This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1055). In the Cray-fish, and also in the Lobster, the secerning organs or testes, when examined in situ, are found to occupy the dorsal region of the thorax, lying upon the posterior part of the stomach.
(1056). Examined superficially, the testes would seem to form but one mass, consisting of three lobes (fig. 207, a a, b); but on investigating the minute structure of the organ, it is found to be made up of very delicate secreting-tubes that give origin to two excretory ducts (c c.) After numerous convolutions, which form a kind of epididymus (d), each duct, becoming slightly dilated, terminates by a distinct orifice (f), seen upon the basal articulations of the last pair of ambulatory legs. There is no intromittent apparatus visible; but, according to Milne-Edwards*, the extremity of the excretory duct, by undergoing a kind of tumefaction, may be protruded externally, so as to become efficient in directing the course of the fecundating fluid.
(1057). In Crabs, the mass of the testis is exceedingly large, but in its essential structure similar to that of the Cray-fish, and the external opening of its excretory duct is found to occupy the same situation; in some genera, however, instead of being placed upon the first joint of the last pair of legs, the orifices of the male organs are found upon the abdominal surface of the last thoracic ring itself.
(1058). In the male Brachyura1 the so-called false feet constitute the external sexual organs; and Mr. Bate has several times taken Carcinus mamas in the act of copulation, under which circumstances he distinctly saw these styliform processes inserted within the vulvae of the female. These false feet consist of two pairs, the largerbeing anterior, and attached to the first abdominal ring, - the less, or posterior, to the second ring. In all, except the edible Crab, the second pair is very small, apparently rudimentary, and lie with their extremities inserted posteriorly into the larger pair. But in Cancer pagurus, though slight, they are equally long with the first pair, and have a joint, peculiar to this Crab, situated near their centre, in addition to one, common to others, attached to the basal joint. The orifice of this pair is slightly frilled: it lies posteriorly against the first pair, which are the most important. These latter are styliform, and attached by a hinge to a calcareous continuation of the dermal membrane of the abdomen. From the first joint of the fifth pair of legs a membranous tube (the vas deferens) passes out and enters at the second joint of the so-called false feet, continuing through, and terminating at the apex in an oval slit.
Internally the tube is derived immediately from the testicle.
(1059). Mr. C. Spence Bate believes that Crabs have more than one brood to a single impregnation by the male, and that the male can only-impregnate the female immediately after the shedding of the exuviae. "For days previously the male may be seen running about and hiding himself under stones, holding the female by one or more of his legs, the carapace being pressed against the sternum of the male. In this relative position they continue until the female throws off her calcareous clothing, when connexion immediately follows, and continues perhaps for a day or two".
* Cyclop, of Anat. and Phys., art. "Crustacea".
1 "Notes on Crustacea," by C. Spence Bate, Ann.&Mag. Nat. Hist., 2 ser. vi. p. 109.
(1060). The female generative organs of Crustacea very accurately resemble those of the male; and in the unimpregnated condition it is not always easy, from a superficial survey of the internal viscera, to determine the sex. In Astacus fluviatilis, the ovaria (fig. 208, a) occupy a position analogous to that of the male testis, and a simple canal derived from each side (b, c) conducts the eggs to the external apertures found upon the first joint of the third pair of legs.
(1061). In Crabs, an important addition is made to the female generative system: - prior to the termination of each oviduct it is found to communicate with a wide sacculus, the function of which is apparently analogous to that of the spermatheca of insects (§ 886), inasmuch as it seems to form a receptacle for the fecundating secretion of the male, in which the seminal fluid remains, ready to impregnate the ova as they successively pass its orifice during their expulsion from the body.
(1062). The eggs are almost invariably carried about by the female until they are hatched, and in order to effect this, various means are provided. In the Decapoda they are fastened by a stringy secretion to the false feet under the abdomen; and a female Crab may generally be readily distinguished from a male of the same species by the greater proportionate size of this part of their body.
(1063). Many of the Decapod Crustaceans, as, for example, the Crayfish (Astacus fluviatilis), do not not seem to undergo material alterations of form, but simply moult at certain intervals, throwing off their old integument and acquiring a new covering. Nevertheless, even in the Decapoda it is certain that great metamorphoses take place in the external appearance of the young animals. Cavolini long since announced that the embryo of Cancer depressus exhibited at birth a singular and uncouth appearance, of which he gave a very tolerable representation*; and Mr. Thompson has rendered it certain that, even in the development of the common Crab, so different is the outward form of the newly-hatched embryo from that of the adult, that the former has been described as a distinct species, and even grouped among the En-tomostraca, under the name of Zoea pelagica. On leaving the egg, the young Crab presents a curious and grotesque figure (fig. 209): its body is hemispherical, and its back prolonged upwards into a horn-like appendage; the feet are scarcely visible, with the exception of the last two pairs, which are ciliated like those of a Branchiopod, and formed for swimming. The tail is longer than the body, possesses no false feet, and the terminal joint is crescent-shaped and covered with long spines. The eyes are very large, and a long beak projects from the lower surface of the head.