4 THE ENTOMOSTRACA OF 



These are divided by Latreille into two orders, the Lophyropa and Phyllopa ; the 

 former of which he has subdivided into three sections, Cladocera, Ostracoda^ and 

 Copepoda, the types of which are the common Daphnia (water-flea), Cypris, and 

 Cyclops of the ponds and ditches. It is to the second of these sections that our atten- 

 tion is at present directed. 



The recent genera of this section have been well defined, their distinctive characters 



being taken from the number and position of the limbs, and other soft parts of the 



animal. The Ostracoda have a bivalve shell or carapace, the valves being united 



at their dorsal margins by a ligamentous hinge and muscles, and which, when 



shut, perfectly inclose the body and limbs of the animal. This peculiar carapace, 



although resembling in general form and in its adaptation to the animal the bivalve 



shell of the Conchifera, is essentially different from it, being analogous to the carapace 



or large dorso-thoracic tegumentary piece of the decapodous Crustaceans. We may 



remark that the framework or solid parts of the Crustacea consist of a series of rings, 



the normal number of which is twenty-one. M. Audouin has demonstrated that each 



ring is composed of eight elementary pieces, and is divisible into two arcs, the superior 



^ f. or dorsal and the inferior or ventral, each arc being formed 



^ ° ^ of four pieces. The tergum, or upper surface of the dorsal 



"^^ # arc, is formed of two of these pieces united in the median 



^^■^ line, and the superior arc is completed at the sides by two 



Diagram of the elementary parts of a otlicr picccs, the flancs or epimerul pieces. Thc inferior 



ring of the aermo-skeleton of Crustacea. t. ■> ^ j- j. 



D, dorsal arc ; t, t, tergal pieces ; e, e, Qxc is similar in Composition, having two sternal pieces in 



epimeral pieces ; v, ventral arc is.s.s.s,.-, ■,. ■,• n ^ t i , • i ? • i • i 



sternal and episternai pieces ;p,p,inser- the median Ime, flanked Joj two eptstemal pzeces, which 



tions of the extremities. ■, ,■, 



meet the epimera. 



The carapace of Crustacea is generally formed from the superior arc of the third or 

 fourth cephalic rings of the tegumentary skeleton by excessive development of either 

 the tergal or the epimeral pieces. "In Limnadia, Cypris, &c., the pieces which are 

 analogous to the epimeral or lateral pieces of this cephalic buckler acquire a great 

 extension, whilst the tergal portion of the arc to which they belong continues rudi- 

 mentary or proves entirely abortive, so that they constitute two large valves, containing 

 the whole body of the animal, and bearing a considerable resemblance to the shells of 

 certain acephalous molluscs." (Milne Edwards, in Todd's Cyclopaedia of Anatomy 

 and Physiology, art. Crustacea.) 



These Entomostraca have two pairs of jaws, with a pair of mandibles, and a lower 

 lip or sternum (so called), two pairs of antennae, the lower pair being denominated by 

 Milne Edwards the " pediform antennge," two or three pairs of feet, and a tail formed 

 of two laminae. The superior antennae are plumed and natatory, and in some genera 

 the pediform antennae are likewise adapted for swimming ; but in others these inferior 

 antennae are unprovided with setae and like true legs are used only in creeping. The 

 posterior pair of legs, except in Cythere, are not protruded from the shell like the 



