Having by one of these methods reached a suitable resting-place, the proscolex now proceeds to surround itself with a cyst, and to develop a vesicle, containing fluid, from its posterior extremity, when it is called a "scolex" (fig. 114, 2, and fig. 113, g). In some of the Taeniada the scolices are called "hydatids," and it is these, also, which constituted the old order of the "Cystic Worms." When thus encysted within the tissues of an animal, the "scolex" consists simply of a taenioid head, with a circlet of hooklets and four "oscula" or suckers, united by a contracted neck to a vesicular body. It contains no reproductive organs, or, indeed, organs of any kind, and cannot attain any further stage of development, unless it be swallowed and be taken for the second time into the alimentary canal of a warm-blooded vertebrate. It may increase, and produce fresh scolices, but this takes place simply by a process of gemmation. The series of changes, however, whereby the scolex is converted into the "strobila," or adult tape-worm, cannot be carried out unless the scolex gain access to the alimentary canal of a warm-blooded vertebrate. In this case, the scolex attaches itself to the mucous membrane of the intestinal tube by means of its cephalic hooklets (when these are present) and suckers. The caudal vesicle now drops off, and the scolex is thus converted into the "head" of the tape-worm. Gemmation then commences from its posterior extremity, the first segments being immature. As the first-formed joints, however, are pushed further from the head by the constant intercalation of fresh articulations, they become sexually mature, thus constituting the "proglottides" of the adult tape-worm with which the cycle began. To the entire organism, with its "head" and its mature and immature joints (" proglottides"), the term "strobila" is now applied.

Fig. 114.   Morphology of Taeniada. 1. Ovum containing the embryo in its leathery case; 2. Cysticercus lotigicollis; 3.

Fig. 114. - Morphology of Taeniada. 1. Ovum containing the embryo in its leathery case; 2. Cysticercus lotigicollis; 3. "Head" of the adult Taenia solium, enlarged, showing the hooklets and cephalic suckers; 4. A single generative joint, or proglottis, magnified, showing the dendritic ovary (o), the generative pore (a), and the water-vascular canals (b); 5. A portion of a Tape-worm (strobila), showing the alternate arrangement of the generative pores.

In the development, therefore, of the tape-worm, we have to remember the following stages :

1. The ovum, set free from a generative joint or proglottis.

2. The proscolex, or the minute embryo which is liberated from the ovum, when this latter has been swallowed by any warm-blooded vertebrate.

3. The scolex, or the more advanced, but still sexually imperfect embryo, into which the proscolex develops, when it has encysted itself within the tissues of its host. (Under this head come the so-called "Cystic Worms.")

4. The strobila, or adult tape-worm, into which the scolex develops itself, when received into the alimentary canal of a warm-blooded vertebrate. The strobila is constituted by the " head," and by a number of immature and mature generative segments or joints, termed the "proglottides."

The subject will perhaps be more clearly understood by following the development of one of the common tape-worms of man - viz., the Taenia solium. Commencing with an individual who is already suffering from the presence of this parasite, one of the most distressing symptoms of the case is found to be the escape of the joints of the animal from the bowel. These joints are the ripe "proglottides," containing the fecundated ova. When the ova - which are microscopic in size - are liberated by the decomposition of the proglottis, they may gain access to water, or be blown about by the wind. In many ways, it is easy to understand how one of them may be swallowed by a pig. When this occurs, a "proscolex" is liberated from the ovum, and bores its way through the walls of the stomach, to become a " scolex." It now takes up its abode, generally in the muscles, in which position it was originally described as a cystic worm under the name of Cysticercus cellulosae (fig. 113, g), constituting what is commonly known as the " measles " of the pig. In this state the scolex will continue for an indefinite period; but if a portion of " measly " pork be eaten by a man, then the scolex will develop itself into a tape-worm. The scolex fixes itself to the mucous membrane of the intestine, throws off its caudal vesicle, and commences to produce " proglottides" instead, becoming thus the "strobila" of the Taenia solium, with which we originally started. The other common tape-worm of man - viz., the Taenia mediocanellata - is derived in an exactly similar manner from the "measles" of the ox. In like manner, the tape-worm of the cat (Taenia crassicollis) is the mature form of the cystic worm of the mouse (Cysticercus fasciolaris); the Taenia crassiceps of the fox is derived from the Cysticercus longicollis of the vole (Arvicola terrestris); one of the tape-worms of the dog (T. serrata) is the fully developed form of the Cysticercus pisiformis of hares and rabbits; another of the tape-worms of the dog (T coenurus) is the adult of the Coenurus cerebralis, which gives rise to the "staggers" of the latter animal; and another (T. echinococcus) spends its larval stage in the tissues of man; while T. marginata of the dog and wolf is the mature stage of the Cysticercus tenuicollis of the ruminants and of the pig. On the other hand, the embryo of the "Russian tape-worm" (Bothriocephalus latus) is not "cystic," but is ciliated and furnished with hooklets, whilst the scolex apparently leads an independent life in water, and its intermediary bearer (supposed by some to be a fish of the salmon tribe, or by others to be a fresh-water Annelide) is at present unknown.

Besides being liable to the attacks of various species of adult tape-worms, man is not uncommonly attacked by " scolices," or the larval forms of the tape-worms of other animals. Thus, what are professionally termed " hydatids" are really the scolices of one of the tape-worms of the dog (the Taenia echinococcus}. The "strobila" or adult worm (fig. 115, A) is only about a quarter of an inch in length, and is singular in consisting of only four segments, including the " head." The last segment is sexually mature, and the head is furnished with hooklets and suckers. The egg gives rise, when swallowed by man, to a "proscolex," which bores its way through the walls of the stomach, and finds a lodgment in some solid organ - very commonly the liver. The primitive scolex now consists of a spherical vesicle (fig. 115, C) with a thick laminated external covering enclosing a central granular mass. This mass ultimately forms a cellular membrane lining the outer laminated envelope, and from it are developed numerous secondary "scolices," each of which is attached to the parent cyst by a pedicle, and is furnished with a crown of hooklets and four suckers, but is destitute of a caudal vesicle (fig. 115, D). In fact the parent cyst may be regarded as morphologically composed of the coalescent caudal vesicles of the contained "heads" or "Echinococci" The parent cyst may grow to a great size, and is filled with a clear fluid, whilst instead of producing simple "Echinococci," it may bud off numerous " brood - capsules," each of which develops in its interior a group of Echinococcus heads (fig. 115, B), or it may produce numerous secondary cysts, which may repeat the process, and all of which may incessantly produce "brood-capsules" and

Fig. 115.   A, Sexually mature Taenia echinococcus, showing the head with its hooklets and suckers, and the three succeeding proglottides, the last containing the reproductive organs (enlarged); o Ovary; v Water vessels. B, Interior of a portion of a hydatid cyst, showing the brood capsules and included Echinococci (from Man). C, Young Echinococcus, about six weeks old, showing the thick laminated outer capsule and the inner granular contents. D, Single Echinococcus (from Man), showing the hook lets, suckers, contained

Fig. 115. - A, Sexually mature Taenia echinococcus, showing the head with its hooklets and suckers, and the three succeeding proglottides, the last containing the reproductive organs (enlarged); o Ovary; v Water-vessels. B, Interior of a portion of a hydatid cyst, showing the brood-capsules and included Echinococci (from Man). C, Young Echinococcus, about six weeks old, showing the thick laminated outer capsule and the inner granular contents. D, Single Echinococcus (from Man), showing the hook-lets, suckers, contained "calcareous corpuscles," and pedicle. All the figures are enlarged. (After Spencer Cobbold and Wilson.)

"Echinococci." The disease known as "hydatids" in the human subject is, therefore, indicated by the presence in the liver or other solid organ of a strong membranous cyst, often of large dimensions, filled with a transparent watery fluid, and having attached to its interior innumerable minute secondary "scolices," or "brood-capsules," or containing daughter cysts produced by a peculiar form of gemmation. In some countries, as in Iceland, the disease is very common, and it is of a very serious character, as the growth of the tumour is apt to gravely interfere with the vital functions, or even to produce fatal results. The symptoms, of course, depend upon the position occupied by the cyst, and the importance to life of the organ affected.