74 
BRITISH ENTOMOSTRACA. 
which the animal deposits the eggs after laying them, 
and where they remain till hatched, and ready to be 
launched into the world. The seventh segment is pro- 
vided with two filaments, which have an articulation about 
the middle of their length, like those of the rami. In the 
last segment we perceive two dentated arches, between 
which is situate the anus. Beyond this it contracts in 
size, and terminates in two horny hooks, the last of which 
is the longest. The whole of the body, except the first 
segment, as I have already said, is free and unattached, 
and the insect can extend it beyond the valves at pleasure, 
the two hooks at its extremity serving well for enabling 
it to clear the interior of the valves. It seems also to clear 
the feet from any particles of mud or dust adhering to 
them ; and Schceffer thinks it may also assist in bringing 
before the mouth objects of food. He says also, that 
perhaps the motions of the insect are partly regulated by 
the strokes of this body, or tail, as he calls it, and certainly 
it is in almost constant motion when the animal swims. 
On the back, in the first segment of the body, we see an 
ovoid-shaped vesicle (t. VIII, f. b, i), possessed of very rapid 
contractions ; this is the heart. According to Jurine, there 
springs from its anterior extremity an arterial vessel, which 
contracts in an opposite manner to the heart itself, curves 
immediately from its origin, and goes backwards, follow- 
ing the direction of the intestinal canal. Gruithuisen 
describes the heart and circulation at greater length. He 
says, there are two hearts, one venous, the other arterial : 
the venous supplies the intestines and other parts of the 
body with blood ; the arterial supplies the head and parts 
connected with it, its branches making the circuit of the 
shell on the anterior edge, and collecting near the pos- 
terior inferior part into one large trunk, which runs along 
the back of the shell, and returns to the arterial heart 
again. 
The legs are five pairs, all differing in many respects 
from each other, and serving a different purpose than as 
organs of locomotion. The first pair (t. VIII, f. e) is 
