Page:VCH Leicestershire 1.djvu/137

 CRUSTACEANS which the outer branch is unjointed. In one genus, however, Chonopeltis, Thiele, the first antennae are wholly wanting, contrary to the character almost universal in the crustacean class. But this genus shares with Argulus the peculiarity that the second maxillae are transformed into suckers. There is a twofold reason for speaking of this as a case of transmutation, because in the third genus, Do/of s (Audouin), these maxillae form, not suckers, but strong hooks, and in the larval Argulus foliaceus the transition stage has been observed from a claw to a sucking-disk. Mr. Wilson very justly suggests that though the uncinate form of maxilla anchors the parasite strongly to its host, the suckers are a better contrivance, because they are so much more easily fastened and unfastened. By alternately holding fast with the sucker on one side while that on the other is released and advanced, the owner is enabled to move rapidly and yet securely over the surface of a fish. Whether A. foliaceus (Linn.), which is said to be generally distributed, actually occurs in Leicestershire I am not in a position to affirm, but of the larger and seemingly much rarer A. coregoni (Thorell), Mr. Scourfield says, 'Canon Norman possesses specimens of this species, which were taken by Mr. Dodds on the Barbel, in Leicestershire ; it has not previously been placed on record as British.' 9 While A. foliaceus in the female attains the length of only six or seven millimetres, and has an elliptical carapace, the lobes of which reach the fourth pair of legs, this sex in A. coregoni is thirteen millimetres long, fully half an inch, with a nearly orbicular carapace, which leaves the fourth pair of legs entirely uncovered. The smaller male has a still more rounded carapace completely covering all its legs. The respiration appears to be dependent on the general surface, rather than on any special appendages. It should be noticed that the movable eyes, the habit of depositing the eggs instead of carrying them about in ovisacs, and the position of the sucker- disks, are characters strongly differentiating the Argulidae from all uncon- troversial families of parasitic Copepoda. The genus Argulus is also dis- tinguished by having a venomous stilet in its oral siphon, which apparently prevents a fish from retaliating on its persecutor, when otherwise opportunity offers for swallowing it. To prevent the destructive multiplication of these little vampires in closed waters, Mr. Wilson points out that in such vivaria it is inexpedient to keep only valuable fishes. It is the little insignificant kinds that feed with useful voracity on the larval Argulus The antlered Cladocera owe their title to the two-branched setiferous second antennae, which are their swimming organs. Mr. Garnar's list of the species taken in the immediate neighbourhood of Leicester was sent, in a letter dated 6 April, 1903, to Mr. Scourfield, who has kindly supplied a copy of it annotated by himself for use in this chapter. The number of species is twenty-four, Seddington Reservoir being specified as the locality for three of them, namely, Macrotbrix /aticornis, Ilyocryptus sordidus^nft. Lathonura rectirostris* Mr. Scourfield's own collections were made in the Charnwood Forest district at Whitsuntide, 1906, the localities examined being ' Groby Pool, ponds near Beacon Hill and Bardon Hill ; Old Fish Pond, Grace Dieu Priory, and various little roadside and farmyard ponds, &c.' Sixteen species are named in this list, seven of them additional to those examined by the late 9 Journ. Queketl Micros. Club, 40, April, 1 904. 10 Proc. U.S. Nat. Mus. xxv, 652. 99