324 



SCIENCE-GOSSIP. 



FISH-LICE. 



By Frederick Noad Clark. 



THE cve&t\vi:e, Argulus foliaceus,'h.QVQ illustrated 

 belongs to the sub-division Entomostraca of » 

 tbe class Crustacea, and is commonly known as the 

 fish-loiise. It is fovmd parasitic on most of our 

 fresh-water fishes, notably the pike, carp, roach, 

 and stickleback. It was from the latter hosts I 

 obtained my specimens. The life history of A. 

 foliaceus is still somewhat obscui-e, but I have had 

 the good fortime to observe its habits from the 

 ova to nearly an adult stage. 



The grounds of the Paddington Infirmary 

 occupy a portion of the left bank of the Grand 

 Junction Canal, and being upon that side on 

 which the barges do not touch, the bank and 

 svirrounding vegetation are still in a wild and un- 

 molested state of natui-e. Here, in the summer 

 months, may be found large nimibers of the 

 smaller dragon-flies, Agrioii, etc., as well as several 

 species of aquatic hemiptera, such as Nepa and 

 others. It was here, in the latter part of April 

 1896, that the sticklebacks were in abundance. 

 My attention having been dra-\vn to some " insects " 

 infesting these fish, an examination found them 

 to be the parasite Argulus foliaceus. For the 

 past year or so I have been unable to procure 

 any specimens, owing to the scarcity of the stickle- 

 backs, due probably to the increased pollution of 

 the canal. In the year mentioned, both host and 

 pai'asite were numerous, and now, when unable to 

 procure material, one regrets lost opportunities. 

 About one in every four or five fish examined was 

 found to have an Argulus attached, and in some 

 cases two or three. 



Argulus foliaceus. — Female, x 12. 



The favourite position of attachment is the pos- 

 terior parts of the body of the fish, although I 

 have found them in the neighbourhood of the 

 gills, and in one case upon the eye itself. No 

 difiiculty is experienced in detaching the parasite. 



This done, it commences to swim with a darting 

 motion, turning somersatilts in the water at 

 intervals. Very noticeable, too, is the incessant 

 motion kept iip by the swimming feet, whether at 

 rest on the side of the glass vessel in which it was 

 kept, or when in pursuit of its j)rey. I have 

 observed, on several occasions, a fish to take the 

 parasite in its mouth, but shortly afterwards to 

 eject it uninjured. 





' '.V ' 





Argulus foliaceus. — Male, x 20. 



In size the adult female Argulus is from five to 

 six millimetres in length, and from three to four 

 millimetres in breadth — the male being much 

 smaller. The females were the more numerous, and 

 could readily be identified by the presence of two 

 black spots, one on each lobe of the abdomen. 

 This animal is of a slightly greenish colour, and 

 almost transparent. On the ventral surface are 

 situated the feeding and locomotive organs, the 

 dorsal sua-face being somewhat convex in shape. 

 The apparatus for attachment to its host is seen 

 to be very perfect for the purpose. In addition 

 to an armament of spines, which covers the under 

 marginal surface of the carapace, attention will at 

 once be drawn to the pair of " suckers," that, at 

 first sight, are not unlike the pro-legs of lepidop- 

 terous larvae. These are short, fleshy, cylindrical 

 appendages, with a ciliated margin. They are 

 provided with powerful muscles which contract at 

 will, so forming a pneumatic sucker, and thiis 

 enable the parasite to firmly adhere to the 

 smooth surface of the fish. The eyes are of a 

 dark-purple coloixr ; they will be observed a little 

 in front of the suckers, but nearer to the middle 

 line, and resemble in some degree the compound 

 eyes of insects. The antennae are situate close to 

 the eyes, and consist of two joints terminating in 

 a pointed hook, together with a three-jointed 

 appendage, which has been described by some 



