144 
BRITISH ENTOMOSTRACA. 
The body of the animal is completely inclosed within 
a shell of two valves, of a horny-cretaceous substance, 
which in general appearance closely resembles that of a 
very small mussel, so much so that, as Midler remarks, a 
person at first sight of this insect would suppose that it 
was a parasite inhabiting the shell of some small mollusc. 
The substance of these valves is compact and very brittle, 
and seems 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 per- 
fectly 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 ligamentous 
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 pub era, and asks “ an 
ovula?” but there is no connexion between them and the 
ova. No other author has mentioned them ; they are to 
be met with perhaps in all the species. 
The body of the insect (t. XVIII, f. la) consists of two 
rounded portions of unequal size, connected together by 
a narrow space, and having on their upper surface a 
transparent body, which is the matrix. From the anterior 
or thoracic portion spring the two superior antennae, im- 
mediately above which is situate the eye, the inferior 
antennae, the organs of the mouth, and the first pair of 
feet. From the posterior or abdominal portion spring 
the second pair of feet and the tail. 
The eye is single, fixed, and in the form of a black 
sessile tubercle, in which we can discover no traces of 
crystallines. 
The superior antennae are inserted immediately below the 
eye, and have their origin near each other (t. XVII, f. 1$). 
In general they consist of seven articulations,* from the 
* Jurine says eight. 
