130 BULLETIN OF THE BUREAU OF FISHERIES. 



balance of nature in his search for food among the fishes of the sea enters as a 

 factor tending toward the extinction of any species is hard to estimate. Theoreti- 

 cally his mightiest efforts are, in the aggregate, so slight that they appear to my 

 mind to be a negligible quantity. If the economic use of fish by man does enter 

 as an appreciable factor in the struggle for existence, then anything which will open 

 up uses to which- sharks may be put will be in the way of restoring the balance 

 which has been disturbed and of correcting this particular evil of cestode parasites 

 in the flesh of food fish. 



The skin of the shark has some commercial value as shagreen, glue may be 

 made of some of the waste parts, oil from the liver, etc. In addition to these uses 

 the use of sharks as food is a matter of considerable economic importance. Young 

 sharks are quite as good eating as many of the fish which are sold in our markets 

 and, indeed, they themselves form an important addition to the food fishes in some 

 countries. Unfortunately there is a strong prejudice against shark meat, especially 

 in this country, and when it is called dogfish the prejudice is intensified, although 

 dogfish, in the opinion of many who have tried it, is quite as good eating as sque- 

 teague. Properly prepared it would be hard to tell it from canned salmon. It 

 must be admitted, however, that old shark is not good eating. 



If, then, an industry could be built up which would make it worth while for 

 fishermen to expend the same energy and develop the same ingenuity in the capture 

 of sharks as is done in the capture of fish which are now used as food, there would 

 result a means of profit where there is now actual loss. A factor would also be 

 introduced in the life experience of sharks which might tend toward reducing their 

 numbers. If the number of sharks were reduced there would follow a correspond- 

 ing reduction in the amount of such parasitism among teliosts as is caused by 

 cestode worms. 



PROBLEMS SUGGESTED. 



A few problems are suggested by the occurrence of these parasites in the flesh 

 of the butterfish for which no final solutions are proposed in this paper. 



1. Time required for the development of a mature scolex. 



It is possible that this may be ascertainable by means of feeding experiments, 

 although there are many practical difficulties in the way. Some information is 

 probably obtainable by a very careful examination with the aid of the microscope of 

 small butterfish of known age. Small fish were found to be free from cysts in the 

 flesh, or to have very few and those immature. The larger fish were for the most part 

 found to have many parasites. Furthermore, the cysts in the smaller fish were 

 white, those from the larger fish yellowish, showing thus a difference in age. 



2. Distribution of the cysts in the flesh. 



The rather uniform distribution of cysts in certain favored locations is doubtless 

 dependent primarily on the vascular system, and particularly on the arterial 

 circulation. 



When liberated from the egg shells by the action of the digestive juices the 

 embryos are very small and, upon penetrating the mucous membrane of the stom- 

 ach and intestine and entering lymph spaces or capillaries, would be carried in the 

 circulation as easily as blood corpuscles. The distribution of the cysts indicates 



