208 



THE MICROSCOPE AND ITS REVELATIONS. 



FIG. 411. 



is almost entirely marine, and some species of it may almost invariably be 

 met-with in little pools among the rocks between the tide-marks, creep- 

 ing about (but not swimming) amongst Confervas and Corallines. There 

 is abundant evidence of the former existence of Crustacea of this group, 

 of larger size than any now existing, to an enormous extent; for in cer- 

 tain fresh-water strata, both of the Secondary and Tertiary series, we find 

 layers, sometimes of great extent and thickness, which are almost entirely 

 composed of the fossilized shells of Cyprides; whilst in certain parts of 

 the Chalk, which was a marine deposit, the remains of bivalve shells re- 

 sembling those of Cythere present themselves in such abundance as to 

 form a considerable part of its substance. 



605. In the order Copepoda, there is a jointed shell forming a kind 

 of buckler or carapace that almost entirely incloses the head and thorax, 



an opening being left beneath, 

 through which the members pro- 

 ject; and there are five pairs of 

 legs, mostly adapted for swimming, 

 the fifth pair, however, being rudi- 

 mentary in the genus Cyclops, the 

 commonest example of the group. 

 This genus receives its name from 

 possessing only a single eye, or 

 rather a single cluster of ocelli; 

 which character, however, it has in 

 common with the two genera already 

 named, as well as with Daphnia 

 ( 606), and with many other En- 

 tomostraca. It contains numerous 

 species, some of which belong to 

 fresh- water, whilst others are ma- 

 rine. The Fresh- water species often 

 abound in the muddiest and most 

 stagnant pools, as well as in the 

 clearest springs; the ordinary water 

 with which London is supplied fre- 

 quently contains large numbers of 

 them. Of the marine species, some 

 are to be found in the localities in 

 which the Cythere is most abun- 



mose setae of tail; B, tail, with external egg- dailt, whilst OUiei'S inhabit the Open 



SSt^'ySdt!' ' successive sta * es of develop - ocean, and must be collected by the 



Tow-net. The body of the Cyclops 



is soft and gelatinous, and it is composed of two distinct parts, a thorax 

 (Fig. 411, a) and an abdomen (), of which the latter, being compara- 

 tively slender, is commonly considered as a tail, though traversed by the 

 intestine which terminates near its extremity. The head, which coa- 

 lesces with the thorax, bears one very large pair of antennae (c), possess- 

 ing numerous articulations and furnished with bristly appendages, and 

 another small pair (c?); it is also furnished with a pair of mandibles or 

 true jaws, and with two pairs of 'feet-jaws,' of which the hinder pair is 

 the longer and more abundantly supplied with bristles. The legs (e) are 

 all beset with plumose tufts, as is also the tail (/,/) which is borne at 

 the extremity of the abdomen. On either side of the abdomen of the 

 female, there is often to be seen an egg-capsule or external ovarium (B); 



