The matrix (q) thus receiving a continual supply of ova, becomes gradually distended, until it occupies almost all the interior of the body, and branches out in different directions into caecal pouches at points where the least resistance is offered, until finally the skin of the proglottis becoming as tightly distended as the matrix, both are ruptured, and the ova escape in this artificial manner. The vagina (fig. 69, g g) is a large canal, having, like all the organs belonging to this apparatus, distinct parietes. It commences externally (f) in the immediate vicinity of the male organ, penetrates to the centre of the body, and, bending at an angle, makes its way backwards to the space that separates the two ovaria (germigenous organs, i.) Its length is invariably in correspondence with that of the penis of the male apparatus. At the extremity of the vagina is situated the copulative sac (h), a small vesicle with very delicate parietes, the contents of which abound in spermatozoa.

Such being the anatomical arrangement of the different parts of this somewhat complex apparatus, it now remains to take a brief survey of their physiological import in the performance of the generative function. In the living Entozoa it is sometimes not difficult to see the germigenous and the vitelligenous organs opening into a common canal, and each of them pouring their product into its cavity; and if a specimen is selected in which the parts are in full activity, and the compression used be such as to render the organs transparent without putting a stop to their action, the germs may be seen to arrive, one by one, at regular intervals, before the opening of the vitelligenous organ, which, contracting forcibly, expels a certain quantity of the vitelline substance, in which the germ becomes enveloped, having previously, on passing the orifice of the copulative sac, become impregnated by contact with the spermatozoa therein contained. As the vivified ovum advances onwards it receives its outward envelope, arrives in the matrix, and is there retained until birth is accomplished by the destruction of the animal.

A question has often arisen in relation to the manner in which the act of copulation is effected in animals presenting this remarkable hermaphrodite condition of the generative system - a question to winch Professor Van Beneden has been able to give a satisfactory solution. In a specimen of Phyllobothrium luca, he had ocular demonstration that the individual was self-fecundating. Its penis became unrolled, and passed immediately through the vulva into the vagina, into which it was deeply inserted. Active peristaltic movements of the vaginal tube were very manifest, and spermatozoa were seen abundantly in. its interior, these being subsequently conveyed by peristaltic action into the copulatory pouch. The penis, after some considerable time, is withdrawn, returns into its pouch, and all the organs assume their previous condition.

(347). In studying the progressive development of the egg in the Taeniae and other Cestoid worms, it is only necessary to remember that all the ova contained in the same segment are of the same age, and that the age of the segments increases progressively, from the head to the opposite extremity of the elongated body, to enable the observer to select ova in any stage of their development in order to submit them to examination under the microscope.

(348). In their general structure, the eggs of the Taenioid Entozoa are similar to those of the other classes of Invertebrate animals; and the segmentation and breaking-up of the yelk proceed exactly in the same manner.

(349). On arriving at maturity, however, a series of phenomena of the highest possible interest begin to develope themselves, which we will proceed to describe with as much conciseness as the subject will allow*. The worm, when it emerges from the egg, instead of being-composed of a series of segments, consists simply of the first segment, or head, as it is called, of the compound worm, variously armed with hooks, suckers, or bothria, according to the genus, to which is appended a short caudal extremity, wherein but slight traces of any internal organ are apparent. In this condition it has received the name of Scolex, and may be regarded as a sort of root from which all the rest of the animal is developed, much in the same way as the Planulae of the Acalephae are segmented off from their Hydra-like parent (§ 304.) In this condition the "Scolex" exists for some time, and in some instances, as, for example, in the Tetrarhynchi, clothes itself in a kind of sheath formed by a mucous exudation derived from the surface of its body.

(350). We have already pointed out the similarity of structure that exists between the armature of the "head" of the Cysticereus (fig. 66) and that of the Taenia (fig. 67); but the reader, from a comparison of the two figures, would scarcely be prepared to expect that the one was the Scolex of the other. Siebold had, indeed, satisfied himself that the arrangement of the horny circlet of Cysticercus fasciolaris, found in the liver of the mouse, entirely corresponded with that of Taenia crassicollis, that inhabits the intestines of the cat. If young Taenias and Cysticerci be carefully examined, and compared with other forms, it is satisfactorily seen that the Cysticercus is merely the Scolex from which a Taenia may be developed, and that its vesicular portion corresponds exactly with the similar vesicles of some Tetrarhynchi in a like state of development. A Taenia, says Van Beneden, might probably very well acquire its complete development, without assuming the vesicular form, as is proved by the Paradoxical Taenia; but, for that, it would be necessary that the germ should be deposited in an intestinal tube. The same is the case with the Tetrarliynchi: here also the body becomes quite out of proportion to the size of the head whilst the germ remains amongst the peritoneal folds of the fishes in which they are found, just as the Cysticerci do whilst imbedded in the peritoneum or amongst the muscles.