570 PROCEEDINGS OF THE NATIONAL MUSEUM. tol. 47. 



Their free swimming period is, of course, the only time that their 

 enemies can get access to them, and this period is so brief that not 

 many of the larvae are killed. 



But the brevity of this period also operates as a regulator to pre- 

 vent the parasites from becoming too numerous. For unless they 

 find a host during this period they quickly perish. The shorter the 

 period, therefore, the fewer will be those whose search is successful. 

 In the crowded confines of a stock pond, on the contrary, practically 

 every larva is sure of finding a host. And all too often the gills of the 

 fish become so crowded with parasites that they can not perform 

 their normal functions, and the fish are suffocated. Among fresh- 

 water hosts the various species of trout and salmon, the white fish 

 of the Great Lakes region, and the bass and perch are the ones most 

 infected, the parasites being confined to the two genera Salmincola 

 and Achtheres. 



Among salt-water fish many members ot the large family of Gadidae, 

 particularly the cod and haddock, the mullets (MugiHdae) the rays 

 (Dasyatidae), the skates (Rajidae), and the sharks are the most com- 

 mon hosts. 



Food. — These parasites feed upon the blood of their host, as is 

 shown by the fact that they fasten to the gills or fins. They are also 

 furnished with mandibles which were evidently designed for piercing 

 the skin of their host, and whose margin is cut into saw teeth, which 

 not only penetrate the flesh but also lacerate the wound and thus 

 stimulate the flow of blood. Adult females are frequently obtained 

 with their alimentary canal filled ^vith blood, but the color of the 

 latter is quickly removed by the digestive fluids. It is a question 

 whether the male ever eats anything at aU, and if he does, what con- 

 stitutes his food. The following facts and considerations have an 

 important bearing on this question: 



1. The adult male possesses a pair of mandibles as large, as power- 

 ful, and as suited for laceration as those of the female. He has also a 

 well-developed esophagus and stomach, but in all those examined 

 there is no trace of an intestine or anus. 



2. There are no digestive glands connected with the alimentary 

 canal, but the stomach is lined with large gland cells, which are filled 

 with a liquid that undoubtedly aids in digestion. The abundance 

 of these cells more than compensates for the lack of glands, and would 

 be amply sufficient to digest such food as blood. 



3. The excretory glands are exceptionally large and are provided 

 with good-sized ducts. It would seem as if they could take care of 

 all the residue that would be left undigested. 



4. The male does not live on the body of the host, but clings tightly 

 toson?e convenient place on the body of the female, and no specimens 

 have ever been found on the fish's gills, although repeated search has 

 been made for them. Males have been found cHnging to the second 



