416 Mr. R. Gurney on the 



at all events, as concerns the North-American species ; but 

 it appears that the genus Thersitina, which he was unable 

 personally to examine, is still involved in some obscurity, 

 owing to conflicting statements as to the structure of the 

 single species, T. gasterostei. As I have had the oppor- 

 tunity of obtaining abundant material of this species, it 

 seems worth while to give some account of its structure and 

 life-history. 



Occurrence. 



Thersitina gasterostei is found in abundance under the gill- 

 opercula of sticklebacks (Gasterosteus acideatus) in the ditches 

 containing somewhat brackish water in the neighbourhood of 

 Yarmouth, but I have never found it in quite fresh water. 

 It seems to show a decided preference for the fhree-spined 

 stickleback {G. acideatus), so that, in a ditch in which every 

 specimen of that species is infested, many specimens of the 

 ten-spined stickleback (G. pungitius), though the commoner 

 of the two, may have no parasites at all, and it never has so 

 many as the other species. On G. acideatus I have found as 

 many as forty specimens of Thersitina under one operculum, 

 and it is not uncommon to find one operculum smothered 

 with the parasites while the other is nearly free from them. 

 The parasite is found under the posterior part of the gill- 

 operculum, clasping the mucus which covers the skin, but 

 not apparently fixed to the skin itself. When specially 

 numerous, specimens may be found attached to the mucus of 

 the gills themselves, and I have even found them clinging to 

 the pectoral and dorsal fins and to the tail. 



Breeding Periods. 



Towards the end of October the adult females found are 

 without egg-sacs and usually the ovary is also empty ; but a 

 large number are immature, not having reached the swollen 

 condition of the adult. It seems probable that the adults do 

 not survive the winter, and it is very noticeable that the 

 number of adults found decreases from August onwards. 1 

 believe that the last generation of young, hatched about the 

 beginning of October, fix themselves towards the end of the 

 month, and pass the winter and early spring in a more or less 

 dormant condition. All the evidence points in this genus, as 

 in Ergasilusj to the conclusion that the female is fertilized 

 once and for all during the free-swimming stage. I have 

 never once found a male beneath the operculum of the host, 

 but I havo seen spermatophores attached to a free-swimming 



