OCR Text |
Show 130 CRUSTACEA. rostrum less salie~t; the valves narrower and less gibbous ~uperiorly, and gaping in front in such a manner as to present a Wtde and al. most circular opening. The antennre are much larger and have the appearance of being furnished with two horns bent _underneath, which are considered by Muller as the organs of generatton. Straus could not discove1· these sexual parts, but he remarks that the little nail ter•minating the last joint of the two antel'iot· feet-or the second, if we suppose the oars to be the fi1·st-is much larger than those in the female, that it has the form of a Yet·y large hook with a strong outward curvature, and that the seta of the third joint is also much Ionger; 'it is by means of these hooks that he seizes the female. The mammillre or the sixth segment of the abdomen are much smaller, and at an early age have the form of tubercles. The inferior antcnnre excepted, which are longest, the two sexes are nearly alike, and the two valves of theil· shell terminate in a stylet, dentated beneath, arcuated below, and neal'ly as long as the valves. Every time the animal changes its tegument, this stylet becomes shorter, so that in the adult it forms a mer·e obtuse point. The males pursue their females with much ardour, and several frequently unite in their advances to the same individual. A single copulation fecundates the female for several successive generations, and for a period of six months, as ascertained by Jurine. Straus, remarking that the orifices of the ovaries are placed very deeply under the valves and that consequently no part of the body of the male could reach them, suspects that he has no copulating organ, but darts the fecundating fluid under the valves of the female, whence it finds its way to the oval'ies; analogy however seems to dis· prove this conjecture( 1 ). J urine saw them in actu, for a period of eight or ten minutes. The male, first placing himself on the back of the female, seizes her with the long threads of his anterior feet; he then seeks the inferior margin of her shell, and approximating the aperture of his own to that of the latter, he introduces the threads, as well as the hooks of these same feet. He now brings his tail in contact with that of his companion, who at first, refusing to comply, flies with her amorous mate, but finally yields. Little granulated bodies of a gt·cen, rose, or brown colour, according to the season, gradually ascend into the matrix and become eggs. J urine observes, that the males of the D. pulex are but few, when compared to the • number of females; that they are extremely rare in spring and sum· · mer, but less so in autumn. About the ~ighth day after they are hatcht!d, the young Daphnia ( 1) See Jurine, Hist. des Mon. p. 106, et seq. BRANCHJOPODA. 131 • 't first ch;nge of tegument, and repeats the same process effects 1 s . . 1 d. . . l d e1·ery fi ~· e or six days, according to the mcreasec or Imlms 1e tempera ture of the weather; it is not. merely the body and valves wh.tc h 1o s e their et>idermis, the b.r anchHc and setre of. the .o ars under- . the same operation. It IS only after the thu·d change that gt heoyrn ga r e fitted to continue their species. At first the. female lays but a single egg, then two or three, gradually augmentmg the num-b Vhich in the D. magna amounts to fifty-eight. The day after er, ' . . she has produced her ova, the female changes her skm, and m the teguments which she abandons may be found the shells. of the eggs h has previously laid. The next moment a new batch 1s produced. Ts hee young from each set of eggs are generally of one sex, an d 1· t 1· s rare to find two or three males proceeding from that which pro.duced females, and vice versa. But in five or six of these brood's, m the summer, one at most consists of males. Individuals are frequently remarked whose integuments are of a milky white, opaque and thickened'; they do not however appear to be affected b~ it, and. on the renewal of the shell, but slight rugous traces of th1s alteration are perceptible. . . These animals cease to propagate, and no longer cast their skms on the approach of winter; they perish before the extr.eme cold h~s arrived. The ova contained in the ephippia, and which were la1d during the summer, are hatched on the first approach of the vernal heat; and the ponds soon abound again with countless Daphnire. Some naturalists attribute the occasionally sanguine tinge of these waters to the presence of myriads of the D. pulex, but Straus says he never remarked the fact, and that this species is at all times hut slightly coloured. Morning and evening, and even during the day in cloudy weather, they keep on the surface; but in the heat of summer, or when the sun darts his rays directly upon the pools which they inhabit, they descend to the depth of six or eight feet; frequently, not one is to be seen on the surface. Their mode of natation is by little bounds, of a greater or less extent, according to the length of their oar~, and in proportion to the projection of the shell which covers the bo~y, an increase of its size impeding their movements. According to Straus, their food consists exclusively of small parcels of vegetable substances which they find at the bottom, and frequently of Confervre. They always refused the animal substances he presented to them. He repeatedly saw them swallow their own freces, carried along by the current formed by the action of their feet, which directs their ordinary aliment towards their mouth. They use the hooks which terminate the extremity of theil· tail to clean their branchi~. Daphnia pulex; Monoculus pulex, L.; Pulex aquaticus arborea- |