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A HISTORY OF NORTHAMPTONSHIRE which have been described as ' intermittent parasites.' Their hosts, so far as at present known, are always fishes or frogs. Upon these they fasten and suck their juices, to the serious detriment of tender tadpole or the smaller kinds of fish. Then, when a grateful feeling of replenish- ment supervenes, they relinquish their hold of the now anaemic victims, and swim about with activity in freedom to get up an appetite for another meal. They are classed in an order called Branchiura, which contains the single family of the Argulidce. Of this the only repre- sentative known in England is the Argulus foliaceus (Linn.), the discovery of which in Northamptonshire is thus notified by Mr. H. F. Tomalin : — ' Occurrence of the Argulus foliaceus upon the common " trout," Sal mo far 10. 'The trout was caught at Holdenby in June of this year [1883], and thirteen of these parasites were obtained from it ; they were how- ever near to the abdominal fins, and not fixed just under the pectoral fins as is more common. The occurrence of this parasite upon the trout is very unusual. Thompson, in his Natural History of Ireland, records it as having been obtained from this fish, and it has more recently been recorded in The Field.' After an account of the classification, Mr. Tomalin continues : ' The fish-argulus may be found on various fresh- water fish, the two species of sticklebacks being the more general ; they swim gracefully, and have a strong resemblance to the seeds of plants. The female is larger than the male, and is distinguished by the black spot on each side of the abdomen ; the width is about two lines. They occasionally leave the fish, but not for a long time.'^ Mr. Beeby Thompson, in a letter already quoted, alluding to Mr. Tomalin's capture, says : ' The same species, I think, I have had on gold- fish in my aquarium — possibly introduced by sticklebacks.' Since gold- fish are of the carp tribe, the Argulus foliaceus, which both in England and Germany is often called the carp-louse, would find itself at home upon them. As measurement by lines or twelfths of an inch has gone out of fashion, it may be well to observe that a width of two lines is equivalent to a little over four millimetres, and that the width was mentioned by Mr. Tomalin, rather than the length, probably because these little greenish semi-transparent vampires are disc-like in shape, and not to any striking extent longer than broad. A very remarkable feature in their organization is the transmutation of their second maxillae into a pair of circular suckers. In this respect they differ from the species of the companion genus Dolops, in which these maxillae end in strong hooks instead of suckers. The Argulida were at one time arranged with the parasitic Copepoda, as in Mr. Tomalin's notice, and to some of the so- called fish-lice of that group they show a marked superficial resemblance. But a different classification is now accepted. Even Claus, the dis- tinguished German zoologist lately deceased, who argued for their removal from the Branchiopoda, does not venture to include them among the genuine Copepoda. In regard to their habits he is not quite at one ' Jourti. Northampton Nat. Hist. Soc, vol. ii. p. 292 (1882-83). 104