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Show 128 CRUSTACEA. bearded: if this be so, we do not see why these appendages may not concur in the process of respiration( 1 ), a property confined by Straus to the following ones, because the latter have, besides, a lamina on the inner side, which, with the exception of the two last, is edged with a pectinated series of set~, that according to the figures of Jurine and Randohr are also bearded. The structure of the two last feet is somewhat different, and Randohr distinguishes them by the name of claws. The abdomen, or body properly so called, is divided into eight segments perfectly ft·ee between its valves, and is long slender, recurved at the extremity, and terminated by two small hooks directed backwards. On the superior surface of the sixth segment is a range of four papillre forming indentations, and the fourth presents a sort of tail(2). The ovaries are situated along the sides between this segment and the first, and open separately near the back into a cavity-matrix, Jurine-formed betwixt the shell and the body, in which the ova remain for some time after they are produced. Muller has given the name of ephippium, or saddle, to a large, obscure! and rectangular spot, which at certain periods, and particularly m summer, appears, after the females have changed their tegument, on the superior part of the valves of the shell, and which he attributes to disease. According to Straus this ephippium presents two oval, diaphanous ampullre, placed one before the other and forming with those of the opposite side two small oval capsules,' opening like that of a bivalve. It is divided, as are also the valves of which it forms a part, into two lateral halves, united by a suture along their superior edge; its interior exhibits another similar, but smaller one, with ft·ee edges, provided it be not the superior that is attached to the valves, the two halves of which, playing upon each other as if hinged, present the same ampullre as the exterior lids. Each capsule contains an egg with a greenish and horny shell, other'~ ise sim~Iar to an ordinary ovum, but requiring a greater length of tlme for lts development, and being destined to pass the winter in stat.u q~o. When the animal is about to change its tegument, the eph1ppmm, as well as its ova, is abandoned with the exuvice of which it constitutes a part, and which protect them during 'the ( 1) Acco~ding to Straus, Cypris and Cytbere are not true Branchiopoda, inas· much as then· feet are ?ot provided with branchia:; but, as we have ah·eady ob· served, the seta: and hau·s of the two anterior ones and those of the antenna: may exercise the f~ncti~ns of br~nchire as well as those of the pal pi and first jaws. (2) We om1t var1ous deta1ls of the organization, because some can only be compr~ hended by means of drawings, and others appear common to most of the Bran· ch10poda. BRANCHIOPODA. 129 . f m the cold. The heat of spring hatches them, and ~oung ..,flnter ro . . t • f h · ·are produced exactly s1m1lar to t 1ose whtch come rom t e Da~hnlre ggs Schreffer affirms that they will remain for a long ordtnary e • . I . h . I' f 1 . d ·n a desiccated state w1thout osmg t e v1ta tty o t 1e germ, perto 1 . 1. . b J . Or those preserved 1n that cone 1t10n y urme was ever but none . b d They are entirely free, or do not adhere to each other m bate e • d · J · h h t e·tr pe cull'ar cavities. In summer, accor m. g to urme, .t ey may be hatched in two or three days. In the chmate of Par1s, '~here Straus observed them at all periods of the year, they requ1re at least one hundred hours. The fretus, twenty-four hours after the production of the ovum, is a mere rounded and unfor.med mass, on which, when closely examined, may be seen obtuse rudtments of arms · the form of very short and imperfect stumps glued to the body; 10 'ther head nor eye is perceptible; and as yet, the green or reddish ne1 . 1 · 1 body dotted with white, like the egg, exhibits no motiOn. t 1s on y at the ninetieth hour, and when the eye has .appeared, ancl the arms and valves are elongated, that the fretus begms to move. By the hundredth hour it is very active, and finally, at the hundred and tenth it only differs from the newly hatched anim~l in the s.etre of the oars which are still glued to their stem, and m the tail of the valves which is bent under and received between their inferior edges. Towards the end of the fifth day, the tail, which terminates the valves in the young animal, and the setre of the arms become ~ree, and the feet for the first time begin to move. The young bemg ready to make their appearance, the mother lowers her abdomen and they dart out. Newly laid eggs deposited in a glass jar, where they were observed by Straus, were developed in this order. Jurine has also furnished us with the result of his analogous observations upon the successive changes in the embryo Daphnire, but made during the winter, and as the eggs were not hatched till the tenth day, he could cousequently detect their development with more precision. The ovum, onthefirstday, presents a central bubble, surrounded by smaller ones, with coloured molecules in the intervals. These bubbles and molecules appear destined to form the organs by approximating to':"ards. the centre, and finally disappear. The form of the fretus begms to be defined on the sixth day; on the seventh the bead and fe~t are distinguishable; on the eighth appears the eye as well as. t~e mtes .. tine; on the ninth the network of that eye begins to be v1s1ble, and the bubbles have entirely disappeared, the . central one e~cepted, which contains the alimentary canal under the heart; on the ten~h the development of the fretus is terminated,, the y~ung Daphma. issues from the matrix and for a moment remams motionless. The males of those species at least observed by Straus, are very distinct from' the females. The hea.d is proporti.O nably shorter; t ll e VoL. III.-R |
