Mounted as a preparation for the microscope.
The worm has been removed from its connective tissue sac or capsule, and the head with a small portion of the neck has been evaginated by gentle pressure from the vesicle, or proscolex, within which it is generally retracted. The vesicle has delicate walls, and in life is distended by a liquid which contains only a trace of albumen and is little more than a solution of salts, chiefly of sodium.
Under the microscope, using a 1/2-inch objective, the principal features of the head may be readily made out: the slightly projecting rostellum: the circlet of hooks and the suckers. The hooks are arranged in alternation, one large with one small. The free part of a large hook is longer and somewhat straight as compared with the corresponding part of a small hook. The imbedded portion or root is in both cases forked, but the anterior (or rostellar) branch of the fork is of remarkable length in the large hooks.
In the neck, especially in the part not completely evaginated, may be noticed numerous clear rounded bodies. These are the calcareous bodies, or concretions: see ante, preceding preparation, p. 227. They do not exist in the walls of the vesicle.
The life-history of Taenia serrata is briefly as follows. The ripe proglottides are scattered among the grass by the way-side, etc, and are swallowed by a Rabbit, the soft tissues being digested in the stomach, and the chitinoid shells containing the hexacanth embryoes or proscolices set free (cf. p. 228): or the proglottides decay naturally and set free the contents of their uteri, which are then eaten with herbage. The chitinoid shell of the proscolex, under the combined influence of warmth and the gastric juice, becomes brittle, and either break up spontaneously or is broken by the movements of the embryonic hooks. The embryo, now free appears to bore its way into the tissues: and it has been found in the portal blood by Leuckart. The embryoes of T. marginata have been similarly found by Leis-ering in the portal capillaries of the Lamb. The parasite sojourns for about four weeks in the liver, in which it creeps about. It then escapes into the abdominal cavity, and becomes encapsuled either on the omentum or mesentery. The Cysticercus of some Tapeworms, e. g. of T. solium, is generally found in the connective tissue of muscles, etc.
In these instances the hexacanth embryo probably migrates through the tissues, not through the blood-current.
The changes undergone by the embryo are as follows. It grows in size, and in the case of C. pisiformis becomes elongate, and the hooks are lost. The central cells enlarge and become clear. The subcuticular muscles are differentiated, and at a later period a system of more deeply placed muscles. Between the two layers of muscles intervenes a layer of cells. It is from a meniscus-like thickening of this cell-layer, which is developed at the anterior end, the end where the hooks remain attached in C. Arionis, that the head and neck of the future Tapeworm are developed. The head may be formed however, as in Archigetes Sieboldi, at the opposite end, the one at which Moniez asserts that it is always formedl. At the same time a network of excretory vessels appears. It opens to the exterior by a posteriorly placed pulsatile vesicle. The finer vessels end in ciliated funnels. When the organism has attained a length of 4 mm. the central clear cells break down and give rise to a central accumulation of liquid.
In some instances this change takes place at a much earlier period: in others, e. g. in Piestocystis or in the Cysticercus Taeniae ellipticae, or of Tetrarhynchus, in a word, in the majority of Cestoda the central cells do not thus liquefy2.
When C.pisiformis is about 2 mm. long, the meniscus of cells above-mentioned begins to develope. It grows inwards, pushing before it the deep layer of muscles which form a receptaculum capitis. When it has reached a certain size, a depression appears externally, and thenceforth the meniscus grows inwards as a hollow cone. The cavity of this cone widens at its inner or deep end, and here the ros-tellum, hooks, and suckers are developed; but it is only when the cone is wholly or partly evaginated that they appear in their ordinary shape. In the position in which they are developed they are, as it were, inverted. When the head and neck are evaginated the Cysticercus appears to be divisible into three parts, (1) the head and neck proper, or scolex; (2) the basal part of the invagination or hollow cone, and (3) the proscolex or vesicle. When such an encapsuled Cysticercus is transferred to the stomach of a dog, the inclosing cyst is digested together with the parts, (2) and (3) supra: and only the head and neck pass on into the intestine.
Here the head attaches itself by its hooks and suckers, and in forty-eight hours growth has proceeded so far that there are well-marked indications of 12-18 joints.
1 Leuckart states that the excretory system of Archigetes opens externally near the posterior end of the scolex or sexual worm, i. e. the end to which the proscolex or vesicle is attached. The excretory system of the scolex and proscolex, e. g. of a Cysticercus, opens externally at the end of the proscolex opposite to that at which the scolex is attached. If Leuckart's statement is correct, it shows that the scolex of Archigetes develops from the proscolex in an unusual position and tends, therefore, to invalidate the assertion of Moniez referred to in the text.
Moniez states that the proscolex when 1 cm. long and less than 1 mm. broad divides transversely, the two parts being connected by a slender pedicle which ruptures. The anterior half develops the head: a rudiment of a head (?) was observed only once in the posterior half. The point of attachment of the pedicle to the anterior half is marked by a persistent depression, the foramen caudale. The fate of the posterior half is not known for certain. Non-division of a Cysticercus is, according to Moniez, the reason why the proscolex sometimes attains such large dimensions.