REPRODUCTIVE ORGANS. - The organs of generation are easily to be discerned in most of the Crustacea, but the analogy between these parts in the male and female is so great in many genera as to need the most careful examination in order to discriminate between the two. Generally, however, the males may be discerned by their having cue or more pairs of limbs especially modified to assist in the marital act.
With the exception of the Cirripedia the two sexes appear never to exist together in the same individual among the Crustacea.
The small size and great dissimilarity of the males of some of the parasitic genera caused them to remain long unknown, and led to the error of supposing the females to be hermaphrodite, as Darwin has shown to be really the case in the Cirripedes. But even in this division 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 performance of this one function of reproduction (Darwin, eirripedia, Ray Soc. 1851).
Bilateral symmetry generally prevails among the members 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 spermatozoa are discharged on reaching the efferent openings, 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 specially 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 Spence Bate has frequently taken Carcinus mamas with these styliform appendages deeply inserted within the vulvm of the female. He has also shown the existence of a vas deferens in these false feet (Ann. and hag. Nat. Hist., 2nd series, vol vi., p. 109). • The ovaries in the crab resemble four cylindrical tubes placed longitudinally iu the thorax, and divided into two
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' on each side, the neck of which opens externally in the sternal pieces of the fifth thoracic smite, which bears the third pair of walking appendages.
In the Anomoura and Macroura there arc no copulatory pouches, and the vulvm open on the basal joint of the third pair of ambulatory legs. It is possible, therefore, that in these forms the fecundation of the ova does not take place until the eggs are actually extruded, which we know to be the case in Limn- : lets, and probably in same other forms, and as is also the case in fishes.
arrived, when the brood in most cases is dispersed.
This is not, however, always the case, for whilst examining 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 Thysanopoda the eggs and young arc contained in a pair of oval sacs dependent 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 attached to each of the four anterior pairs of legs of the thorax. In the genus Podoceras 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 of 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 Cammarus, they appear to quit the maternal pouch and return to it as to a place of safety. Cap-relic carries its young attached to its body ; the female Arcturus supports them adhering to her large antenna.
In Daphnia, besides the several groups of ova which are successively 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 ephippiuns or saddle, and is placed on the dorsal surface of the Daphnia, but within the shell of the parent. Another structure, similar to the cploppium, and called the "internal ephippiurn," placed within it, is found to contain two bivalved capsules, in each of which a fertilised egg is lodged, which remains in a passive 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 Lo almost beyond the power of figures to express.3