1200 BULLETIN OF THE BUREAU OF FISHERIES. 



tomcod, and fish of similar habits, have small distomes encysted in the skin 

 and in the fins. As these are almost always all removed in preparing the fish 

 for cooking, they need cause no more thought, even to the ultra fastidious, 

 than other accidental debris that may be caught by the slimy epidermis of the 

 fish. On fishes inhabiting small fresh-water lakes this form of parasitism is 

 common. The bearer of the adult stage of these skin parasites is commonly 

 some fish-eating bird. 



CESTODA. 



This order is represented by many genera and species among the entozoa 

 of marine fishes. The sharks and skates harbor a long list of adult cestodes in 

 their alimentary canals, especially in the intestine or spiral valve. The mature 

 joints of these cestodes, each filled with hundreds, even thousands of eggs, are 

 cast into the water in vast numbers along with the fseces of the host. It is a 

 peculiarity of these free segments that they may continue living for some time, 

 even many hours, in sea water. In the water they are likely to be eaten by 

 such fish as feed on small worms, Crustacea, and the like. The adult stage of 

 any of these cestodes of the sharks and skates is limited to a few closely related 

 species, or, in some cases, apparently to a single species. They are, on the 

 other hand, capable of living on a large number of intermediate hosts. A little 

 reflection on the contrasted conditions to which the adult and the larval stages, 

 respectively, of a cestode are subjected will serve to explain this difference. 



In the adult stage the cestode passes its whole existence in the alimentary 

 canal of its host. It has become adapted to a highly specialized set of condi- 

 tions. Hosts differ specifically not only in respect to their morphological char- 

 acteristics but in their physiological characters as well. Thus a given cestode 

 may find the juices of the alimentary canal of a tiger shark kindly while it finds 

 the juices of the alimentary canal of any other shark fatal to its developn;ent. 

 There is also some difference in the character of the food. The latter might 

 seem to account for the difference between the parasites of a shark whose diet 

 consists mainly of crustaceans, and one which has a strictly fish diet. On the 

 other hand, sharks which feed on practically the same food, if they are not 

 closely related morphologically, maybe found to harbor a different set of cestode 

 parasites. For example, the entozoa of the dusky shark {Carcharhinus obscurus) 

 and those of the blue shark (C. viilbcrti) comprise practically the same species. 

 When the list from either of these sharks is compared with the list from the 

 sand shark {Carcharias littoralis), some constant differences at once appear. 

 There is one species of cestode (Crossobotlirittm laciniaium) which is almost 

 invariably present in the sand shark and usually in considerable numbers. It 

 has not been found in any other species of shark or skate. Furthermore, very 

 few of the long list of cestodes from the dusky shark have been found in the 

 sand shark. 



