4 
THE ENTOMOSTRACA OE 
j ^ 
D 
p 
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 whieh is twenty-one. M. Audouin has demonstrated that each 
ring is composed of eight elementary pieces, and is divisible into two arcs, the superior 
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 
other pieees, the flancs or epimeral pieces. The inferior 
arc is similar in composition, having two sternal pieces in 
the median line, flanked by two episternal pieces, which 
meet the epimera. 
The carapaee 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, &e., the pieces which are 
analogous to the epimeral or lateral pieces of this cephalie 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 aeephalous molluscs.” (Milne Edwards, in Todd’s Cyclopaedia of Anatomy 
and Physiology, art. Crustacea.) 
These Entomostraca have two pairs of jaws, wuth 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 antennae,” 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 Cy there, are not protruded from the shell like the 
Diagram of the elementary parts of a 
“ ring" of the dermo-skeleton of Crustacea. 
D, dorsal arc ; t, t, tergal pieces ; e, e, 
epimeral pieces ; v, ventral arc ; s, s, s, s, 
sternal and episternal pieces ; p, p, inser- 
tions of the extremities. 
