Page:Popular Science Monthly Volume 8.djvu/704

684 of the creature from one animal or host to another. The eggs are swallowed by some animal, usually a vegetable feeder, they hatch within its body, enter its tissues, and remain in a state of incomplete development until transferred to the stomach of another animal which has eaten the flesh of the first one. Here development recommences, and goes on to completion, when the process of reproduction begins. Each species of worm has its particular animals, through the agency of which these changes occur, and, if in its passage it gets off the proper track, that is, enters the wrong animal, it must either perish, or, as sometimes happens, find its way by a second transfer into the body of its destined host. The tapeworm of man. Tœnia solium (Figs. 23 and 24), is a member of this group, belonging to the Cestoidea, or ribbon-like worms. These cestoids are found in all classes of vertebrate animals. They exist in two principal forms. The first or vesicular form resembles somewhat in appearance the finger of a glove partly drawn inward. In this shape they are always lodged in the midst of the flesh, or in a closed organ, surrounded by a cyst, and

thus circumstanced the worm is harbored by a host which is to serve as a vehicle to introduce it into its final host. It is a parasite on a journey, and usually bears the name of Cysticercus (Fig. 25). In the second shape it is like a ribbon, it attains a great length, always occupies the intestine, and is mainly occupied in producing eggs, which it