The common tapeworm, tonic solium, derives its Linnatan title from the idea that it is always a solitary worm. Although this is commonly, it is not by any means always the case: Klichenmeister has several times found two or three together, and cases are recorded in which 30 and even 40 worms have been expelled from one patient. The full-grown. tapeworm (strobila) has been known from the earliest times, and is described by Hippoc rates, Aristotle, and.Pliny• but its organization and mode of development have only been properly understood during the last few years. The segments of which it is com posed vary in size, and number froth 800 to 1000, the earlier immature ones being extremely narrow, and the sexually mature joints commencing at about the 450th segment. From 10 to 35 ft. may be regarded as representing its ordinary length; its breadth at about the widest part being one-third of an inch. The head, which is seldom seen in the tape worms exhibited in our museums, although the evacuation of the head with the rest of the worm is not very rare, is very small and globular (about the size of a pin's head), with black pigment ingrained in it. On examining it with a low magnifying power, it displays four circular sucking disks, in front of which is a conical proboscis, armed with a double crown of hooks, from 23 to 28 in each circular row. The head is succeeded by a very narrow neck, nearly half an inch in length, which is continued into the anterior or sexually immature part of the body, in which traces of segmentation first appear in the form of fine transverse lines, which are gradually replaced by visible joints. These joints or segments represent the body, and each mature segment contains both male and female organs of generation; and in addition to these structures, the entire series of joints is traversed by a set of vascular canals constituting the so-called aquiferous sys tem, which consists of two main channels, one passing down on either side of the worm, and both being connected by transverse vessels, which occur singly at one end of every joint. It is only in the alimentary canal of man or some other animal that a tapeworm of any kind can attain to sexual maturity: and in all of these the eggs are fecundated before being discharged. The expulsion of the eggs may take place in any of the following ways: First, the mature segments separate from each other, and passing out of the body, either with the ordinary evacuation of the bowels or independently, become decomposed, and set free the enclosed eggs. The single joints thus discharged undergo violent contraction after being expelled, which led to their being formerly mistaken for a distinct species of worm, to which the title vermes cucurbitini was applied, from their resemblance to a pumpkin seed. There is a figure in Aitken's Medicine, 3d ed. vol. i. p. 815, showing the joints of a tanks, mediocanellata (which will be presently described) of the natural size, in various stages of contraction; and on examining the recently discharged excrement of a con stipated dog, the same phenomenon may be very frequently observed. Secondly, the eggs may be discharged through the genital pore by pressure from any cause. It is only thus that we can account for the occasional (but very rare) coexistence of a cysticercus cellulose (the embryo of the worm) and an adult tapeworm in the intestinal canal of the human subject—an association which constitutes one of the most serious dangers which the matured worm can inflict upon its host, and one of the strongest indications for its removal. Thirdly and lastly, the mature joints sometimes appear to undergo disintegra tion within the intestine, and to liberate the eggs; but the conditions under which this disintegration occurs are unknown. In reference to the ultimate fate of the embryos in oro, that are liberated in the intestinal canal, Dr. Cobbold has informed the author of this article in a private communication, that, in his opinion, they do not migrate in the living host, except when by regurgitation they occasionally get into the stomach, when, after their shells have been dissolved by the gastric juice, the young organisms commence their wanderings. The mature segments are usually expelled from the human bowel at the rate of six or eight a day. Their vitality is prolonged by moisture, which favors the distribution of the liberated eggs over grass and other vegetables, or in water, which may be used as food or drink by animals. For a full description of the eggs we must refer
to Dr. Cobbold's work. It is sufficient here to remark that, in their mature condition, they " present a globular figure, and are easily recognized by theitremarkably thick shell, which surrounds the six-hooked embryo. They present an average diameter of of an inch, the shell itself measuring about of an inch in thickness. After a while, by accident as it were, a pig coming in the way of these embryos, or of the proglottides, is liable to swallow them along with matters taken in as food. The em bryos, immediately on their being transferred to the digestive canal of the pig, escape from the egg-shells, and bore their way through the living tissues of the animal, and having lodged themselves in the fatty part of the flesh, they there rest to await their fur ther transformations or destiny. The animal thus infected becomes measled, its flesh constituting the so-called measly pork. In this situation the embryos drop their hooks, and become transformed into the cysticercus celluloscs. A portion of this measled meat being eaten by ourselves, either in a raw or imperfectly cooked condition, transfers the cysticercus to our own alimentary canal, in which locality the cysticercus attaches itself to the wall of the human intestine, and•having secured a good anchorage, begins to grow at the lower or caudal extremity, producing numerous joints or buds to form the strobila or tapeworm colony."—Cobbold, Entozoa, p. 221. In its fully mature stage the measle presents the appearance of an elliptical hydatid, varying in size from that of a pea to that of a small kidney-bean, the average diameter being one-third of an inch. On dis secting or breaking up a measle it will be seen that the great vesicular portion constitutes the bladder-like caudal extremity of the cysticercus, while the head, neck, and body can be drawn out so as to exhibit a vermiform character.
In the article GENERATIONS, ALTERNATION OF, it was stated that the group of phe nomena included in that term would be further illustrated in the history of the tape worm. From what has been already shown it appears that "we have a simple alterna tion of generation in which the immediate product of the proglottis (or sexually matured zooid individual) is a six-hooked brood; by metamorphosis the latter becomes trans formed into the cysticercus, having a head with four suckers, and a double crown of books; and by gemination the latter gives rise to a whole colony (strobila) of individuals, the greater part of which are destined to become sexually mature—zooid individuals or proglottides. It will be observed, therefore, that the product of a single ovum is, in the first instance, a single non-sexual embryo; in the second phase, it becomes a non-sexual cysticercus (these two phases together constituting the protozooid); in the third change it gives off, by budding, numerous gemmules, most of them destined to be sexually ma ture individuals (or deuterozooids), in this way resembling their original parents. The relation and nature of these developmental changes may be further simplified by placing the various life phases in a tabulated form as follows: (a.) Egg in all stages. ' 1 Six-hooked embryo = proscolex. }-Protozooid. c. Resting larva, or cysticercus (telce) cellulose; (seolex). J d.) Immature tapeworm. .) Strobila, or sexually mature teenier solium. .) Proglottis (cucurbitinus) = free segment = deuterozooid. —Cobbold, Entozoa, pp. 221, 224.
How long a tapeworm can naturally exist in an intestinal canal is not known; but there is doubtless a period at which the parasite snontaneouslv senarates from the tato..
tinal mucous membrane of its hosf- –a period probably coinciding with the shedding and non-renewal of the circlet of hooks. When this separation occurs, the whole length of the worm is expelled, in the same manner as if the parasite had been first killed by the administration of a vermifuge medicine. From this history of the structure and life history of this organism, which applies with slight difference in minor points to all other tapeworms, we proceed to describe the injurious effects which the worm in its adult and larval produces on man, and the precautions which should he taken to prevent its entrance into the system; while the discussion of the means of expelling it when it has once found a lodgment in the intestinal canal, will be postponed to the article on FERMI