.OBSERVATIONS CONCERNING FISH-PARASITES. 103 



A dibothrium (D. manubriforme Lt.) which I found in the billfish (Tetrapterus 

 albidus) was, in the subsequent summer, found to infest also the closely related 

 species, the sailflsh (Histiophorus gladius). This dibothrium, while it bears some resem- 

 blance to 1). plicatum of the swordtish (Xiphias gladius), is specifically different from it. 



A singular cestode {Thysanocephalum crispum Lt.), found in 1883 and again in 1889 

 in the tiger shark ( Galeocerdo maculatus), is apparently peculiar to that species. Like- 

 wise another cestode, which I have found repeatedly from 1882 to 1889 in the land 

 shark (Garcharias littoralis), and which I have described under the name Crossoboth- 

 rium laciniatittn, appears to be peculiar to that host. 



In general I may say that in a majority of the species which I have examined 

 from which no entozoa had been already reported, I have found one or more entozoa 

 that proved to be new to science. 



While it would, of course, be entirely improper to set up peculiarities of this kind 

 as criteria for the determination of species, 1 venture to say that the helminthologist 

 who finds two species of fish, which ichthyologists have made generically different, 

 habitually harboring identical kinds of entozoa, under similar circumstances, would 

 not be surprised to see these genera united by the next disturber of ichthyological 

 nomenclature; and, conversely, if two species which systematists have regarded as 

 being closely related should be found to harbor a completely diverse set of entozoa 

 under similar conditions of migration, food supply, etc., sufficient reason would thereby 

 be afforded for asking that the morphology of the two species in question be reviewed. 



Certain natural groups of animals, as orders for example, are infested by certain 

 genera or even families of entozoa which are peculiar to them. 



A.notable example of this is furnished by those remarkable cestodes which have 

 four eversible spinose proboscides, belonging for the most part to the two genera 

 Rhynchobothrium and Tetrarhynehus and forming the family Tetrarhynchidce. The 

 Tetrarhynchidw, in their adult form, are confined to the Elasmobranchii, sharks and 

 skates. For this limitation of their habitat other vertebrates can not be too thankful. 

 The proboscides are often relatively long, abundantly long enough in most cases to 

 penetrate the mucous and submucous coats of the alimentary canal of their host. 

 They are armed with multitudes, often a thousand or more, of sharp, recurved hooks, 

 so that when they penetrate the stomach or intestinal walls of their host they can be 

 dislodged only with great difficulty. They often burrow a little pit in the intestinal 

 wall, where sometimes a cluster of scolices will be lodged together. I have occasion- 

 ally seen the stomach and intestine of a large shark with several ulcerated pits due to 

 the presence of such parasites. As a rule, however, the presence of a moderate number 

 of intestinal parasites in the adult stage does not apparently cause much discomfort 

 to the host. The sharks, for example, which I have examined, and I have examined 

 a goodly number, all appeared to enjoy excellent digestion; their accessory digestive 

 organs being in nearly every case clean and bright. Indeed, if a parasite were to cause 

 much mortality by its presence in its final host it would not be long, as things go in 

 the economy of nature, until it would either cause the destruction of the species to 

 which its host belongs, or at least makes it exceedingly rare, and thus imperil its own 

 existence as a species. Hence, among the entozoa of fishes, as among those of man 

 and the domestic animals, it is the immature stage of the parasite that causes most 

 mischief, and it works that mischief not on the final but on the intermediate host. 



