A CESTODE PARASITE IN THE FLESH OF THE BUTTERFISH. 127 



It is a significant fact that in all cases where a comparison was made between 

 fish of the same length and depth, the one with numerous parasites and the other 

 with few, the result was a less weight for the fish having the greater number of 

 parasites. It seems reasonable to conclude, therefore, from this loss of weight, 

 that the vitality of the fish is affected by the presence of cysts in the flesh. 



LIFE HISTORY OF THE PARASITE. 



The round of life of this parasite may be epitomized thus: 

 The adult stage is passed in the spiral valve of some shark. Ripe proglottides 

 containing large numbers of eggs escape from the intestine of the shark along with 

 the feces, and are eaten by various fi.shes. The eggs give rise to embryos, which 

 make their way into the body of the intermediate host, where they become encysted. 

 The encysted embryo vdtimately develops into a scolex with characteristic sucking 

 disks and proboscides armed with hooks. Wlien a fish that has these cysts in its 

 tissues is swallowed by a shark which is adapted to become the final host of this tape- 

 worm the connective tissue cyst is digested off , the scolex is liberated, and attaches 

 itself to the mucous membrane of the shark's intestine. It there grows to the adult 

 tapeworm, producing segments which separate easily from the strobile as they 

 ripen, and pass off with the feces bearing eggs to begin another generation. 



SPECIAL CASE OF BUTTERFISH AND HAMMERHEAD SHARK. 



Food of the hammerhead fihark. — A more particular consideration of tlie case 

 afforded by the butteiiish is justified by the high percentage of fish which were found 

 to be infected. That the butterfish is the principal source from which the adult 

 tapeworm of the hammerhead comes is based on the following considerations: 



The hammerhead is carnivorous; fish and squid are the only food which I have 

 found in the stomachs of those which I have examined. Butterfish are abundant, 

 and, being good eating, are doubtless often eaten by the hammerhead. 



Practically all of the buttei-fish which were examined for cysts in the flesh, except 

 small specimens, were found to be infested, and a large proportion of them harbored 

 enormous numbers of cysts. Although s]:>ecial search for these cysts was confined to 

 the past season, there is reason for believing that the large proportion of parasitized 

 fish then found is not exceptional. My notes made at the time of collecting in 1899 

 and 1900 show the same distribution of cysts in the flesh of this fish as was found in 

 1904, and there is no reason for thinking that they would not have been found in 

 abundance in other butterfish in previous years if search had been made for them. 



Sharks and buttei'fish arc taken together in the fish traps, thus showing an 

 association in their natural surroundings sufficiently close to permit their being final 

 and intermediate host, respectively, of the same cestode parasite. 



In view, therefore, of the ver}^ large proportion of butterfish which harbor cysts 

 of Otobothrium crenacoUe, the conclusion is justified that the butterfish is the principal 

 source from which the supply of adult tapeworms is kept up. 



It may be mentioned here for the comfort of eaters of fish that parasites in the 

 flesh of food-fish are of very unusual occurrence. I had looked for them repeatedly 

 in most of the food-fish of the Woods Hole region for many years without finding 



