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 



