418 Mr. R. Gurney on the 



figured by Kroyer is an adult female in the free-swimming 



S t (1 0TG 



The male (PI. XII. fig. a and PI. XIII. fig. /) closely 

 resembles the free-swimming female, but is readily distin- 

 guished from it by the possession of the large sickle-shaped 

 maxillipedes and the form of the abdomen, the genital segment 

 generally having a refringent appearance, owing to the con- 

 tained spermatophores. The body is also somewhat more 

 slender and the abdomen longer. The first thoracic segment 

 is fused with the head, but the succeeding four segments 

 are all distinct. The abdomen consists of five segments, the 

 genital segment being as long as the remaining four together 

 and bearing a seta on either side at its posterior angles. The 

 fourth segment has a ventral ring of cilia. The furcal rami 

 are, as in the female, short and conical, bearing a very long 

 seta and three short subequal ones with a very characteristic 

 arrangement (PI. XIII. fig./). In the free-swimming 

 female the abdomen consists of four segments only, and the 

 openings of the oviducts on the dorsal surface are very 

 conspicuous. The appendages of the male do not differ from 

 those of the female, with the exception of the presence of the 

 maxillipede. 



The Appendages. 



First pair of antennae. — Unlike Ergasilus, the first antenna 

 of Thersitina consists of five distinct joints, of which the 

 proportional length is, on an average, as follows : — 



Joints 12 3 4 5 



Length 1G 7 8 6 5' 



The joints bear numerous very transparent setse, one of which, 

 on the posterior face of the third joint, is conspicuously long. 



Second pair of antennae. — These are strong prehensile 

 appendages composed of four joints, the last joint being in 

 the form of a strong claw with a stout accessory claw at its 

 base. In the male the whole appendage is more slender 

 than in the female and the accessory claw is very small. 



Mouth-parts. — The mouth-parts are exceedingly difficult 

 to follow out even in the larva, owing to their minuteness 

 and crowded position, and there seems to be no adequate 

 published account of them. I have found it best to boil the 

 adult female in caustic potash before examination, since it 

 is then comparatively easy to see the appendages in their 

 natural positions. There are then visible three pairs of 



