42 BRITISH PARASITIC COPEPODA. 



three- jointed, and bearing a few stout apical spines. 

 Thoracic legs nearly as in Ergasilus. 



In the male the cephalic segment is not so much 

 enlarged, and the body is more elongated. 



1. Thersitina gasterostei (Pagenstecher). 

 (Plate XXV, figs. 1-6 ; Plate LI, fig. 6.) 



1861. Thersites gasterostei Pagenst. (94) vol. xvii, p. 118, pl.vi, figs. 1-9. 

 1863. Ergasilus gasterostei Kroyer. (71) p. 233, pi. xii, figs. 2, a h. 

 1892. Thersites gasterostei Canu. (29) p. 245, pi. xxiii, figs. 13-18. 



1899. Ergasilus gasterostei Bassett-Smith. (8) p. 444. 



1900. Thersites gasterostei T. Scott. (112) p. 146, pi. v, figs. 1-7. 

 1905. Thersitina gasterostei A. M. Norman. Museum Normanianum, 



III, Crustacea, second edition, p. 41. 



Female. Cephalic segment considerably dilated, 

 and, when viewed from above, concealing the remain- 

 ing thoracic segments as well as part of the abdomen; 

 genital segment of the abdomen longer than the 

 combined length of all the other abdominal segments. 

 Caudal rami short. 



Antennules very short, tolerably stout, and com- 

 posed of five joints, which are sparingly setiferous. 

 The formula shows their proportional lengths. 



Numbers of the joints . . .12345 



Proportional lengths of the joints 16 10 11 7 8 



Antennae short and stout, and each furnished with 

 a tolerably strong terminal claw. Mandibles with a 

 bilobed biting part. First maxillipeds short, simple in 

 structure, and provided with a few small spines. 

 Second maxillipeds small, basal part enlarged, ter- 

 minal part more slender, forming a curved arm which 

 is furnished with a few apical spines. 



The first three pairs of swimming-legs with both 

 rami three- jointed, but the outer ramus rather shorter 

 than the inner. In the fourth pair the outer ramus is 

 only two-jointed, the elongated first joint consisting 

 probably of two coalescent joints ; the inner ramus 

 three-jointed as in the preceding three pairs. Length 

 about 0*8 mm. (- 3 - - of an inch). 



Habitat. Parasitic on certain fresh- water fishes, 



