6 



devoured the Arguli with great greed, and in a few days the 

 tanks were comparatively clear again. 



According to Baird, A. foliaceus, L. is frequently found 

 parasitic on the stickleback in the South of England, it seems 

 strange therefore that this fish should be made use of to get 

 rid of the same parasite on others. Mr. Carrington kindly 

 sent me some specimens alive for study, and it was then that 

 I found that although there were a few specimens of A. 

 foliaceus present, by far the greater number belonged to A. 

 coregoni of Thorell. This latter species is readily distinguished 

 from the former by its much greater size, and by having the 

 tail lobes pointed instead of rounded at the apex. As I am 

 not aware that this species has hitherto been recorded as 

 British, I made inquiries as to whether any fish other than 

 those from British waters had been placed in the tanks. It 

 appears that the only foreign fish introduced were some 

 American black bass, all of which had died months before the 

 parasite made its appearance. Now, although there are 

 several species of Argulus found in America, most of which 

 are peculiar to it, I am not aware that A. coregoni has ever 

 been recorded from there, so that there appears no reason to 

 suspect its introduction here. 



For the guidance of those of your readers who may care 

 to follow the subject, I subjoin a summary of the leading 

 characters of the British species of Argulus. 



A. foliaceus, L. Distinguished by its smaller size (length 

 6-7 mm., breadth 3-J mm.), by basal joint of second pair of 

 antennae being armed with a strong spine, and by having 

 apices of tail lobes rounded, Baird (Nat. Hist. Brit. 

 Entomostraca, 1850, pi. 31) does not correctly indicate the 

 basal joint of first pair of antennae, which does not appear to 

 differ essentially from the same in coregoni. Claus, however 

 figures it correctly. Sexual appendages on second, third, and 

 fourth pair of legs of $ differ considerably from those of $ of 

 coregoni. (See Claus, Zeitschr. f. w. Zool. B. 25, 1875, pi. 

 xvii.) 



A. coregoni, Thorell. Known by its much larger size — 

 length 10J ( <? ) 13 ( ? ) mm., breadth 8 ( $ ) 8£ ( ? ) mm.— by 

 having but a very small projection at base of second antennae 

 (as Thorell describes but does not so correctly figure), and by 

 having apices of tail lobes lanceolato-accuminate. Habitats 

 (not British) chiefly on the salmonidae, but Claus has also 

 found it on the sand-eel. (See Thorell, Ofver. Vetsakad. 1864.) 

 Leydig describes another European species A . phoxini (Archiv. 

 f. Naturg., 1 871) found on Phoxinus Icevis, but Claus considers 

 it merely a small example of A. coregoni, without even any 

 subordinate modifications produced by its different habitat. 



