The reason of this situation of the vulva seems to be the fixed condition of the head of this species of Strongylus. In both sexes it is commonly imbedded so tightly in a con densed portion of the periphery of the lung as to be with difficulty extracted; the anal extre mity, on the contrary, hangs freely in the larger branches of the bronchi, where the coitus, in consequence of the above dispo sition of the female organs, may readily take place.
In the Strongylus armatus the two oviducts terminate in a single dilated uterus, and the vulva is situated at the anterior extremity of the body, close to the mouth.
We find a similar situation of the vulva in a species of Filaria, about thirty inches in length, which infests the abdominal cavity of the Rhea, or American Ostrich. The single portion of the genital tube continued from the vulva is one inch and a quarter in length ; it then divides, and the two oviducts, after forming several interlaced convolutions in the middle third of the body, separate ; one ex tends to the anal, the other to the oral ex tremities of the body, where the capillary portions of the oviducts respectively com mence.
In the Ascaris Lumbricoides the female organs (fig. 96) consist of a vulva, a vagina, a uterus, which divides into two long tortuous oviducts gradually diminishing to a capillary tube, which may be regarded as ovaries. All these parts are remarkable in the recent animal for their extreme whiteness. The vulva (d, fig. 72,) is situated on the ventral surface of the body at the junction of the anterior and middle thirds of the body, which is generally marked at that part by a slight constriction. The vagina is a slightly wavy canal five or six lines in length, which passes beneath the in testine and dilates into the uterus (k, Jig. 96). The division of this part soon takes place, and the corium extend with an irregularly wavy course to near the posterior extremity of the body, gradually diminish ing in size; they are then reflected forwards and form numerous, and apparently inextricable, coils about the two posterior thirds of the intestine. Hunter has successfully unravel led these convolutions, and each of the tubes may be seen in the preparation in the Ilunterian Collection to measure upwards of four feet. The generative organs contained in the female, or longer branch of the Syn gamustrachealis, have a cor responding structure with those of the Nematoidea.
The capillary unbranched ovary and uterus are double, as in Ascaris, Spiroptcra, Filaria, and most Stron gyli. The vulva is in the form of a transverse slit, and is situated at the ante rior third of the body, im mediately below the attach ment of the male branch. In the Nematoidca the male individual is always smaller, and sometimes dis proportionately so, than the female. At the season of reproduction the anal ex tremity of the male is at tached to the vulva of the female by the intromission of the single or double spi eulum, and the adhesion of the surrounding tumid la bia; and, as the vulva of the female is generally si tuated at a distance from either extremity of her body, the male has the appearance of a branch or young indi vidual sent off by gemma tion, but attached at an acute angle to the body of the female.* In the Iletcroura andro phora of Nitzch (Ilersch and Criiber's Encyclopa die, th. vi. p. 49, and th. ix.
tar. 3. f. 7.) the male maintains an habitual con. nexiun with the female, which has a horny pre hensile process for the purpose of retaining the male in this position. here there is no conflu ence of the substance of the bodies of the two sexes; the individuals are distinct in their RI perficies as in their internal organization. But this singular species offers the transitional grade to that still more extraordinary Entozoon, the Syngamus trachcalis, in which the male is orga nically blended by its caudal extremity with the female, immediately anterior to the slit-shaped aperture of the vulva, which is situated as usual near the anterior third of the body. By this union a kind of hermaphroditism is produced ; but the male apparatus is furnished with its own peculiar nutrient system ; and an indivi dual animal is constituted distinct in every respect, save in its terminal confluence, with the body of the female. This condition of animal life, which was conceived by Hunter as within the circle of physiological possibilities, (see Anim. (Economy, p. 46,) has hitherto been only exemplified in this single species of Ento zoon ; the discovery of the true nature of which is due to the sagacity and patient research of Dr. Charles Theodore Von Siebold.