OSTBACODA 353 



organs are distinguished by their compactness. A heart is usually absent. 

 A single median eye or a pair of eyes close together is usually present, 

 but the Cypridinidae have an additional pair of large compound eyes. 

 Ostracods are unisexual animals, most of which lay eggs which they either 

 attach to water plants, as in the case of Cypris, or, as in Cypridina, carry 

 between the shells until they hatch. Cypris and certain other genera are par- 

 thenogenetic, in certain species no males having yet been discovered. The 

 Cypridae and Cytlicridae are born as nauplii, in which the bivalved shell 

 is already present; the other ostracods are bom later than the nauplius 

 stage. The Ostracoda live on or near the bottom of both salt and fresh 

 water, where they crawl or swim actively about and feed principally on 

 small animals. Entocythere is parasitic on the crayfish. The order con- 

 tains 2 suborders and more than 1,400 species, of which the gi'eat majority 

 are marine, the Cypridae being the only family which lives almost exclu- 

 sively iji fresh water. 



Key to the subordei's of Ostracoda: 



Oi Second antennae apparently uniramose 1. Podocopa 



O2 Second antennae biramose, one branch large, the other minute ; marine. 



2. Myodocopa 



Suborder 1. PODOCOPA.* 



Second antennae apparently uniramose and bearing sharp bristles, 

 which are of use in swimming or walking; heart absent; shell without 

 antenna! sinus: 2 families; mostly in fresh water. 



Key to the families of Podocopa: 



Ci The 2 pairs of thoracic feet dissimilar, second pair bent back. . . .1. Cypridae 

 Ca The 2 pairs of thoracic feet and the second maxillae all locomotory and 



similar 2. Cytheridae 



Family 1. CYPRIDAE. 



Shell thin; first antennae 8-jointed, but appearing 6 or 7-jointed, and 

 with long bristles; second antennae apparently uniramose, the exopodite 

 being a minute plate with 3 bristles, and leg-shaped and 3 to 6-jointed, 

 with several long natatory bristles on the second and also the last 

 joint; usually a single eye present; manibular palp 4-jointed; first max- 

 illa with a 2-jointed palp; second maxilla small, with a short palp, which 

 is prehensile in the male; first pair of legs locomotory, the second bent 

 back and not locomotory; abdomen with a furca; genital organs large, 

 extending into the space between the shells: about 350 species, mostly 

 in fresh water. 



* See "Contributions," etc., by R. W. Sharpe, Bull. 111. St. Lab., Vol. 4, 1897. 



