152 PROCEEDINGS OF THE NATIONAL. MUSEUM vol.88 



folds wliicli would have been difficult to interpret if other specimens 

 had not been at hand. The vas deferens was traced into the neck 

 where it lay ventral to the esophagus and the right of the uterus. 



Some small, yellowish masses, collected June 27, 1910, encysted on 

 the viscera proved to contain trematode ova, 0.017 by 0.010 mm., asso- 

 ciated with granular material. The ova resemble those of trematodes 

 from the gills of the bonito. 



Slender, filiform fragments collected by Dr. C. W. Hahn, July 21, 

 1911, under the mucous membrane of the inner wall of the gill cham- 

 ber, also fragments from a cyst. These fragments are narrow and 

 ribbonlike ; ova, 0.015 by 0.009 mm. 



DIDYMOZOON species 



Plate 23, Figures 311-314 



Following are records of trematodes probably to be referred to this 

 genus but which do not present sufficient characters to admit of specific 

 determination. 



Hosts. — Frigate mackerel {Auxis rochei), chub mackerel {Pneu- 

 matophorus grex)^ common mackerel {Scomber scomhrus), rudderfish 

 (Sei'iola zonata). 



Record of collections.— Cy^t on gills (U.S.N.M. No. 8388), collected 

 July 12, 1912, from frigate mackerel; length, 4 mm.; diameter, 1.5 

 mm. The cyst contained portions of uterus and vitellaria ; ova with 

 rather thin shells, much crowded, about 0.018 by 0.009 mm. The speci- 

 men was doubled on itself and represented a length of about 6.65 mm., 

 breadth, 0.63 mm. 



From the chub mackerel (fig. 313) : A small yellow cyst, collected 

 August 15, 1908, in muscle tissue beside the anal aperture of the host. 

 Three yellow cysts, collected in August, 1908, from gills of one fish, 

 and one from another (U.S.N.M. No. 8389). These cysts contained 

 slender trematode fragments filled with ova about 0.012 by 0.006 mm. 

 Fragments mounted in balsam aggregate about 25 mm. in length, 

 and vary in breadth from 0.05 to 0.26 mm. One specimen complete, 

 length about 18 mm., breadth varying from 0.05 to 0.28 mm.; oral 

 sucker, terminal, about 0.07 mm. in diameter; pharynx not distinct, 

 being hidden by ova. 



From common mackerel (figs. 311, 312) : Two yellow, globular 

 cysts on pyloric caeca (U.S.N.M. No. 8390), collected August 10, 

 1906. One cyst contained a threadlike trematode, broken into frag- 

 ments which aggregated a length of about 180 mm., and, for the 

 most part, 0.07 to 0.14 mm. in breadth ; ova, 0.014 by 0.008 mm. Two 

 heads were found. Diameter at anterior end, 0.14 mm., at 0.28 mm. 

 from anterior end, 0.29 mm., at 1 mm. from anterior end, 0.15 mm. 

 The second cyst contained a large number of eggs. These frag- 



