250 Mr. Baird on British Entomostraca. 



or abdomen, is quite free and unattached within the valves of 

 the shell. It is slender and long, and is divided, according to 

 Straus, into eight segments, the first of which is the largest, 

 and is the only one which is attached to the valves. At the 

 second segment the abdomen suddenly diminishes in vertical 

 diameter, sinking down and leaving above a strong projection, 

 formed by the first segment. From this projection, through- 

 out the rest of its extent, the body is unattached to the shell, 

 and leaves a vacant space between it and the edge of the 

 valves, into which the insect deposits the eggs after laying 

 them, and where they remain till hatched and ready to be 

 launched into the world. The seventh segment is provided 

 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 ter- 

 minates 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 ex- 

 tend 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 SchaefFer 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 insect 

 swims. On the back of the insect, in the first segment of 

 the body, we see an ovoid-shaped vesicle, possessed of very 

 rapid contractions ; this is the heart (plate ix. fig. 4.) Ac- 

 cording to Jurine, there springs from its anterior extremity 

 an arterial vessel (a), which contracts in an opposite manner to 

 the heart itself, curves immediately from its origin, and goes 

 backwards, following the direction of the intestinal canal. 

 Gruithuisen describes the heart and circulation at greater 

 length. He makes two hearts, one venous, the other arte- 

 rial : 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 



