104 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



III. 



It will be worth our while now to consider some of the injurious effects of the 

 presence of entozoa among - fishes. 



I have found the larvae of many of the Tetrarhynchidce encysted in a number of 

 food-fishes of our coast, such as Pomatomus, Cynoscion, Scomberomorus, Lobotes, 

 Caranx, Gadus, etc. These were invariably blastocysts, inclosed in cysts of connect- 

 ive tissue and each containing an embryo tetrarhynch more or less developed. They 

 occur in the body cavity, where they are either attached to the serous covering of the 

 viscera, peritoneum or mesentery, or they may be buried in the liver or spleen or lying 

 between some of the coats of the stomach or intestine. 



The scolex of a tetrarhynchus of the dusky shark (Carcharinus obscurus) is quite 

 common in the squeteague or weakfish (Cynoscion regal is), where it may be found 

 under the submucous coat of the stomach and intestine, often dotting the whole length 

 of the alimentary tract with small specks. If one of these specks be punctured in 

 July and August (the only mouths in which I have collected them) a small squarish or 

 roundish scolex of a tetrarhynch is liberated. The abundance of these scolices in a 

 single host explains the occasionally great abundance of the adult stage in the pyloric 

 division of the stomach and the spiral valve of the dusky shark. That is, in such 

 cases the shark has made a meal off of one or more fishes which were infested with the 

 larval tetrarhynch in the encysted stage. 



The cycle of life of these forms has not been proved in all cases, but from what is 

 known of other members of the order it may be thus summarized: The larval 

 tetrarynchs having been liberated by the action of the digestive fluids of the shark 

 attach themselves to the walls of the stomach and intestine of this their final host, 

 where they develop into the mature form, and whence chains of proglottides, contain- 

 ing numerous ova, escape. Some of these in due time may be themselves eaten by a 

 squeteague or other fish, in which the ingested ova, instead of yielding to the action of 

 the digestive fluids along with other food, undergo normal development, are hatched 

 into minute six-spined embryos, which, boring through the tissues and finding lodg- 

 ment somewhere in the body cavity of the intermediate host, there develop into a 

 blastocyst in which a larval tetrarynch by and by makes its appearance. The larva, 

 apparently, does uot usually have the power of escaping from the blastocyst unless it 

 is swallowed by the proper final host. These encysted forms in the squeteague, blue- 

 fish, etc., where only a few individuals are present, probably do little harm. It must 

 be remembered, however, that the nourishing of every cyst is at the expense of a 

 certain amount of vitality of the host; as the vitality of the host is diminished, the 

 chances of its being caught by an active enemy are increased. Parasitism thus tends 

 to perpetuate itself. The life-history of all these forms that require the intermediation 

 of two hosts which stand toward each other in the relation of eater and eaten, has 

 become adapted to the various vicissitudes and interrelations of the hosts. 



Sometimes the presence of the parasite must work positive injury to its host. 

 A case in point is that of a parasite which infests the liver of the large suntish (Mola 

 rotunda). I have been able to verify the statements of Oobbold concerning this para 

 site {Tetrarhynchus repens) and have seen the liver of a large mola tunneled in various 

 directions by this parasite until the whole organ was little more than a sac of worms. 

 Such a state of aft'airs must occasion a decided loss of vitality to the sunfish, and 



