This order comprises the internal parasites, called Tape-worms (Cestoid worms), and the old order of the "Cystic Worms" (Cystica); the latter being now known to be merely immature forms of the Tapeworms.
The Taeniada are Scolecids in which the body of the adult is elongated and composed of flattened joints, the anterior extremity ("head") armed with hooklets, or suckers, or both combined. There is no mouth or alimentary canal, and the young pass through a metamorphosis. The mature animal is hermaphrodite.
In their mature condition, the Taeniada (see figs. 113 and 114) are always found inhabiting the alimentary canal of some warm-blooded vertebrate animal; and they are distinguished by their great length, and by being composed of a number of flattened joints or articulations. These joints are not, however, an example of true segmentation, nor do they really constitute the Tape-worm; the true animal being found in the small, rounded, anterior extremity, the so-called "head," or "nurse," whilst the joints are simply hermaphrodite generative segments, which the "head" throws off by a process of gemmation. The "head" (fig. 114, 3, and fig. 113, e), which constitutes the real Tape-worm, is a minute, rounded body, which is furnished with a circlet of hooks or suckers, or both, whereby the parasite is enabled to maintain its hold upon the mucous membrane of the intestines of its host. No digestive organs of any kind are present, not even a mouth; and the nutrition of the animal is entirely effected by imbibition. The nervous system consists of two small ganglia, which send filaments backwards; but there is considerable obscurity on this point, and it has been asserted that the nervous system is entirely wanting, or that there is only a single ganglion. The "water-vascular system" consists of a series of long vessels which run down each side of the body, communicating with one another at each articulation by means of a transverse vessel, and opening in the last joint into a contractile vesicle. It thus appears that all the joints are organically connected together. Whilst the "head" constitutes the real animal, it nevertheless contains no reproductive organs, and these are developed in the joints or segments (fig. 114, 3, and fig. 113, h), which are produced from the head posteriorly by budding. After the first joint, each new segment is intercalated between the head and the segment, or segments, already formed; so that the joints nearest the head are those latest formed, and those furthest from the head are the most mature. Each segment, when mature, contains both male and female organs of generation, and is therefore sexually perfect. To such a single segment (figs. 114, 4, and 113, h), the term "proglottis" is applied, from its resemblance in shape to the tip of the tongue. The ovary is a branched tube, which occupies the greater part of the proglottis, and opens, along with the efferent duct of the male organ, at a common papilla, which is perforated by an aperture, termed the "generative pore." The position of this pore varies, being placed in the centre of one of the lateral margins of the proglottis in the common Tape-worms of man (Taenia solium and T. mediocanellata), but being situated upon the flat surface of the segment in the rarer Bothriocephalus latus. These two elements - namely, the minute head, with its hook-lets and suckers, and the aggregate of the joints, or proglottides - together compose what is commonly called a "Tapeworm," such as is found in the alimentary canal of man and of many animals. The length of this composite organism varies from less than an inch to several yards.
Fig. 113. - Morphology of Taeniada, a Head and a few following segments of Taenia mediocanellata; b A few segments of the same further removed from the head; c and d Segments progressively further removed from the head, - all of the natural size ; e Head of the same, enlarged; h A single proglottis of the same, with its branched ovary and lateral genital pore, enlarged two diameters; f Embryo of Taenia bacillaris, with six hooklets ; g Cysticercus cellulosae, with its hooklets and suckers, its wrinkled neck, and its caudal vesicle, enlarged. (After Leuckart, Van Beneden, and Weinland.)
Singular as is the composition of the mature Tape-worm, still more extraordinary are the phenomena observed in its development, of which the following is a brief account:
"Proglottides," or the sexually mature segments of a Tapeworm, are only produced within the alimentary canal of man, or of some other warm-blooded vertebrate. The development of the ova which are contained in the proglottides, cannot, however, be carried out in this situation; hence the comparative harmlessness of this parasite, and hence the name of "solitary worm," which is sometimes applied to it. For the production of an embryo, it is necessary that the ovum should be swallowed by some animal other than the one inhabited by the mature Tape-worm. If this does not take place, the fecundated ovum is absolutely unable to develop itself. To secure this, however, the dispersion of the ova is provided for by the expulsion of the ripe proglottides from the bowel, all their contained ova having been previously fertilised. After their discharge from the body, the proglottides, which for some time retain their vitality and possess some power of movement, decompose, and the ova are liberated (fig. 114, 1), when they are found to be covered by a capsule which protects them from all ordinary mechanical, and even chemical, agencies, which might prove injurious to them. In this stage, the embryo is often so far developed within the ovum that its head may be recognised by its possession of three pairs of siliceous hooklets. For further development, it is now necessary that the ovum be swallowed by some warm-blooded vertebrate, and should thus gain access to its alimentary canal. When this takes place, the protective capsule or covering of the microscopically minute ovum is ruptured, either mechanically during mastication, or chemically by the action of the gastric juice; and the embryo is thus liberated. The liberated embiyo (fig. 113, f) is now called a "proscolex," and consists of a minute vesicle, which is provided with three pairs of siliceous spines, fitted for boring through the tissues of its host. Armed with these, the proscolex perforates the wall of the stomach, and may either penetrate some contiguous organ, or may gain access to some blood-vessel, and be conveyed by the blood to some part of the body, the liver being the one most likely.