The Cestodes or Tape-worms are exclusively endoparasitic Platyhelminthes living, in the adult condition, in the alimentary canal of Vertebrates, with the exception of _Archigetes_ (Fig. 37), which may become mature in the body-cavity of _Tubifex_. In relation with this wholly parasitic existence, the Cestodes exhibit certain characteristic modifications in structure and mode of development, such as the formation, by the segmentation of the "neck," of a (usually) long chain of "proglottides" or joints, which form the "body" of the Cestode; and the entire absence of an alimentary tract, both in the larva and adult. As an adaptation to the fixed mode of life, the anterior end (head, scolex) is modified to form an adhering organ. Various adaptive forms of larvae are known. These live in the internal organs of one or more intermediate hosts, and are transferred to the final host passively during a meal. Lastly, there is the curious metamorphosis by which the adult is formed from a portion (scolex) of the larva.[94] {75}[Illustration: FIG. 36.—_Echinobothrium affine_ Dies., from the intestine of _Torpedo_, × 43. _hd_, Head; _hk_, hooks; _hl_, lobes of the head; _ov_, ovary; _pe_, penis; _ps_, penis-sheath; _te_, testes; _ut_, uterus; _vag_, vagina; _yg_, yolk-glands. (After Pintner.[95])] _Taenia solium_, from man (Fig. 39, B), or _Echinobothrium_ (Fig. 36), from an Elasmobranch fish, is fixed to the mucous lining of the intestine of its host by means of a radially-constructed apparatus of four suckers and a circlet of hooks (Fig. 39), which are borne by the "head" or "scolex," being that part of the worm which is directly derived from part of the larva, and which contains the central, commissural portion of the nervous system. Firm adhesion to the host's intestine is necessary, in order to avoid the loosening action of the peristaltic movements of the intestine as the food passes along. The heads of different Cestodes exhibit a marvellous variety of suckers and hooks, from a mere muscular depression in _Schistocephalus_, to the compound proboscides of _Tetrarhynchus_[96] which is found in Elasmobranchs. The jointed body, often of enormous length (up to 20 yards in _Bothriocephalus latus_), is usually separated from the head by a slender neck, from which the proglottides are segmented off from behind forwards, and become more and more individualised as they recede farther away from the neck by the intercalation of younger joints. Thus in Fig. 36 the mature, distal proglottis has passed through all the stages represented by the other segments. {76}[Illustration: FIG. 37.—_Archigetes sieboldii_ (_appendiculatus_), from the coelom of _Tubifex rivulorum_. × 40. _app_, Persistent larval appendage; _go_, genital pore; _hk_, persistent larval hooks; _ov_, ovary; _sc_, sucker; _te_, testes; _yg_, yolk-glands. (After Leuckart.)]