The neck and body of the worm are more or less flattened. One surface, that beneath which the testes lie, is, judging from the analogy of the Trematoda, considered to be dorsal; the other, to which the vagina is approximated, and on which the genital apertures are sometimes situated, as ventral.
1 The life history of the non-segmented genus Caryophyllaeus is not known. It may be noted that it has recently been asserted that Taenia serrata may undergo its development within the intestines of one and the same host; but the evidence is not conclusive, and such an occurrence is not probable.
2 Taenia echinobothrida (Megnin) from the Fowl and wild and tame Pigeons has the four suckers covered with minute hooks. According to the same authority, hooks, suckers, and even the scolex may atrophy when sexual maturity is attained. See on both points, Journal de l'Anat. et Physiol, xvii. 1881.
There is a cuticle, with a subjacent layer of transverse elastic (? muscular) fibres, a layer of vertical fusiform cells, and a more or less granular matrix, which forms the bulk of the substance of the body. This matrix imbeds cells of various kinds, nuclei, and the muscles of the body. It is traversed by irregular lacunae, generally held to represent the coelome. The muscles form one or more longitudinal layers, and an inner circular layer, inclosing a core of matrix in which lie the main nerves, excretory vessels and genitalia. There are also dorso-ventral fibres; see pp. 225-6. The whole worm is endowed with an extraordinary power of contraction and extensibility. For the calcareous bodies which are absent in the proscolex, see p. 227.
The nervous system of the Taeniadae, so far as is known, conforms more or less closely to that of T. serrata (p. 226). It is always situated in the anterior part of the head. It consists in Bothriocephalus latus, in another species of the genus parasitic in the dog, and in Ligula of a transverse commissure, containing ganglion cells in the former, and of two inconsiderable lateral ganglia from which a stout lateral nerve passes backwards on each side. In B. latus each lateral nerve gives origin to eight nerves which accompany it, four on either side. The principal nerves give filaments to the layer of subcuticular cells. Isolated ganglion cells have been said to occur in the lateral nerves; so too ganglionic swellings in each joint (p. 226). There are no organs of special sense. The pigment specks seen on the head of the scolices of certain species of Tetrarhynchus or on the neck of Echinobothriurn, do not appear to be degenerate viscual organs1.
The excretory system has been found, wherever it has been accurately investigated, to consist of ciliated funnels appended to delicate canaliculi, of main longitudinal canals with or without an intermediate network of finer canals. Ciliated funnels (p. 581) are found in all parts of the body, but most plentifully in a zone superficial to the central core of matrix, or in T. lineata within the layer of circular muscles. The canalicules may unite inter se, and open either conjointly or separately into a superficial network of fine canals', e. g. in Tetrarhynchus, Bothriocephalus; Caryophyt/aeus, or directly into one of the main longitudinal canals, e. g. Scolex Trygonis pastinacae. These canals take a longitudinal course. They number four - two on each side, one dorsal, the other ventral - in the Taeniadae, Tetrarkynckidae, and Tetrapkyllidae. As to the first-named, the dorsal canals have a tendency to atrophy, and may even disappear, and this is especially the case with the large tapeworms that infest man l. The four canals are connected in the head by a simple or plexiform ring; at the posterior margin of each joint by a circular vessel, or when the ventral main canals alone persist by a cross vessel.
In some instances the anastomoses are more complicated (Riehm). A valve, formed by two opposing folds of the walls of the canal, guards the aperture at either end of the cross vessel. The Pseudopkyllideay however, and Caryopkyllaeus possess as a rule a larger number of main canals, 10-24, or even more, according to the genus or species; and Fraipont has drawn a distinction between ascending and descending canals. The former have a smaller calibre, contents rather granular, and are non-contractile; the latter are larger, with clear contents, and distinctly contractile. The two sets of vessels are connected by a network of vessels intermediate in size between them and the canalicules (supra). And it must be carefully noted that the different systems of vessels, i. e. main canals, intermediate network, and canalicules, invariably retain their own proper calibre and never graduate the one into the other. Their walls are formed by a delicate membrane. Those of the main canals are surrounded, according to Pintner, by an epithelium, the cells of which contain yellow droplets.
In Solenopkorus similar cells occur in the matrix in the neighbourhood of the canals, but not in contact with their walls (Griesbach); whilst in Taenia lineata flattened granular epithelioid cells immediately invest them (Hamann). The point is one requiring further investigation. The excretory system opens externally by a posteriorly placed pulsatile vesicle, with which the main canals are connected, e. g. in Caryopkyllaeus; some species of Botkriocepkalus, etc.; and when joints have been detached either by a shortened cross vessel, as in Taeniadae according to Leuckart, or by separate openings, one for each of the main canals, apparently the more usual mode. The pulsatile vesicle is always present, according to Pintner, at the end of the body of the scolex. The main canals have also been observed to open externally at the margins of the body by short tubes; on the head, e. g. in Tetrarkynckus; on the neck as well, e. g. Scolex Trygonis pastinacae, some Taeniae and also on the joints, e.g. Botkriocepkalus punctatus, Triaenopkorus, Taenia osculata. Two such openings in every joint, one on each side, have been observed in the immature Schistocephalus (Riehm 1). These 'foramina secundaria' are probably more common than is usually supposed.