The Scolex, when fully formed, has its own individual development arrested at this point; but it now begins to give off buds, of which the body (Strobile) of the Entozoon is composed.

* Vide " Recherches sur les vers Cesto'ides," par P. J. Van Beneden, Nouveaux Mem. de l'Academie de Bruxelles for 1850.

(351). The Scolex, therefore, in this stage of development is synonymous with the "head," or, as it might as well be called, the "root" of the worm; and as long as this root, head, or Scolex remains unexpelled from the body, it will continue to give origin to fresh segments or joints, ad libitum.

(352). Gradually the tail of the Scolex, or the body of the worm, is developed; and as soon as this has attained a certain length, transverse markings begin to make their appearance, segments are formed, separated from each other by slight indentations, and the internal organs appropriate to each segment are progressively evolved. When the segments have attained to maturity, or, in other words, when the gemma has grown into an adult worm (Proglottis of Van Beneden), the indentation separating each from the one preceding it increases in depth, until, being reduced to a mere pedicle, the segments are successively thrown off as so many distinct animals. From the above account, therefore, it is evident that the last, or caudal, segment is always the oldest, the newly-formed segments continually pushing the others from before backwards.

Scolex; b cd, Strobile; e, Proglottis.

Fig. 70. «, Scolex; b cd, Strobile; e, Proglottis.

(353). Most frequently the mature segment or Proglottis is detached, as stated above, and becomes an independent worm: nevertheless this probably does not invariably happen, some apparently remaining permanently connected together, and laying their eggs without having enjoyed a separate existence, as is the case in various forms of Asci-dians and Polyps.

(354). While the segments of the Strobile remain conjoined, they seem to enjoy a complete community of life and of movement. Some species especially may be observed to become suddenly dilated in one region and contracted in another - these alternate movements, passing along the entire length of the animal, giving precisely the same appearance as is witnessed in many Annelidans when they make violent efforts for progression, - a circumstance which will readily explain how Taeniae are frequently met with having their bodies tied in complicated knots - a very puzzling phenomenon to the older helminthologists.

(355). The Proglottis, on becoming detached from the general community, is provided with all its organs; nevertheless its development becomes still further advanced: it even completely changes its shape; the angles of the segment become effaced, the whole body rounded, and its movements, moreover, more extensive: nay, as Van Beneden assures us, not only does the Proglottis continue to grow, but sometimes it becomes as large as the entire Strobile - a circumstance which frequently causes a Cestoid at this age to be mistaken for a Trematode Entozoon.

(356). Many thousands of eggs must be produced from such multiplied sources of reproduction; and yet, how are they preserved and replaced in circumstances favourable to their development? Fortunately it is rare to meet with more than one of these creatures, at the same time, taking up a residence in the same individual; and, in fact, the species which has specially been the subject of our description is often called, par excellence, "the solitary worm," from this circumstance. Yet what becomes of the reproductive germs furnished in such abundance? Do they, as was the opinion of Linnaeus, live in a humbler form in stagnant waters and marshes, until they are casually introduced into the body of some animal, where, being supplied profusely with food and placed in a higher temperature, they attain to an exuberant development? Or are the germs thus numerous in proportion to the little likelihood of even a few of them finding admission to a proper nidus? To these questions we can only reply by conjectures; and, interesting as the subject is, few are more entirely involved in mystery.

(357). In some Taeniae, as for example in T. serrata, which is found in the intestines of dogs, M. Dujardin has pointed out that the ova, instead of being, as Rudolphi supposed, more delicate and frail in their substance than the Entozoa themselves, are defended by envelopes so strong that, thus protected, they may be dispersed in prodigious numbers in various situations, and escape destruction until conveyed into a nidus proper for their development.

(358). To form some idea of the number of ova furnished by a single Taenia of this species, it must be considered that it is furnished successively with at least two hundred segments, which in the aggregate will produce for each Taenia 25,000,000 of eggs. The mature segments arc found loose in the intestine of the dog, and are able to move about with considerable quickness, creeping sometimes at the rate of three inches in a minute, by the contractions of which they are capable. If one of these be placed in a flask, or under a moist glass bell, they will soon begin to crawl about upon its surface, leaving a sort of milky track wherever they pass, in which, by the aid of a lens, innumerable eggs may be detected. Under these circumstances they will exist for several days, until they are entirely emptied of their ova and reduced to half their original bulk, when, their destiny being accomplished, they perish. Therefore it cannot be doubted that, when expelled naturally from the intestines of the animal in which they live, they are able to deposit their ova in a similar manner.

(359). Many interesting facts relative to the development of the intestinal worms have been recently brought to light, and promise to lead to still more important discoveries. In 1840 M. Miescher announced, to a meeting of naturalists at Bale, the discovery that several genera of Entozoa undergo most extraordinary metamorphoses, whereby their form and character are completely changed. A Filaria, met with in a fish, became changed into a flat, oval, leaf-like worm, in fact a Planaria; from the interior of the Planaria there subsequently issued a Tetra-rhynchus, armed with four long proboscides; and lastly, the last-mentioned form probably gave birth to a Boihriocephalus.