The scolex of Ligula and Schistocephalus developes joints and immature sexual organs while still within the first host, a Stickleback; that of T. crassicollis (= Cysticercus fasciolaris of the Mouse) developes non-sexual joints under similar conditions which however are digested when transferred to the stomach of the Cat, in this instance the final host. But in other Cestoda joints and sexual organs appear only when the scolex reaches its last resting-place.

Though many Cestoda are known, the life histories of relatively few have been traced. The most important tapeworms inhabiting Man are - T. solium, the Cysticercus cellulosae of the Pig, etc. and man himself; T. mediocanellata, the Cysticercus Bovis of the Ox; Bothriocephalus latus, the non-sexual state of which inhabits the Pike and Burbot (Lota vulgaris), perhaps however only as intermediate hosts; and T. Echinococcus, but in the non-sexual state only. The chief forms found in the Dog are given on p. 225. The Cat gives shelter to T. crassicollis (supra) and T. elliptica (P-225).

1 It is possible that there is more than one species of Coenurus. The identity of the two forms occurring in the Sheep and Rabbit has not been established, and there are others known. 2 Villot has proposed to classify the cystic forms of Taeniae as follows: I. Cystic forms properly so-called. ' The caudal vesicle originates from the Proscolex by simple growth and structural modification, without the production, strictly speaking, of any new part.' Cysticercus, Coenurus, Echinococcus.

II. Cystic forms ' in which the caudal vesicle originates from the Proscolex by budding, i. e. by the addition of a new part' = Cysticercoid forms. There is a head, the future Scolex; a body and caudal vesicle, or cystic portion; and a blastogen = Proscolex, 'which preserves its autonomy and embryonic characters.' There are two subdivisions.

A. The caudal vesicle is formed by endogenous budding. Polycercus, the blastogen gives origin to a number of individuals which are contained within it; P. Lumbrici, the Echinococcoid form from the Earthworm mentioned in the text. Monocercus, a single individual is formed and contained within the blastogen; M. Arionis ( = Cysticercus Arionis) where Villot considers what is usually held to be an adventitious cyst as the blastogen; M. Glo?neridis, from Glomeris limbatus; and perhaps some others.

B. The caudal vesicle is formed by exogenous budding. Cercocystis, a single individual is produced by the blastogen and remains in connection with it; C. Tenebrionis (Stein, Z. W. Z. iv. 1853, p, 205); Staphylocystis, the blastogen produces a colony which remain in connection; St. bilarius & St. micracanthus, both from Glomeris limbatus. Urocystis, the blastogen produces a series of individuals which are detached; U. prolifera from Glomeris limbatus. Cryptocystis, the blastogen forms a single individual which is detached; C. Trichodectis ( = Cysticercus T. ellipticae), the strob ila of which is T. elliptica of the Dog (Melnikow, A. N. 35, 1, 1869, p. 62). See Villot, A. Sc. N. (6), xv. 1883.

Braun has proposed to divide the non-sexual forms of Cestoda into (I) Cysticerci with a vesicle containing much liquid; (2) Cysticercoidei with little fluid in the vesicle; (3) Plerocerci, forms with a small but solid Proscolex; (4) Plerocercoidei, forms with a solid elongated Proscolex.

The duration of a Cestode's life is unknown. It probably varies. The pro-scolices of Echinococcus (= hydatids, acephalocysts) have been known to persist for thirty years, and the strobila of T. mediocanellata five, six, seven, nine, or even eleven years. A Cysticercus cellulosae (= T. solium) has been observed in the eye for twenty years.

Archigetes Sieboldi, which occurs in a sexual scolex stage in the coelome of Tubifex rivulorum, an Oligochaete, is about 3 mm. long. It consists of an oval body, the scolex, about I-I-3 mm long, and a cylindrical tail, the proscolex, which is attached to a pit at the end of the body, and bears at its other end three pairs of hooks. Both parts are capable of motion. The scolex has two grooves, one on either side the head. There are eight longitudinal excretory vessels, and a vesicle opening at the insertion of the tail. The testes are placed anteriorly; the vitellaria, one on each side of the body. The genital aperture is ventral, and is the common exit of the vas deferens, a vagina or oviduct, and a uterus separate from the latter. Leuckart, Z. W. Z. xxx. 1878, Suppl.; Gruber, Z. A. iv. 1881.