In the formation of their head, these resemble tape-worms to such a degree, that even in 1836 Johannes Muller proposed to unite them in a single order, with two subdivisions. Light has, however, been since thrown upon the subject, which warrants us in going a step further, pronouncing these cystica with tape-worm heads to be in truth nothing more than errant cestoda, which, owing to their deviations, have sickened, declined, and remained sexless.

The lime corpuscles found upon them, and especially upon the cysti-cereus, are the same as those occurring upon tape-worms. They have been erroneously held to be ova, and in reality rather represent an outer skeleton formation. These cystica, within textures, are almost always distinctly encysted; that is, shut up within a capsule effused from the textures. In free spaces, - for example in the ventricles of the brain, this is not the case. This adventitious outer cyst is not to be confounded with the cyst proper to the animal itself. They frequently perish, especially through inflammation of the external cyst, being either mechanically crushed by, or corroded and destroyed in, the product. In the sequel, the complicated contents of the outer cyst, after having suffered many changes, progressively thicken, and eventually cretify, en masse, within the shrivelled capsule.

The unequivocal proof of the previous existence of an animal in such obliterated cysts is furnished by ddbris of the animal cyst; by hooklets, from the coronet of hooklets, which have resisted the corrosive agency; and lastly, by the presence of the lime corpuscles before alluded to.

In man occur:

The cysticercus eellulosus, consisting of a conical, snow-white, transversely rugous body, and of a vesicle which constitutes its caudal extremity. The vesicle is oval, spherical or square, - in muscles, cylindrical, parallel to the muscular fibres, - and of the size of a pea or a haricot bean, - in rare instances, for example, in the ventricles of the brain, of a hazel-nut. When the animal is retracted into this vesicle, it appears as a white, spherical, solid body, seated somewhat eccentrically on its inner surface, whilst upon the vesicle itself is observable, externally, a delicate point-like fold or depression at the same spot. When the animal is external to the vesicle, a condition easily brought about by puncturing the vesicle, and pressing the hardish spherical body between the finger and thumb, a pore becomes perceptible which leads to the interior of the animal pouch. Taking the size of the caudal vesicle at the ordinary one of a pea, the animal itself, that is the trunk, would about equal the diameter of the vesicle, both together measuring from six to twelve lines in length. The neck is short, very thin, and, like the body, wrinkled. Upon it is seated the largish, bulb-shaped, or rhomboidal head, upon which there is at each angle a circular suction-cup; and midway between these a proboscis, cone-shaped in its protruded state, with, at its extremity, a coronet of hooklets consisting of a double row [about thirty-two in all], which, when retracted, pack up into a funnel-shaped cup. The two circles of hooklets are identical in shape; those of the outer circle are however much smaller than the others, whilst both are so disposed that the larger and smaller hooklets alternate with each other.

The above-mentioned transversely wrinkled, anterior portion of the creature appears as an almost structureless, feebly striated membrane, to which a profusion of fine, black-contoured molecule adheres. It is, moreover, studded with a multitude of roundish or oval, whitish, smooth, sharply contoured, shining, lesser or bigger corpuscles, of from one-eightieth to one-thirtieth of a millimetre in diameter. They are most numerous about the middle part; near the neck and head their number greatly diminishes, whilst, close to the caudal vesicle, they suddenly and entirely disappear. They lie superimposed in several layers, those of the outer stratum being only loosely adherent to the animal, so that they may be very easily scraped away. Treated with hydrochloric or with acetic acid, they dissolve under the copious development of carbonic acid, leaving an organic base-substance behind. In the solution, oxalic and sulphuric acids create a precipitate.

The caudal vesicle consists of the same homogeneous, indeterminate, granulated mass, besprinkled with countless small and larger fat-molecules. The contents of the caudal vesicle consist of a watery, neutral fluid, holding but a scanty portion of albumen.

Wherever the cysticercus occurs in textures, it is inclosed within a second cyst of fibrous texture. When magnified it appears as a delicately-fibred membrane, permeated by delicate blood-vessels, and easily rendered transparent by acetic acid. Where the cysticercus occurs free within a cavity, as within the ventricles of the brain, it is uninvested, showing the outer cyst, in other localities, to be adventitious.

When the creature perishes, as frequently happens from disease of the outer cyst, the caudal vesicle becomes semi-opaque, collapsed, its contents turbid, displaying the said lime-corpuscles and hooklets, which, together with a granulate mass, are found floating in its fluid. The entire creature softens and liquefies, afterwards condenses, and eventually settles into a cretaceous concrement. Meanwhile the outer cyst shrivels and dwindles into a thick-membraned capsule, for the isolation of the said concrement.

The cysticercus cellulosus occurs in the brain, in the striated muscles, including the heart, and in the areolar tissue. It also occurs, free, without its outer envelope, in the ventricles of the brain, and in the chambers of the eye. It sometimes occurs in the muscles and brain simultaneously, in great multitudes.

Even in the brain it is usually borne imperceptibly. When present there in great numbers, however, it often occasions vertigo, and the case has happened of its proving fatal by setting up inflammation in its vicinity.