542 CRUSTACEA. 



latter whicli possess a great number of " fin-feet," swim with an 

 easy gliding movement, sometimes on their back alone (as is the 

 case with BrancMpus), sometimes with equal facility on the back, 

 belly, or sides (as is done by Artemia mlina, the " brine shrimp"). 

 Some of the most common forms of both tribes will now be 

 briefly noticed. • 



367. The tribe of Lophyropoda is divided into two orders ; of 

 which the first, Ostraooda, is distinguished by the complete en- 

 closure of the body in a bivalve shell, by the small number of 

 legs, and by the absence of an external ovary. One of the best- 

 known examples is the little Oypris, which is a common inhabi- 

 tant of pools and streams ; this may be recognized by its posses- 

 sion of two pairs of antennae, the first having numerous joints 

 with a pencil-like tuft of filaments, and projecting forwards 

 from the front of the head, whilst the second has more the shape 

 of legs, and is directed downwards; and by the limitation of its 

 legs to two pairs, of which the posterior does not make its ap- 

 pearance outside the shell, being bent upwards to give support 

 to the ovaries. The valves are generally opened sufficiently 

 wide, to allow the greater part of both pairs of antennae, and of 

 the front pair of legs, to pass out between them ; but when the 

 animals are alarmed, they draw these members within the shell, 

 and close the valves firmly. They are very lively creatures, 

 being almost constantly seen in motion, either swimming by the 

 united action of their foot-like antennae and legs, or walking 

 upon plants and other solid bodies floating in the water. Nearly 

 allied to the preceding is the Cythere, whose body is furnished 

 with three pairs of legs, all projecting out of the shell, and 

 whose superior antennae are destitute of the filamentous brush ; 

 this genus 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, creeping about (but not swimming) 

 amongst Confervse 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 certain 

 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 de- 

 posit, the remains of bivalve shells resembling those of Cythere, 

 present themselves in such abundance as to form a considerable 

 part of its composition. In the order Copepoda, there is a jointed 

 shell forming a kind of buckler that almost entirely encloses the 

 head and thorax, an opening being left beneath, through which 

 the members project ; and there are five pairs of legs, mostly 

 adapted for swimming, the fifth pair, however, being rudimentary 

 in the gemis Cyclops, the commonest example of the group. 

 This genus receives its name from possessing only a single eye. 



