The relation of both to each other, and the import of the last-named animal in particular, will become manifest from the following description:

(A.) Echinococcus

Within a sac of fibroid texture is inclosed a solitary, independent, thoroughly distended vesicle, containing a limpid, serous fluid; or else inclosing, as a parent vesicle, other similar vesicles of various size, in various numbers, spherical or flattened by mutual compression, either floating at large in the contained fluid, or sessile upon the inner membrane of the said parent. Its size varies from that of a vesicle just cognizable, and as big as a poppy- or a millet-seed, to the magnitude of a goose's egg and more. In number it may amount to hundreds, so that the serous contents of the parent vesicle are reduced to a minimum. Generally speaking, the lesser filial vesicles are sessile, whilst the larger ones are free.

In very voluminous sacs it is common to find that the parent vesicle appears to be wanting. Either it is mixed up with the younger vesicles, split up, collapsed and dissolved into scattered shreds, or else it has disappeared in the excessive attenuation consequent upon its enlargement.

In their unimpaired vegetation, these vesicles are filled to distension, are elastic, and impart to the touch a sense of tremulous fluctuation, as does the parent cyst replete with them [hydatid tremulousness]. They consist of a substance resembling coagulate albumen, separating into several layers, partly diaphanous, partly white and opaque, frequently accumulated in the inside to considerable thickness, and into gibbous projections. Moreover, they contain a limpid serosity identical with the contents of the parent cyst. When the vesicle is punctured, this fluid gushes forth in a column, and on an incision being made, the parietes of the vesicle become suddenly inverted. The substance of the latter is a stratified, homogeneous, very fine-granular, structureless mass, whilst their contents exhibit a few lustrous fat-drops, some scattered or agglomerate, elementary granules, and glebous coagula.

These vesicles occasionally contain others similar, of a third, and the latter again in rare instances of a fourth generation.

On a narrower inspection of the inner surface of these vesicles, we perceive in many of them, a whitish, opaque, gritty efflorescence, whilst with the aid of the microscope we here discover densely-nestled animalcules, which prove, by the most various changes of shape, that they long continue to live on in the dead subject. A few of them are even found free in the above-mentioned fluid.

This entozoon is from one-ninth to one-third of a millimetre long, and from one-twelfth to one-fourth of a millimetre broad. It has a taenioid head, with four lateral suction-pores, and a proboscis garnished with a double coronet of hooklets. The head is distinguished from the thicker, spheroid trunk, by an annulate indentation. From the proboscis a longitudinal striation runs to the posterior part, and, commencing from these striae, the body of the creature is transversely striated. The posterior termination is a transverse cleft, in which is inserted a cordlike formation, by whose means the creature maintains its seat upon the vesicle. Between the striae of the trunk are spherical or oval, limelike corpuscles, resembling those upon the cysticercus.

In its developed state the creature appears in the above form. It is met with, however, under various other shapes. Thus it appears as an elongated sphere, in the centre of which the coronet of hooklets appears perspicuous when the head is retracted. Or it assumes the shape of a heart, or of a pitcher, or even of a horse-shoe.

The abode of this echinococcus in mankind is, according to our own experience, invariably internal to, and never external to, the vesicles.