ORDER OSTRAPODA ; CYPRIS. 271 



to give support to the ovaries. The food of these little animals 

 consists of dead (but not putrid) animal matter, conferva, &c. ; 

 but they will not attack living animals that are well and strong, 

 although they are often seen to attack worms, &c., when 

 wounded and weak, and even to prey on one another. When the 

 ponds and ditches in which they live dry up in summer, they 

 bury themselves in the mud, and thus preserve their lives as long 

 as the mud retains any moisture, becoming as active as ever 

 when the rain falls, and again overflows their habitations. After 

 long-continued droughts, however, when the mud becomes very 

 dry and hard, they perish ; but the race is then kept up by the 

 eggs, which are capable of resisting this influence. These little 

 creatures are very lively ; being almost constantly seen in 

 motion, either swimming by the united action of their antennae 

 or anterior feet, or walking upon plants and other solid bodies 

 floating in the water. When alarmed, they draw their antennaa 

 and legs within the shell, and close its valves so firmly, that 

 there is no possibility of opening them. An allied genus, Cythe- 

 rina, distinguished by the possession of three pairs of legs, is an 

 inhabitant of salt water. 



809. There is abundant evidence of the former existence of the 

 minute Crustacea of this group, to an enormous extent ; and 

 their size was much greater. The largest existing species of 

 Cypris does not exceed one-sixteenth of a line in length. But 

 in certain strata of the Secondary and Tertiary formations, which 

 appear to have been deposited by fresh water, we find layers, 

 sometimes of great extent and thickness, which are almost en- 

 tirely composed of the fossilized shells of Cyprides, many of 

 them exceeding a line in length ; and in the Chalk, which was a 

 marine deposit, the remains of bivalve Crustacean shells, proba- 

 bly belonging to the genus Cytherina, are frequently to be 

 found in great abundance. 



