With regard to the modifications presented by the respiratory organs of the Onisci, which like the Gecarcini live far from water, nothing certain is yet known. The opinion that the abdominal false limbs, which serve as respi ratory organs among the Isopoda in general, are here vesicular, and perform the office of lungs internally, whilst their external surface acts in the manner of gills, still requires to be confirmed.
§ 6. Of Generation.
Sexual organs are readily demonstrated in the whole class of Crustaceans, but those of the two sexes never exist in the same indivi dual. The doubt which at one tirne pre vailed in regard to this fact, and which mainly arose from no other than females of certain species having ever been taken, is at once put an end to by the circumstance of the con siderable dissimilarity in their external form, which occurs between the males and females of these species; this dissimilarity indeed is, in some instances, so great that naturalists were led into the error of regarding the male and female of the same creature not only as be longing to different species, but even to diffe rent genera. Oviparous reproduction is also a constant character of the class.
Generally speaking, the reproductive appa ratus, whether in the male or in the female, is perfectly distinct, especially at the period when the organs composing it are in a state of acti vity ; and one of the most remarkable facts which the careful study of this part of the structure of the class has afforded, is their com plete state of doubleness ; on either side of the body we find an organ perfectly distinct, and often wholly independent of its fellow ; to such an extent, indeed, is this carried, that among the facts with which modern science has been enriched in regard to the structure of the Crus tacea, one by no means the least interesting is that in which an animal of this class was actu ally found presentingin either half of its body a different sex, each apparatus complete in every one of its conditions, and even with the whole of its modifications.'" Another fact, not less striking, is that of the analogy which exists, at least among the more perfect Crustaceans between the male and the female reproductive organ. This similarity is so great that the simple inspection of the organ is not alone sufficient to inform us always of its true nature, which in some instances can only be ascertained by the most carefnl examination.
The male apparatus consists essentially of an organ the secreting instrument of the fecunda ting fluid, and of an excretory canal variously modified. These two parts are contained within the thorax along with nearly the whole mass of other viscera, and never extend lower down than the last ring of this region of the body. They are not always very distinct from one another, and it frequently happens that the testis and the excretory canal are confounded inextricably under the form of a single tube, nearly identical in its structure from beginning to end. The length of this canal is occasionally very great and variously convoluted and con torted, so that its relations with the other tho racic viscera become excessively multiplied. This peculiarity we observe very well in the Maja and the Cancer pagurus (see .fig. 418). The canal, which throughout is single, is capillary at its commencement, but increases gradually in its dimensions to its termina tion.
In the Astacus fluviatilis, on the contrary, the two portions of the male reproductive apparatus are perfectly distinct, and severally completely developed. The testis (a, .fig. 432) consists of capillary secerning vessels, which are readily demonstrable, and presents three lobes, two of which lie forwards upon the sides of the stomach, and one backwards underneath the heart. From these three lobes two excretory canals (b) take their origin. In the Edriophthalmia the male organ is com posed of two or three elongated vesicles, which terminate in a common excretory canal.
It is in the Cancer pagurus perhaps that the male organ of generation is most highly de veloped. It occupies of itself' a large por tion of the thorax (fig. 418). The testis pre sents the appearance of a kind of grape cluster, formed of four principal lobes which, studied minutely, are found to be niade up of an infinity of extremely delicate ver micular canals, contorted so as to form great numbers of pellets. This first portion of the organ is situated in front of the thorax, and terminates in a primary large convoluted vessel lying on the side of the stomach ; be hind and in connexion with this we perceive the vas deferens, properly so called ; it is a canal of considerable size, much convoluted, and of a milky white colour; it traverses the thorax, still twisting about, penetrates the cell of the last pair of ambulatory extremities, and opens outwardly on their basilar piece. This indeed is the situation in which the copulatory organs of the Crustacea generally appear. Still, in many Brachyura of the Catometopa family, the Ocypoda and Grapsus for ex ample, the external opening of the male gene rative organ is found on the sternal part of the last thoracic ring; and there are even several of these animals in which the efferent canal, after having attained the external surface in the basilar articulation of the last pair of ambu latory extremities, returns inwards, and pene trates by a small groove, which conceals it until it has attained to that portion of the sternum which is hidden by the abdomen ; an example of this occurs in the Gonoplax. In the ordinary state the excretory canal termi nates on the edges of tbe opening, but at the instant of sexual intercourse the extremity of the canal undergoes a kind of erection, and by becoming folded upon itself like the finger of a glove, projects externally, so as to form a kind of penis adequate to the intromission of the fecundating fluid. This latter circumstance was long unknown to naturalists, who were accustomed to look upon the members of the first and second, abdominal rings as the ex ternal male instruments. These two pairs of extremities, in fact, (fig. 433,) are distinguished from the rest by their shape, which is styli form, and their structure, which is tubular, being composed of two horny lamina2 eonvo /uted one upon another, the first including the second. But direct observation has de prived them of all claim to be considered as fulfilling any office of so much consequence in the economy of the Crustacea as that of con veying the fecundating fluid from the body of the male into that of the female. At the most, they can only be regarded as organs of excita tion, and which the animal may perhaps em ploy at the same time to guide the male into the female organ.