Fig. 67. Taenia solium: a, head; b, c, d, segments of the body.

Fig. 68. Immature segment of Taenia solium: a, lateral canals; b, ovary; c d, accessory glands; e, lateral sucker.

(343). The alimentary canal seems to be represented by a double tube, which may be traced through the whole length of the body, without any other perceptible communication with the exterior than the minute pore in the centre of the head: at the commencement of every segment, however, there is a cross-branch, which communicates with the corresponding tube of the opposite side (fig. 68, a), so as to facilitate a free distribution of the nutrient fluids *. In some species a delicate vascular network is perceptible in the parenchyma of the body, which may likewise be connected with the nutritive function.

(344). The reproductive organs in the mature segment or Proglottis of a tape-worm, each of which may be considered as an adult animal, consist of a male and of a female apparatus - these two sets of organs being completely distinct from each other.

The male apparatus consists of a testis, a vas deferens, and an intromittent organ, the last of which is lodged in a special sac or pouch.

The testis (fig. 69, a a, b) occupies the middle of the anterior portion of the body, and is of a whitish colour, owing to the spermatozoa contained in its interior. It is composed essentially of a long caecal tube, folded upon itself in close convolutions, and terminating in the vas deferens (c), which reaches to the base of the intromittent organ.

The penis (fig. 69, d), called also by authors cirrus and lemniscus, is very variable in its form in different genera; in its real structure, however, it is merely a prolongation of the vas deferens, just as the latter tube is a continuation of the testis.

* Professor Van Beneden denies the existence of the central aperture or mouth, or that the two lateral longitudinal canals with their intercommunicating trunks constitute an alimentary system; on the contrary, he regards these tubes as secerning organs, the secretion of which is discharged from the terminal segment of the body through a foramen caudale.

In its size it varies considerably; it consists of two muscnlar coats invaginated one within the other, and unrolls itself like the finger of a glove, until it acquires its full length. The external surface, which is internal when in a state of repose, is covered with minute asperities or rough points; when fully retracted, it is lodged in the pouch, e.

(345). The female generative system is composed of an ovary, which produces the germ (ger-migene), of an organ which secretes the vitelline globules (vitelligene), of ducts from these two organs, and of a matrix, a copulative vesicle, vagina, and vulva.

(346). It seems to have been by no means a rash supposition on the part of Siebold, that in some Entozoa there might exist a double set of glands for the production of the ova, one appropriated to the formation of the germ, the other to the secretion of the vitellus. In the Cestoid forms, according to Van Beneden, the proper ovary or germigene (fig. 69, i) is situated at the posterior part of the body, occupying about one-third or one-quarter of its width. This organ is double, being exactly repeated on the right and left of the median line, the two being united by a central commissural canal*: when empty, the presence of this organ is discoverable with difficulty, on account of its extreme delicacy. Its appearance varies much: sometimes it is a bag surrounded with slight depressions (culs-de-sac); sometimes the whole viscus is divided into lobes, and has the appearance of an ordinary gland, whilst occasionally it is entirely made up of long caecal tubes united together, and opening at the same point.

Fig. 69. Diagram representing the fully-developed generative system of the Proglottis of a Cestoid Ento-zoon (after Van Beneden.) a, testis; o, commencement of ditto; c, vas deferens; d, penis; e, sac of the penis; f, orifice of vagina; g, vagina; h, copulative vesicle; i, germigenous organ, or ovary (represented on one side only); l,germiduct; m, point at which the vitelline globules enter the germiduct; n, vitelligenous organ, or vitelliduct; o o, transparent vesicles, developed at a very early period; p, oviduct; q, matrix; r, longitudinal canals; s, the skin; t, cutaneous glands.

* It is represented in the figure upon one side only, to avoid confusion.

On the sides of the body, extending nearly its whole length, are two slender and slightly flexuous tubes (fig. 69, n n), whose presence it is difficult to detect when in an empty condition, but which generally contain in their interior vitelline globules closely aggregated together, which by the peristaltic movements of the tube, aided by ciliary action, are forced onward from before to behind. The two vitelligenous tubes ultimately unite to form a common canal (n), situated near the median line, through which the vitelline globules enter the germiduct at the point marked m. On passing the opening of this canal, the germ becomes suddenly invested with a layer of vitelline globules, and, being thus transformed into an ovum, is carried onwards through the flexuous canal, or proper oviduct (p), into the matrix (q), becoming invested in its passage through the oviduct with an outer covering that represents the egg-shell.