THEKSLTIXA GASTEKOSTEI. 43 



usually on the inner surface of the gill-covers. On 

 three-spined sticklebacks, G-asteroxteus aculeatus, cap- 

 tured in Sinclair Loch, Barra, Outer Hebrides, in 

 May ] 894. On the same species of fish captured in 

 the river Forth near Alloa in February 1896 ; and 

 in brackish pools on Aberdeen links in 1 900. Taken 

 also on a fifteen-spined stickleback, Gastrasa spinachia, 

 captured in Loch Etive, West Coast of Scotland, in 

 May 1896. 



This Copepod, which has been ascribed by some authors to 

 the genus Ergasilus, does not agree very well with some of 

 the characters of that genus. The typical female Ergasilus 

 has the body tolerably elongated, but in Thersitina it is 

 nearly globose ; the structure and size of the antennaa are 

 also altogether different. On account of these and one or 

 two other differences, as, for example, in the structure of the 

 mandibles and second rnaxillipeds, we prefer to retain the 

 species in the genus to which it was ascribed by Pagenstecher. 



Tribe II. CAL1GOIDA G. 0. Sars. 



Head usually in the form of a buckler or shield, 

 with or without frontal plates : in some cases the 

 frontal plates are provided with sucking disks. Thorax 

 consisting of several, usually more or less distinct, 

 rings or segments, with their margins sometimes pro- 

 duced into flattened scales or plates. Abdomen or 

 urosome composed of two or three segments, the 

 genital segment being, in the female, sometimes con- 

 siderably expanded (ex. C aligns), or elongated (ex. 

 Hatschekia). 



Mouth-apparatus consisting usually of a syphon 

 provided with styliform mandibles and other accessory 

 appendages. Usually there are also four pairs of 

 thoracic limbs, but in some genera one or two pairs 

 may be wanting. A fifth pair, more or less rudimen- 

 tary, is also sometimes present. 



The species comprised in this tribe are all, in the adult 

 stage, parasitic on fishes. They are not permanently fixed 

 to their host, but both the female and male possess a certain 



