Fig. 198.—Diagram of a cysticercoid. Cf. figs. 220, [227]. c.v., caudal vesicle or bladder (small); sec. c., secondary cavity caused by the growth forward of the hind-body; t., tail bearing six spines. (Stephens.)
Fig. 199.—Diagram of a cysticercus. c.v., caudal vesicle or bladder; i., invagination of wall of bladder. (Stephens.)
In the case of cysticerci a papilliform invagination forms, projecting into the interior of the bladder (fig. [201]). The layer of cells forming the papilla becomes divided into two laminæ, the outer[279] of which forms a kind of investing membrane (receptaculum capitis) for the papilla. The head and suckers are now developed on the walls bounding the axial lumen of the papilla. The papilla eventually evaginates, so that the receptaculum capitis now forms the inner surface of the hollow head, which eventually becomes solid.
Our knowledge of the development of cysticerci in the wide sense of the word is limited almost exclusively to that of a few true “bladder worms” (cysticerci); in other cases we know either only the terminal stage, i.e., the complete larva, or, exceptionally, one of the intermediate stages, but we are not acquainted with a complete series; the description must therefore be incomplete.
We know from feeding experiments that, after the introduction of mature proglottids or of the fully developed ova of Tænia crassicollis (of the cat) into the stomach of mice, the oncospheres escape from the shell in the middle portion of the small intestine, and a few hours later penetrate into the intestinal wall by means of a boring movement; they have been found in this position twenty-seven to thirty hours after the infection. By means of this migration, for which purpose they employ their spines, they attain the blood-vessels of the intestine; indeed, already nine hours after the infection and later they are found in the blood of the portal vein, and in the course of the second day after infection they are found in the capillaries of the liver, which these larvæ do not leave.
Leuckart, in experimental feeding of rabbits with oncospheres of Tænia serrata (of the dog), found free oncospheres in the stomach of the experimental animal, but not in the intestine: however, he came across them again in the blood of the portal vein. The passage through the blood-vessels to the liver is the normal one for those species of Tænia the eggs of which become larvae in mammals; even in those cases in which the oncospheres develop further in the omentum or in the abdominal cavity (Cysticercus tenuicollis, C. pisiformis), there are distinct changes observable in the liver that lead one to the conclusion that there has been a secondary migration out of the liver into the abdominal cavity. Indeed, one must not imagine that the young stages of the Cestodes are absolutely passive; once they have invaded an organ they travel actively, and leave distinct traces of their passage.
In other cases the oncospheres leave the liver with the circulation, and are thus distributed further in the body; they may settle and develop in one or more organs or tissues. Many oncospheres may, by travelling through the intestinal wall, penetrate through it and attain the abdominal cavity direct; some, perhaps, pass also into the lymph stream. Where there are no blood and lymphatic vessels in the intestinal wall, as in insects, the oncospheres attain the body cavity or its organs direct; in short, they never remain in the intestinal lumen itself, and only rarely—as in Hymenolepis murina of the rat—do they remain in the intestinal wall.