42 BRITISH PARASITIC COPKPODA. 



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

 Thoracic legs nearly as in Erf/asiln*. 



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 gasterostt i Pagenst. (94) vol. xvii, p. 118, pi. vi, figs. 1-9. 



1863. Ergasilus gasterostei Kroyer. (71) p. 233, pi. xii, figs. 2, a 7t. 



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



1899. Ei-gasilus gasterostei Bassett-Smith. (8) p. 444. 



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

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



Ill, 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. 



lumbers 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). 



II a hit fit. Parasitic on certain fresh- water fishes, 



