FLESH PARASITES OF MARINE FOOD FISHES. 1 205 



will enable me to tabulate readily or completely the data which it is desirable to 

 marshal for this particular purpose. The following statements, however, are 

 abundantly warranted from many observations made during previous years, and 

 have some bearing on the immediate question. 



The stomach wall of most squeteagues {Cynoscion regalis) contains a greater 

 or less number of cysts of a definite species of cestode {Tetrarhynchus bisulcatus) 

 which is found in the adult stage in the stomach and intestine of the dusky 

 shark. Furthermore there is found in the cystic duct of the same fish a larval 

 cestode {Scolex polymorphus) , almost always in considerable number. The 

 same parasite is also quite common in the cystic duct of the summer flounder 

 {Paralichthys dentatus). For example, during the past summer I examined 

 a flounder from Menemsha Bight which appeared to be suffering from a case of 

 jaundice. The whole surface was yellow, the unpigmented under side being a 

 decidedly bright lemon yellow. The flesh and the viscera were also yellow. The 

 cystic duct was occluded by a mass which looked something like a .soft tumor. 

 When this mass was cut open it was found to consist of a cluster of these ces- 

 todes. Their heads were buried in the mucous membrane while their bodies 

 effectually stopped the lumen of the duct. Other cases of prevalent parasitism 

 in intermediate hosts could be cited. 



In like manner cases of prevalent parasitism of final hosts are not lacking. 

 Thus every specimen of tiger shark {Galeocerdo tigrinus) which I have examined, 

 about 15 in all, at intervals during many years, has been found to harbor large 

 numbers of a singular cestode (Thysanoccphaluin crispum), a species which has 

 not been found as yet in any other host. Again, nearly every sand shark in 

 the Woods Hole region harbors a species of cestode {Crossobothrium laciniatum) , 

 often in large numbers. 



Plainly, then, all that is necessary to make parasitism, by means of a given 

 species of parasite, affect the majority of the individuals of the host, is to have 

 the source of infection sufficiently widespread, abundant, and pervading in the 

 natural habitat of the infected species. Not only must the final and the inter- 

 mediate hosts, in the case of the cestodes, be related to each other as eater and 

 eaten, but their association together must be otherwise close, else the inter- 

 mediate host will not become largely infected. At present I can see no other 

 explanation of the almost universal prevalence of this parasite in the flesh of the 

 butterfish than that which I gave in the paper cited above. The butterfish 

 must have formed the habit of following sharks, attracted by the bits of food 

 which float off in the water while the shark is feeding. The voracious, fish- 

 eating sharks tear and shake their prey as they eat it, so that there must often 

 be in the vicinity of a shark a cloud of bits and shreds of meat which are greedily 

 sought by smaller fish. This zone of sure, even if it be intermittent, food supply 



