OCR Text |
Show 148 CRUSTACEA. composed of a. single ventricle. The blood, formed of little diapha. nous globules, is impelled forwards in a column which soon divides into four branches, two of which proceed directly towards the eyes, and two towards the antennre; the latter are then reflected back. wards and united to the former, constituting a single column on each side, which descends towards the cup, turns round its base and disappears. A little beneath the t.wo following feet, we ma; distinguish on each side, another sangumeous column whir.h curves outwat·ds, extends along the borders of the shell, and having reached the two penultimate feet, is flexed forwards and ceases to be visible. Another, where, as in the preceding, the blood flows from the ante· rior part of the body to the posterior, traverses longitudinally the middle of the tail; it unites behind with two other currents that may be seen on the edges of the tail, but which flow in a contrary direc. tion, or appear to return the blood to the heart. Jurine avoids using the term vessel, because the blood which is driven into the anterior part of the body, appears to be diffused there in such a manner, as to induce us to believe that its globules, instead of being contained in particular vessels, are dispersed in the parenchyma of those parts. From what we have stated, however, with respect to the circulation in the Decapod a, it is evident, that the blood, in the first instance, is distributed in the Arguli in the same way, and that the currents or columns of which we have just spoken, seem to indica~e the exis· tence of peculiar vessels. This able obsenet·, in fact, subsequently acknowledges that the circulation is not every where carried on in so diffused a manner as in the anterior part of the shell, where, how· eve.r . in. ou.r opinion, it is effectuated as in the Decapod a. The brain ' which 1s situated behind the eyes, appeared to him to be divided into three equal lobes, one anterior and two lateral. The anterior part ~f the stomach gives origin to two large appendages, each divided 1nto two branches, which ramify in the wings of the shell. The brownish coloured aliment they cont.ain renders these ramifications visible. The crecum is provided near its origin with two vermiform appendages. .The excessive ardour of the males frequently induces them to m1stake one sex for the other, or to make their advances to preg· nant or dead females. They are placed in coition on their back to which t?ey cling by ~eans of their feet with cups, for several ho~rs. The per1od of gestatiOn is from thirteen to nineteen days. The ova are smooth, ov~l and milk-white. They are fixed with gluten on stones or other mdurated bodies, either in a straight line or in two ran~es, and from one to four hundred in number; being pressed agamst each other, their form becomes almost hexagonal. Twenty-five days after the extrusion of the ova, and after they P JECILOPOD.\. 149 have assumed a yellowish and opaque·tinge, the eye and parts of the mbryo are perceptible. In about ten days more, the shell opens ~oogitudinally, and the tadpole issues from it, being at this period about three-eighths of a line in length. Its general form is similat· to that of the adult, but the organs of locomotion present a very essential difference. :MUller has described it in this state by the name of /Jrgulus charon. Four oars or long arms, two situated before the eyes and two behind, each terminated by a pennate and flexible pencil of hairs that have a simultaneous motion, by which the animal is impelled by jerks, project from the anterior extremity of the shell: they do not represent the antennre, for they also are visible. The feet with cups are replaced by two stout feet, flexed into an elbow near the extremity, and terminated by a stong hook, with which it clings to Fishes. The only feet proper to the adult, that are developed and free, are those of the second and third pairs, or the two ambulatory and the two first natatory feet; the following ones are as yet fixed to the abdomen. The heart, proboscis, and ramifications of the appendages of the stomach are distinct. After the first change of tegument, which is effected by a laceration of its inferior surface, the oars disappear, and all the natatory feet are visible. In three days more the second change ensues, but without producing any important alteration. But after the third, which occurs forty-eight hours subsequently to the second, these same feet are converted into those with cups, still, however, preserving the terminal hook. At the expil·ation of nine days, there is a new change of skin, and the organs of generation, male and female, are apparent; another change of tegument however is required ere the sexes are fitted for copulation, so that the period of their metamorphosis extends to twenty-five days. Still, however, they have attained but the half of their proper size. For that purpose fresh changes of the tegument, which occur every six or seven days, are requisite. Jurine satisfied himself of the fact, that propagation never ensues without the interven Lion of the male. The females, which he kept separate, perished from a disease which was announced by the appearance of several brown globules, arranged in a semicircle on the posterior portion of the clypeus, and apparently formed in the parenchyma, for they were not dispersed by the change of tegument. .llrgulus foliaceus, Jurine, Jun., Ann. Mus. d'Hist. Nat. VII, xxvi; Monoculus foliaceus, L.; .!lrgulus delphinus, Herro. Jun., Mem. Apter., V, 3, VI, ii; Monoculus gyrini, Cuv., Tabl., Elem. de I'Hist. Nat. des Anim., p. 454; Ozolus gasteroatei, Lat., Hist. Nat. des Crust. et des Insect., IV, xxix, 1-7; Desmar., Consid., L.; Louse of the Stickleback, Baker, Micros., II, xxiv. This species, the only one of the genus that is known, |