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. Miiller 

 has taken notice of them in the pvbera, 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. \d] 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 antennas, 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. \b}. 

 In general they consist of seven articulations,* from the 



* Jurino says eight. 



