296 



BULLETIN OF THE UNITED STATES FISH COMMISSION. 



Distomum sp. 

 [Plato 39, fl<r. 72; Plate 40, figs. 73-75.] 



Among the numerous small distoma found during the summer of 1898, I note briefly a form found 

 on two occasions, but as only a single specimen was obtained in each case formal identification has 

 not been attempted. Both are characterized by having the body armed with minute, scale-like spines, 

 dense on the neck, but becoming sparse posteriorly on the body. One was obtained from a scup 

 (Stenotomus chrysops) August 15, the other from a flounder (FuraHclitliys dentatus) August 25. Since 

 the stomach of the latter contained several small scup, and the distomum was obtained by washing 

 out the alimentary canal of the flounder, the true host of the worm is quite probably the scup. 



Dimensions of living specimens, in millimeters: 



(1) Specimen from scup: Length 0.62, greatest breadth 0.31, diameter of oral sucker 0.09, of 

 acetabulum 0.09, longer diameter of ova 0.076, shorter diameter of same 0.034. 



(2) [U.S.N.M., No. 6521.] Specimen from flounder: Length 1, greatest breadth 0.53, diameter of 

 oral sucker 0.18, of acetabulum 0.18, longer diameters of ova 0.076, shorter diameter of same 0.052. 

 The same specimen mounted in balsam is 1.22 in length and an ovum measured 0.064 and 0.034 in the 

 two principal diameters. 



Diagnostic characters, so far as they can be made out from the latter specimen, are as follows: 

 Body ovate, depressed, whitish in life, covered with short scalelikc spines becoming sparsely scattered 

 posteriorly; neck short with tendency to be constricted behind oral sucker; mouth snbterminal; 

 acetabulum equaling or slightly exceeding month; pharynx longer than broad; oesophagus none; 

 branches of intestine, simple, spacious, extending to near posterior end; testts two, median, back 

 of middle of body, close together, relatively large, broader than long; genital aperture in front of 

 acetabalum, a little to the left, cirrus pouch behind acetabulum ; ovary subglobular lying immediately 

 in front of anterior testis; uterine folds, containing a few (6) relatively large ova, lying between the 

 ovary and acetabulum; vitellaria along lateral margins from the posterior end to acetabulum. 



Immature Distoma encysted in skin of Gunner. 



[Plato 40, fig. 76-81, F. S. N. M. No. 6522.] 



A cunner (Tnutoyolabrux adspcrsns) was examined September 5, in which the general surface of 

 the body, including the fins, was covered with minute cysts. The appearance of the fish agreed in 

 minutest detail with Ryder's description of a similar case observed by him (Bulletin U. S. Fish Com- 

 mission for 1884, pages 37-42). Black pigment cells are very abundant in the vicinity of the cysts, 

 where they make black, opaque masses immediately surrounding the cysts. Pigment is almost entirely 

 absent from the exterior surface of the cyst where the epidermis is tightly stretched. The cysts them- 

 selves are nearly transparent. This is true for the larger cysts. The smaller cysts have pigment 

 cells over their surface, but in no greater abundance than normal. As the cysts grow, the pigment 

 cells retreat from the surface and accumulate about the periphery of the cysts as it is seen in optical 

 section when a scale with these cysts is put under a cover glass and examined with aid of a microscope. 

 The red pigment of the skin continues to be represented over surface of cysts longer than the black. 

 In all cysts observed pigment cells were absent from surface just above the young worm. 



Ryder thought these cysts were duo to the presence of the ccrcaria of some trematode. He does 

 not appear actually to have seen them. Some of the young removed from the cysts proved to be 

 young distoma, thus confirming the general conclusion of Ryder. 



Sections were made of the fins containing numerous cysts, but without throwing any light on 

 the probable identity of the adult species represented by these immature forms. The walls of these 

 cysts, as seen in section, prove to be relatively thick. In one which measured 0.32 by 0.25 mm. in the 

 two principal diameters the wall of the cyst was 0.05 mm. thick. 



The following table gives the dimensions, in millimeters, of living specimens removed from cysts : 



Diameter of a single cyst, 0.36, not including the snrronnding'pigment. 



