Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/677

CRUSTACEA Darwin has found small males parasitic on the female which he has named " complemental males." They are destitute of a mouth, and appear to exist only for the per formance of this one function of reproduction (Darwin, Cirripedia, Ray Soc. 1851). Bilateral symmetry generally prevails among the mem bers of this class, and as a consequence we find always a pair of these organs arranged one on either side of the body, perfectly distinct, and often wholly independent of each other. The male is provided with a paired gland or testes, and two excretory ducts, by which the sper matozoa are discharged on reaching the efferent open ings, usually situated on either side in the basal joints of the seventh pair of thoracic appendages, or the first pair of abdominal limbs. In both the Crab and Lobster the first pair of abdominal appendages of the male are epscially modified to take part in the process of fecundating the female. Milne-Edwards denies that these appendages have any claim to be considered as fulfilling the office of conveying the fecundating fluid to the body of the female, but Sperice Bate has frequently taken Carcinus mcenas with these styliforin appendages deeply inserted within the vulvaa of the female. He has also shown the existence of a vas defer ens in these false feet (Ann. and Mag. Nat. Hist., 2nd series, vol vi., p. 109). * The ovaries in the crab resemble four cylindrical tubes placed longitudinally in the thorax, and divided into two FIG. 23. Side view of Crab (Morse), the abdomen extended and carrying a muss of eggs beneath it ; e, eggs. symmetrical pairs, each opening into a distinct oviduct, yet communicating with each other by a transverse canal and by the intimate union of the two posterior tubes. The oviducts and ovaries are of a whitish colour, and become united to a kind of sac 1 on each side, the neck of which opens externally in the sternal pieces of the fifth thoracic somite, which bears the third pair of walking appendages. In the Anomoura and Macroura there are no copulatory pouches, and the vulvse open on the basal joint of the third pair of ambulatory legs. It is possible, therefore, that in these forms the fecun dation of the ova does not take place until the eggs are actually ex truded, which we know to be the Case in Limu- FIG. 24. A, a few eggs of (he Common Crab enlarged ; B, a single egg greatly enlarged, showing more plainly the hardened thread (/) by which they are attached to each other. This egg also shows the commencement of the development of the embryo. (Morse s Zoology.) las, and probably in s^me other forms, and as is also the case in fishes. If we except Gecarcinus, certain other land-crabs, and 1 The cvpulatory pouches of Milne-Edwards. Limulus, the female does not abandon her eggs after their extrusion. Those of the Decapoda when extruded are coated with a viscous secretion which thickens into threads, and causes the eggs to adhere to each other and to the fine hairs with which the swimmerets of the abdomen of the lobster and the female crab are fringed (fig. 23). Fig. 24 shows the method of attachment of the eggs. Here they are retained securely until the period of hatching has arrived, when the brood in most cases is dispersed. This is not, however, always the case, for whilst examin ing a female Dromia from Australia, the writer discovered more than a dozen young ones adhering to the false abdominal feet of the parent, the young, except in size, agreeing perfectly with the parent. In Mysis the two endopodites of the hinder pair of thoracic feet in the female are developed into a broad plate on either side, and bent under the sternum, thus forming together an incubatory pouch or marsupium, in which the eggs are first deposited, and within which the young are secluded during their minority. In Thysan- opoda the eggs and young are contained in a pair of oval sacs de pendent from the posterior feet, forcibly reminding one of the ovarian sacs in Cyclops, In the Amphipoda the ova are nurtured by the female within a pouch formed by a series of foliaceous plates, one of which is at tached to each of the four anterior pairs of legs of the thorax. In the genus Podoccrus the parent builds a nest in a very bird-like manner, amid the branches of the submarine zoophyte forests, and in one of these Mr Spence Bate met with two broods of different ages, clearly demonstrating that the maternal care for their young is continued long after birth. 2 Similar ovigerous plates are devel oped in the fore-legs of females of the Isopoda. In all these sessile- eyed forms the parent seems specially solicitous for the safety of its young. In Asellus, Talitrus, and Gammarus, they appear to quit the maternal pouch and return to it as to a place of safety. Cap- rella carries its young attached to its body ; the female Archincs supports them adhering to her large antennae. In Daphnia, besides the several groups of ova which are succes sively hatched within the bivalved shell, and excluded during the spring and summer, giving rise to fertile females, there is formed each autumn an opaque layer within the incubatory cavity of the female, which hardens in two pieces like a small bivalved-shell, and is called the ephippium or saddle, and is placed on the dorsal surface of the Daphnia,, but within the shell of the parent. Another structure, similar to the cphi ppium, and called the "internal ephip pium," placed within it, is found to contain two bivalved capsules, in each of which a fertilised egg is lodged, which remains in a pas sive state through the winter, but hatches by the first warmth of spring, giving rise to females only (no males being hatched till autumn), these females in turn giving rise also to as many as six generations of fertile females. Their fecundity is so great as to be almost beyond the power of figures to express. 3 DEVELOPMENT OF THE CRUSTACEA.- In nearly all the Crustacea, the young undergo a series of metamorphoses after quitting the egy. This rule appears to apply more constantly among the truly marine forms. Among the stalk-eyed Crustacea some few species at least quit the egg in the form of their parents, with the full number of jointed appendages to the body. This is the case, according to llathke, 4 with Astacus fluviatilis, the common river cray fish (fig. 25), and according to Westwood 5 in a West Indian land crab (Gecarcinus). The present writer has 2 Spence Bate, 1858, Annals and Mag. Nat. Hist., and Bate and Westwood, Sessile-eyed Crustacea, vol. i. pp. 443-. 3 Baird, British Entomostraca, p. 78. 4 Kathke, Untersuchungen tiler die Bildung und Entioickelung des Flusskrebses, fol. 1820. 5 J. 0. Westwood in Phil. Trans. 1835, vol. cxxv. p. 311. Fritz Mtiller remarks, in reference to Westwood s paper This is a solitary exception of a single species investigated by Westwood. In the same genus Vaughan Thomson found zoea-brood, which has also been met with in other terrestrial crabs (Ocypoda and Oelasimus) The mode of life is in favour of Thomson. "Once a year," says Troschel, "they migrate in great crowds to the sea in order to deposit their eggs, and afterwards return, much exhausted, towards their dwelling-places, which are reached only by a few." For what purpose would be these destructive migrations in species whose young quit the etrg and the mother as terrestrial animals Fritz Miiller, Fads and Arguments fur Darwin, translated by W. S. Dallas, F.L.S.. 1869,

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