94 Thirfi/scrciitli Ainiual Meeting 



salmon. To such must Ix' added also those forms which lie in 

 the pyloric coeca with their rich store of reserve food as well as 

 others which are encapsulated in various parts or move ahout 

 freely in the hody cavity or other closed organs. But even those 

 parasites in the oesophagus, stomach, and pyloric coeca appear 

 to decline in number of species and individuals as the fish re- 

 mains longer in fresh water and climl)s higher in the streams. 

 Finally the salmon harljors only the encapsulated forms and 

 those lying in various entirely occluded organs. The wander- 

 ing in the Ehine constitutes thus a cure for the elimination of 

 parasites from the alimentary canal. Some investigators, in- 

 deed, though without the exact knowledge afforded by these 

 demonstrations of Zschokke, have proposed to explain the mi- 

 gratory instinct of many fishes on the basis of the necessity of 

 relieving themselves of parasites acquired in the ocean. 



The salmon which ascends the streams from the Baltic sea 

 feeds abundantly in fresh water and harbors accordingly some 

 fresh water parasites wdiile the intestinal region does not become 

 free from parasitic infestation. It possesses even a richer para- 

 sitic fauna than the Rhine salmon. 



Other somewhat less extensive observations yield evidence of 

 the same type. Thus the Elbe salmon according to Fritsch 

 (1894) takes alisolutely no food and in a corresponding way is 

 infected by a jjarasitic fauna of a ])urely marine type. Over 

 against this must be placed the record of Mcintosh (1863) that 

 the Tay salmon does feed to some extent during its migrations 

 in fresh water. Even here the nuirine element preponderates, 

 and yet there are not wanting a few parasites recognizable as 

 guests which normally frequent fi'csh water fishes. In the Tay 

 salmon also the parasitic fauna is numerous and varied. !t 

 embraces a considerable number of forms which occur in tlie 

 intestine behind the pyloric coeca just where it will Ije recalled 

 the Ehine salmon is entirely free from parasitic infestation. 

 Both this infestation of the intestine and the large percentage 

 of parasites which may Ije characterized as fresh water forms 

 are regarded by various authors as clear evidence in support of 

 the fact that the Tay salmon unlike its relative of tlu' Ehine 

 does not cease taking food when it enters fresh water. Conse- 

 quently the marine elements in its alimentary canal are gradu- 



