The is a flat, jointed. worm which belongs to the order ces toda. Several varieties of this parasite may be found in the human subject. The most common is the tcenia (the beef tape-worm). The tcenia solium (the pork tape-worm) is also met with. The bothrioce_phalus latus, another species, is not common in the British Islands, although it is less rare on the continent of Europe. There are other varieties, but these, as they are very seldom seen, need not be here considered.
All these worms are flat, segmented creatures, destitute of mouth or ali mentary canal. They grow from the head, which developes a continuous linear series of new joints by a budding process. The joints are quadri lateral in shape. They are at first immature, but as their distance from the head increases, they become larger and more developed. Strictly speaking, the tape-worm is not a single parasite, but a community of indi vidually distinct creatures, of which only the lower or older members (pro glottides) are sexually complete. These contain each their own organs of generation, both male and female.
Between the T. medio-cannellata and the T. solium, the difference is chiefly in the shape of the head. In each, the neck is tapering and thread like, and about an inch in length. This passes gradually into the anterior part of the body, which is sexually immature, and is not distinctly jointed. By degrees the transverse lines, which mark the imperfect divisions of the young segments, become more defined and more widely separated, so that, while the more recent segments, or those nearest to the neck, are much wider than they are long, the older joints, as they become more and more mature, grow to be much longer than they are broad. Each mature seg ment (or proglottis) is about half an inch long by a quarter of an inch broad. It contains an elongated, tubular uterus, branched on either side ; and the male and female organs of generation open by a common perfo rated papilla, which is placed at the border below the middle line, on one side or the other, but not in regular alternation. In a worm eight feet long, the total number of joints has been reckoned at about eight hundred ; but it is not until near the four hundred and fiftieth segment from the head that the joints begin to be sexually mature. The head is globular, and
,pout the size of the head of a small pin. In the T. solium, it forms in front a short cylindrical proboscis (rostillutn) having four projecting suckers decorated by a crown of twenty-six hooklets. In the T. medio-cannellata there is no crown of hooklets or proboscis ; but the suckers are large and prominent, and there is usually a fifth smaller one in the ordinary position of the rostillum.
These worms often grow to a great length and may measure many yards. They infest the small intestine and may number one or more in the same subject. The eggs, which are very numerous, lie in the uterine ducts of the mature segments ; and each contains an embryo which, in the case of the &Tula solium, is furnished with three pairs of hooklets.
The mode of development of the creature is as follows :—The unlike the other worms which have been described, does not pass through all the stages of its growth from the ovum to maturity in the body of the same individual, for the embryo does not develope directly into the perfect worm. There is a transitional stage which requires to be completed in the body of an intermediary. This agent is usually an animal. Thus, when a ripe joint filled with ova is eaten by an animal, it passes into the stom ach. There, the eggs are ruptured, and the embryos (pro-scolices) escape. These embryos have a tendency to perforate the tissues of the animal by whom they are harboured. They may thus make their way into the cellular tissue of a muscle, into the liver or the brain. Thus sheltered, they pass through a metamorphosis, and become the cysticercus or bladder-worm. The cysticercus celluloste of pork consists of a cyst-like body; with a head and neck like those of the fully-developed worm. These are usually in verted within the body. As long as the cysticercus is unmolested it under goes no further change ; but when the flesh of the animal is eaten imper fectly cooked, so that the vitality of the cysticercus is uninjured, the creature at once adapts itself to its new situation, and attaching itself to the wall of the small intestine, developes in the course of a few months into the perfect tape-worm.