184 EVENINGS AT THE MICROSCOPE. 



the adult needs hardly be pointed out any further than 

 by referring to the figure. When they first escape they 

 rarely exceed half a line in length. The body is ovoid, 

 the dorsal shield large and inflated ; on its upper edge 

 and about the middle is a long spine, curved posteriorly, 

 and rather longer than the diameter of the bodv, though 

 it varies in length in different specimens; it is hollow, 

 and the blood may be seen circulating through it. The 

 upper portion of the body is sap-green, and the lower 

 semi-transparent. The eyes are large, sessile, and situ- 

 ated in front, and the circumference of the pupil is 

 marked with radiating lines. The lower margin f the 

 shield is waved, and at its posterior and lateral margin 

 is a pair of natatory feet. The tail is extended, longer 

 than the diameter of the shield ; and is composed of 

 five equal annulations, besides the terminal one ; its 

 extremity is forked, and the external angles are long, 

 slender, pointed, and attached to the last annulation by 

 joints. Between the external angles and on each side of 

 the median line are three lesser spines, also attached to 

 the last ring by joints. Between the eyes, and from 

 near the edge of the shield, hangs a long, stout, and 

 somewhat compressed appendage, which, as the animal 

 moves, is reflexed posteriorly between the claws. Under 

 each eye is another appendage, shorter and slightly more 

 compressed. The claws are in three pairs ; each is com- 

 posed of three joints, and terminates in four long, slender, 

 hair-like appendages. These claws are generally bent on 

 the body, but stand in relief from it. If the animal be 

 viewed in front, the lower margin of the dorsal shield 

 will be found to be waved into three semi-circular fes- 

 toons, the two external of which are occupied by the 

 eyes, and between which the middle one intervenes ; the 

 general direction of the claws will be seen to be at right 

 angles to the body. As the young lies inclosed within 

 the membranes of the egg, the claws are folded on each 



