518 History of British Entomostraca. 
clature and descriptions. The body of the animal is completely 
inclosed within a horny shell of two valves, which in general ap- 
pearance resembles very considerably that of the mussel, so much 
so, that, as Muller says, a person at first sight of the insect, would 
suppose that it was a parasite inhabiting the shell of some small 
mollusque. The substance of these valves is compact and very 
brittle, and they seem to be endued externally with a species of 
varnish, to protect them from the action of the water, as, whenever 
they rise to the surface, the shell becomes perfectly dry and floats 
there in spite of the animal's struggles to again immerse itself. 
The valves are open in their whole circumference, except in the 
middle third of the dorsal surface, where they are united by a liga- 
mentous hinge and muscles, by which the animal can open and shut 
the shell at pleasure. About the middle of each valve, in most 
species, are to be seen a number of small lucid spots, the use of 
which I do not know. Muller has taken notice of them in the pw- 
bera, and asks, " an ovula ?" but there is no connection between 
them and the ova. No other author has taken notice of them ; 
they are to be met with in perhaps all the species. 
The body of the insect (Plate XVI. Fig. 1,) is composed of two 
rounded lobes of unequal size, connected together by a narrow space, 
and having on their upper surface a transparent body, which is the 
matrix according to Jurine. From the anterior lobe of the- body 
(Fig. 1, ,) spring the two antennae, immediately above which is si- 
tuate the eye ; the anterior or first pair of feet ; the organs of the 
mouth, and the second pair of feet. From the posterior lobe, (Fig. 1 . b) 
spring the third pair of feet and the tail. Eye (Fig. 1. c) single, fixed, 
in the form of a black sessile tubercle, in which we can discover no 
traces of crystallines. Antennae (Fig. 1, d. Fig. 2,) two, inserted 
immediately below the eye,, and rising near each other. In general 
they consist of seven articulations, * from the three or four terminating 
ones of which arise several pretty long filaments, vary ing in number in 
the different species. In the larger species we see these filaments 
to be beautifully plumose, a circumstance which has never been 
pointed out by any of the various authors who have written upon 
this genus. Whenever the animal moves, it invariably puts these 
organs into rapid motion, dilating and bringing together again the 
long filaments which spring from near their extremity, and waving 
them to and fro with great rapidity. They are thus said by Mul- 
ler and Straus to act as true fins, and to be the principal organs of 
* Jurine says 8. 
