They are liable to be swallowed by man or by animals in drinking-water, and to be swallowed by animals while grazing. But man and ani mals in general are not suitable soil for them. This is fortunate, else how few would escape becoming a home for them. There is one ani mal which, in this stage, is suited as soil for their development, and that animal is the pig. Let a pig in its eating or drinking swallow some tapeworm eggs. In the pig's stomach the egg is dissolved and the little embryo is liberated from its case. It does not remain in the stomach, but forthwith proceeds, by means of its boring apparatus, to pierce its way through the walls of the stomach and other tissues, till it reaches a place convenient for its settling down. It may get into a blood-vessel and be thus carried to some suitable organ, liver, brain, &c., but it specially selects muscle, and, in par ticular, connective tissue between muscular fibres. Having reached a proper place it loses its boring apparatus and gains the circles of booklets characteristic of the head of the per fect tape-worm, and it develops at the extremity opposite the head a small sac or bladder filled with fluid. In the living state the head and neck are coiled up in the sac (Fig. 115, e), but they project from the sac after death (Fig. 115, 1). It is now called a cysticercus cellulose, and it has now completed its second stage, being incapable of further growth within the body of the pig. In the pig, that is to say, it remains a bladder-worm and will not produce the perfect tape-worm. In the flesh of the pig it appears as a little round or oval sac, the size of a pea or even (Fig. 115, e).
Now a pig may have swallowed only a few eggs or a multitude. If the latter has been the case there will be a multitude of such cysticerci in its flesh, and the pork will have what is called a " measly " appearance. "Measles" of pork are due to these cysticerci or immature tape-worms. Now, before the cystic worm can • produce a tape-worm it must be transferred to the body of some other warm-blooded animal. Suppose a man eats a piece of "measled " pork which has not been cooked well enough to destroy the vitality of the cysticercus, after the piece of meat reaches the stomach the embryo is freed from its cyst by the action of the gastric juice. It thereupon attaches itself by its circle of booklets to the mucous lining of the bowel, the tail vehicle drops off, and it becomes a scolex. Immediately behind the head minute segments are produced by a process of budding. and, in from twelve to sixteen weeks, some segments aay have become quite mature and be passed s joints by the patient, at which time the tape vomit has reached its adult form.
There are thus three main stages in the levelopment of the tape-worm: I. The egg stage in the fully-developed joint r proglottis.
IL The stage of cysticercus, developed from he egg in the body of a pig; and III. The tape-worm stage, produced in man the eating of pork containing the parasite in is second stage. The joints of the fully-de oreloped animal produce the eggs, which must escape from the body of man and gain entrance A the body of a pig to undergo development.
We have seen that it is in the pig particularly ;hat the eggs of the tape-worm attain the second stage of their growth, and that it is specially From eating improperly cooked "measly" pork that persons are liable to get tape-worm. Yet the cysticercus stage may be developed in other %Plurals, and it has been found, though rarely, in the muscles, brain, and eye of man. It is,
nevertheless, the development in the pig that is common and worthy of remembrance. For this reason this form of tape-worm has been called the " pork tape-worm." The symptoms and treatment, and what in some respects is of even greater importance, the prevention of an attack of tape-worm, will be considered when the other forms of the parasite have been described.
Tzenia mediocanellata (Beef Tape-worm). —This tape-worm goes through a series of wanderings and transformations precisely simi lar to those of Tcenia solium. It is, how ever, no longer the pig but the ox or the calf that affords the most suitable soil for the de velopment of its embryo form. Lodged in the intestinal canal of man, it gives off joints just as the former tape-worm. The joints con tain eggs, which are dispersed and find their way ultimately into the stomachs of cattle, whence they work their way, by means of a boring apparatus, into the tissues of the animal and give rise to the "beef measle." This form has, therefore, been called the beef tape-worm. If a piece of improperly cooked beef containing "measles" be eaten by a person, the embryos become free, settle in the bowel, and develop into the Tcenia mediocanellata. But while the natural history of the parasite is similar to that of the pork tape-worm, beef or veal being substituted for pork, in its structure it differs to a considerable extent from Tcenia Viewed by the naked eye the two could not readily be distinguished. The beef tape-worm is, nevertheless, longer, being from 15 to 23, and even 30 feet long, and its joints, or pro glottides are broader. The head also differs from that of Tcenia solium in that it has no proboscis nor circle of booklets. It has, how ever, four sucking discs as shown in Fig. 116, which represents the head.
The absence of the booklets is of considerable significance.
We have seen that the circle of booklets of the pork tape worm affords a means for securely anchoring the head to the mucous membrane of the intestine. So firm is this anchorage that it constitutes one of the difficulties of treat ing the pork tape-worin, the medicines administered often failing to detach the head, and so long as the head remains there is a risk of the tenia going on developing fresh joints. The beef tape-worm, having no hook lets, is less securely fastened and more easily brought away. It is commoner, therefore, to get a specimen of a Tcenia ?nediocanellata, complete with head, than to get a complete Thenia solium, the head, in the latter case, being frequently wanting. Owing to this difference the pork-worm is often called the "armed tape-worm," and the beef-worm the "unarmed tape-worm." It was formerly sup posed that the pork tape-worm was the com monest of all tapeworms, but Dr. Spencer Cobbold declares that the facts warrant him " in styling this worm [the beef tape-worm] the most common of all liable to invade the human body." He also goes on to say "it seems strange to speak of measly beef, and yet, probably more diseased beef exists in this country than sitni larly-affected pork." The beef measly, the cysticercus stage, that is, of the Tcsnia canellata, is smaller than the pork measle—not so large as a pea—and is much more easily looked. The head of the embryo worm within its cyst is, however, a little larger than that of the pork-worm, but, ! like the head of the fully-developed parasite, it has no hooklets. A magnified view of one of the cysticerci of the ox is given in Fig. 117. It has never yet been found in man.