite. It has already been shown in numerous instances 

 that the primary host may also serve as the intermediate host for 

 Proteocephalid species. If one considers now the different habitats and 

 the various kinds of food taken in them it must be admitted that the 

 evidence points toward a confusion of several species under the one 

 name. In the opinion of the writer the validity of many of the records 

 is doubtful. A comparative study of Proteocephalids taken from all 

 these hosts of different localities would perhaps settle these questions 

 as to hosts of P. longicollis. 



Granting that all the salmonids of the list are hosts of this parasite 

 then on the basis of food, distribution, and habitats it seems that the 

 non-salmonids might likewise be its hosts. However, it has seemed 

 that these records relating to the non-salmonids rest on too slender a 

 foundation. Without an exception these records are those of Nufer 

 (1905) whose work has been discussed in the section dealing with the 

 description of P. longicollis. Parasites of these non-salmonids, espe- 

 cially from Lake Lucerne where Nufer was at work, ought to be sub- 

 jected to a careful comparative study. This would doubtless establish 

 or overthrow the validity of the records. 



