Page:The American Cyclopædia (1879) Volume VI.djvu/676

 684 ENTOZOA confined chiefly to the tsenias of animals, and appear to have had little influence with the scientific men of the next century, who fell back again upon the old and easy theory of spontaneous or equivocal generation; and so it remained until 1844, when Steenstrup's theory of alternation of generation was applied to the problem, and Siebold and Dujardin published essays on the connection between the taenife and encysted forms of various animals. Their experiments and those of Kiichenmeister, who must be considered the highest authority on the human helminths, cannot be given in detail here; suffice it to say that tapeworms have been produced in carnivorous animals of all kinds by giving them the encysted forms to eat, and the encysted varieties have been bred in others by administering the eggs or embryos of taeniae. Tapeworms consist of three parts, viz. : head, neck, and colony of joints. The head is a minute object, usually square, and provided with varieties of sucking disks and and in fact are now considered separate in- dividuals. They discharge their eggs either through the genital opening or by self-destruc- tion, which results either from the bursting of their walls or by decomposition. They affect chiefly moist places, and, leaving the manure in which they have been deposited, wander about amid the herbage, and may in this way be devoured ; or they fall into water, and there bursting, discharge their eggs, which are thus borne far and wide, and find entrance to a proper soil for future development. They are not capable of a long continued independent existence, and may even be destroyed within the intestine of their host, scattering their eggs along this canal, though harmlessly ; for Leuckart found by experiment that eggs intro- duced into the intestine before being subjected to the action of the gastric juice remain un- changed ; but that when previously submitted to its influence and then placed within the in- testinal canal, the embryos became free. Pro- glottides may even be swallowed entire by animals which wallow in moist manure, and thus introduce the eggs to their proper dwell- ing place. When once they have set their offspring free, their object is accomplished and they disappear. Each proglottis contains a coronets of hooks, by which it attaches itself to the walls of the intestine. The neck is slender and marked by transverse wrinkles, which gradually are converted into joints. With age these joints increase in number, and finally those first formed become ripe, while new ones are continu- ally given out from the head to supply in turn the place of those dis- charged. Their growth is generally rapid, and some species attain a length of 100 ft., while others are only a few lines long. Considered as a simple individual, FlQ j^^ soli with different Yie w 8 P f the Head. the tapeworm has very limited power of motion, although a distinct layer of muscular fibre is found beneath the skin. This integument is soft, white, moist, and porous ; and through this nutrition is prob- ably carried on by absorption, though the only organs subservient to this function are two pairs of longitudinal canals running along each side of the joints, and united by transverse branches. The skin contains also innumerable roundish, concentrically marked, calcareous corpuscles, recognized only by the microscope, which serve undoubtedly as a sort of skeleton. They possess no nervous system. The sexual organs, however, are remarkably developed. When the' oldest joints have become sexually mature (which period varies greatly in differ- ent species), they pass off spontaneously by the anus sometimes by the mouth of the animal which harbors them. These are flat, quadrangular, yellowish white, and in some species are detached singly, in others by groups. These proglottides, as they are called, are true hermaphrodites, contain the sexual organs and eggs or embryos enclosed within shells, and possess the power of moving about ; so that they have often been mistaken for trematoda, vast number of eggs, so that if one out of the many millions reaches a proper habitation the species will not decrease in numbers. The embryos are enclosed in firm shells constructed to resist a strong pressure from - without, and are either brown or yellowish, and round or oval. They probably cannot undergo a great degree of dryness, heat, or cold, or exist very long in fluid, without the destruction of the animal within. The history of the tcenia so- lium, or common tapeworm, will best serve as an example of the usual method of develop- ment and transformation of the cestoidea, for it has been most fully studied on account of its frequent occurrence in the form of measles, and its important relation to man. This worm is improperly named, since many are sometimes found in the same intestine. It seldom attains a length of more than 20 ft., and is composed of 600 or TOO joints, which when mature con- tain myriads of eggs, and escape singly or at once into the outer world. The eggs being set free find their way into water or manure, and are thus scattered far and wide. One occa- sionally enters the stomach of man on lettuce, fruit, or unwashed vegetables, but more gen-