74 BRITISH ENTOMOSTRACA. 



which the animal deposits the eggs after laying them, 

 and where they remain till hatched, and ready to be 

 lannched 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 SchoefFer 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. VHI, f. e) is 



