A segment with the generative organs in the condition here figured, would be found in either Taenia solium or Taenia mediocanellata, the two common human Tapeworms, at about the 450th segment counting backwards from the head; and the segments would assume the appearance given in Figures 3 and 4, after about 200 more segments in Taenia solium, and 360 to 400 in Taenia mediocanellata.

FIG. 3. Segment of Taenia solium, to show the dendritic outgrowths of the uterus, about twice the natural size; after Leuckart, I.c., p. 387, Fig. 156.

IN this segment the uterus and its contents have increased and encroached so much upon the rest of the generative organs, as to have caused their disappearance. In Taenia solium, its dendritic ramifications have a yellowish colour, and contain aggregations of embryos, such as the one figured at 4, enclosed in a hard resistent shell.

The uterus in the smaller Taeniae enlarges into a simple sac as the ova collect in it. In others again it acquires small lateral ampullae. The Taeniae possess no entrance to the uterus other than the vagina and fertilising canal. In some Cestoda the uterus possesses an aperture of its own. The ova acquire their shell within the uterus; the germ segments, forms the proscolex (Fig. 4) and the shell of the embryo. As there is no uterine aperture the embryo shells with the proscolices are set free by the drying up and dehiscence of the proglottis, or rupture of the uterus, or by digestion of the soft tissues when the proglottis is swallowed entire by the host.

FIG. 4. Embryo or proscolex of an ordinary Taenia, armed, as it is normally in this genus and other Cestoda, with six spines; after Van Beneden, I.c., P1. xxvi. Fig. 27.

SUCH an embryo as this is about three times the size of a human blood-corpuscle, 0.022-0.028 Mm, and when set free from the hard shell, which is not drawn in this figure, by the action of the digestive fluids of its host, it bores and pushes its way from the mucous surface of the stomach either into the blood-vessels, and so passes into the liver, a very common place for the development of the cystic stage, or into the connective tissues. The two spines of the central pair of the three are symmetrical, and, in piercing the tissues, they have an antero-posterior movement. The two pairs of spines again on the right and left are symmetrical with each other. These two pairs move in piercing the tissues much as the fore-limbs do in swimming. The wound made is, on account of the small size of the embryo, invisible; and it has been incorrectly supposed that the embryos find their way into the liver by way of the bile-ducts. The six spines are recognisable up to a certain period, though dislocated from their position, in the more or less distended, cystic or cysticercoid vesicle, into which the proscolex may expand when it reaches its place of lodgment in the tissues. In Cysticercus Arionis the hooks are retained in situ, see Leuckart, op. cit.

Fig. 209, p. 459. Figures of embryos and the history of their migrations are given in the works of Van Beneden and Leuckart referred to.

FIG. 5. Cystic stage in the development of Many-headed Bladder-worm, Coenurus cerebralis, after Van Beneden, l.c., P1. xxvi. Fig. 31.

The hexacanth embryo, figured at 4, has grown greatly after coming to rest in the organ, ordinarily the brain of a sheep, to which it is carried by the blood-current. The six hooks are observed to be dislocated at b, and a number of 'scolices,' the heads of as many future Tapeworms, are developed upon one of the poles of the vesicle. The way in which these heads are formed in the Taeniae is detailed on p. 230. There are 300 to 400 heads in Coenurus. Each head at first points inwards towards the interior of the parent cyst; but by the contractions of the muscular layers of the cyst, as also by those of the intrinsic muscles of the head itself, it may point either outwards or inwards. For the changes that occur when the cystic animal is swallowed by the host, see p. 230 already quoted.

a. Wall of embryonic dilated cyst.

b. Hooks of embryo dislocated by its growth. c 1. Scolex fully protruded.

c 2. Scolex half protruded.

d. Scolex as developed pointing inwards and its tubular body in communication therefore, not with the parent cyst, but with the cavity of the adventitious cyst thrown round the entire organism by the irritated tissues of its host.

For history of Coenurus cerebralis, see Van Beneden, l. c., p. 146; Cobbold, Entozoa, 1864, p. 116 seqq.; Id. Parasites, 1879, p. 333; Gamgee, Report on the Parasitic Diseases of Quadrupeds used as Food, Med. Officers' Privy Council Office Report, v. 1862;Thudichum, ibid., vii. 1865.

FIG. 6. Hydra viridis, with reproductive organs; after Greene, Manual of Coelenterata, 1861, p. 24.

THE animal is drawn, attached by its 'adhesive' or 'pedal' disc to a piece of weed with the oral end downwards, a position ordinarily assumed by it during life. It is much enlarged, its natural size being at the greatest, in a fine example, barely f of an inch.

Hydra differs from the hydroid form of other Craspedota or Hydrome-dusae, (1) in being free and locomotive; (2) in being only temporarily colonial, as the buds formed are eventually set free from the parent and from each other; (3) in being a sexual Hydroid form and not developing a dimorphic sexual zooid or Medusa; (4) in being naked, i. e. devoid of a perisarc. The marine and freshwater genus Protohydra agrees with Hydra in being locomotive and non-colonial.

a. Tentacle. The tentacles are contracted. They form a single circle: in number they vary from six to ten as a maximum in this species, but the odd numbers, seven and nine, are often met with. They are hollow and their cavities are continuous with the gastric cavity.

b. Hypostome, or oral cone. It is conical when protruded, and the mouth is situated at its apex.

c. The line points to the gastric cavity. The outline of this cavity and its extension into the tentacles is dark, owing to the chlorophyl corpuscles contained in the endoderm cells. The ectoderm cells, on the contrary, are transparent, and hence the light outer border both to body and tentacles. The oral and stomachal regions constitute the hydroce-phalis. The peduncle is known as the hydrocope.