ENTOMOSTRACA '. — WATER-FLEAS. 639 



The Segments into which the hody is divided, are frequently very 

 numerous, and are for the most j part similar to each other ; but 

 there is a marked difference in regard to the Appendages which 

 they bear, and to the mode in which these minister to the locomo- 

 tion of the animals. For in the Lophyropoda, or Bristly-footed 

 tribe, the number of Legs is small, not exceeding five pairs, and 

 their function is limited to Locomotion, the Respiratory organs 

 being attached to the parts in the neighbourhood of the mouth ; 

 whilst in the Branchiopoda, or Gill-footed tribe, the same mem- 

 bers serve both for Locomotion and for Respiration, and the num- 

 ber of these is commonly large, being in Apus not less than sixty 

 pairs. The character of their movements differs accordingly; for 

 whilst all the members of the first-named tribe dart through the 

 water in a succession of jerks, so as to have acquired the common 

 name of ' Water-Fleas/ those among the latter which possess a 

 great number of ' Fin-feet,' swim with an easy gliding movement, 

 sometimes on their back alone (as in the case with Branchipus), 

 and sometimes with equal facility on the back, belly, or sides (as 

 is done by Artemia salina, the 'Brine Shrimp'). — Some of the 

 most common forms of both tribes will now be briefly noticed. 



498. The Tribe of Lophyropoda is divided into two Orders ; of 

 which the first, Ostracoda, 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 Cypris, which is a common inhabitant 

 of pools and streams ; this may be recognized by its possession of 

 two pairs of Antenna?, 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 appearance out- 

 side the shell, being bent upwards to give support to the ovaries. 

 The valves are generally opened sufficiently widely to allow the 

 greater part of both pairs of Antenna? 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 Antenna? 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 pro- 

 jecting out of the shell, and whose Superior Antenna? 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 Conferva? and Corallines. — There is abun- 

 dant 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 



