On the Anatomy of Lei^dora hyalina. By H. E. Forrest. 831 



IV. Muscles. 



Probably in no otter animal is muscular action and structure 

 displayed more beautifully than in Leptodora, for on account of its 

 transparency the muscles may all be seen in situ without any dis- 

 section. The most powerfully developed are those of the antennae, 

 which are shown from a dorsal aspect in Fig. 2, /, and from beneath 

 in the larger drawing (Fig, 3, a). The immense power of these 

 muscles requires that the point of attachment should be very 

 firm, but the body wall of the thorax is much too soft to bear 

 the strain, so instead of being attached to it, they are fastened 

 (Fig. 2, v) to the ends of two ligaments which pass right 

 through the thorax, and are only fastened to the body wall by 

 threads of connective tissue. Thus the two opposite antenna- 

 muscles actually pull against one another, and the whole strain is 

 borne by the ligament between them. The six pairs of legs are 

 each provided with a muscle, and their ends too are attached to 

 ligaments between their bases (Fig. 2, w), and the right and left 

 sides pull against one another. A large flat muscle runs down each 

 side of the body, and is attached to each somite, the final attach- 

 ment being to the sides of the terminal somite, almost at the end of 

 the stomach (Fig. 2, r). The rectum (s) is opened and closed by 

 a set of sphincter muscles, and so is the pharynx, as I have already 

 mentioned. The muscles which move the lips are attached at one 

 end to the lips, at the other to the ventral or dorsal sides of the 

 head (see Fig. 1). The eye has six muscles fastened to it at six 

 diiferent points, about equal distances from one another (Fig. 11, d). 

 These are constantly m motion, causing the eye to perform a 

 quarter of a revolution in one direction, then to revolve the same 

 distance in the opposite direction, and this movement does not 

 seem to be under the control of the animal, as it is as incessant, 

 though not so regular as the beating of the heart. The muscles 

 which encircle this last-named organ are distinctly striate (Fig. 12), 

 and distributed over its surface in irregular loops. 



V. Sexual Differences. 

 In Leptodora, as in most of the Cladocera, the female is larger 

 than the male. In August, when I first began to collect them, 

 the females were much more numerous than the males ; but later 

 on the males appeared in goodly numbers, and finally, in October, 

 there were as many males as females, if not more. Now (No- 

 vember) both sexes have entirely disappeared, and I much regret 

 that I am not able to give a drawing of the ovaries, as I neglected 

 to mount a female specimen, supposing I should be able to procure 

 them alive at any time. I have seen them, however, yerj often, 

 so am not at a loss how to describe them. They are elongated 



