EXTOMOSTKACA 



961 



to the tre.sh water, whilst others are marine. The fresh-water 

 species often abound in the muddiest and most stagnant pools, 

 as well as in the clearest springs. Of the marine species some 

 are to be found in the localities in which the Cy there is most 

 abundant, whilst others inhabit the open ocean, and must be col- 

 lected by the tow- net. The body of the Ci/dops is soft and gela- 

 tinous, and it is composed of two distinct parts, a thorax (fig. 720, a) 

 and an abdomen (), of which the latter, being comparatively slender, 

 is commonly considered as a tail, though traversed by the intestine, 

 which terminates near its 

 extremity. The head, which 

 coalesces with the thorax, 

 bears one very large pair 

 of antenna? (c), possessing 

 numerous articulations and 

 furnished with bristly ap- 

 pendages, and another small 

 pair (d) : it is also furnished 

 with a pair of mandibles or 

 true jaws and with two 

 pairs of ' maxilla?,' of which 

 the hinder pair is the longer 

 and more abundantly sup- 

 plied with bristles. The 

 legs (e) are all beset with 

 plumose tufts, as is also the 

 tail (/'. _/') which is borne at 

 the extremity of the ab- 

 domen. On either side of 

 the abdomen of the female, 

 there is often to be seen an 

 egg - capsule (B) ; within 

 which the ova, after be- FIG. 720. A, temale of Cyclops quadricornis : 



ing fertilised, undergo the ?> b ^'> b > iail '< c ' anterma; Aaatemnla; a 

 ,. ,, ., v , ieet ; t, plumose setse of tail. B, tail, with 



earlier stages ot their de- external egg-sacs. C, D, E, F, G, successive 

 velopment. The Cyclops is stages of development of young, 

 a very active creature, and 



strikes the water in swimming, not merely with its legs and tail 

 but also with its antenna?. The rapidly repeated movements of its 

 feet-jaws serve to create a whirlpool in the surrounding water, by 

 which minute animals of various kinds, and even its own young, are 

 brought to its mouth to be devoured. 1 



The tribe of Branchiopoda is divided also into two groups, of 

 which the Cladocera present the nearest approach to the preceding, 

 having a bivalve carapace, no more than from four to six pairs of 

 legs, two pairs of antenna?, of which one is large and branched and 

 adapted for swimming, and a single eye. The commonest form of 



1 See for British forms Professor G-. S. Brady's Monograph of the free and 

 semi-parasitic Copcpoda of the British Islands, published by the Ray Society, 

 1878-80, and Mr. I. C. Thompson's accounts of those collected near the Isle of Man, 

 published by the Liverpool Biological Society. 



3Q 



