The uterus of Bothriocephahus opens posteriorly to the genital sinus, of Triaenophorus anteriorly, and, unlike its genital ducts, on the surface of the joints.

1Quoted by Kiessling, A. N. 48, (I), 1882, note, p. 264.

2According to Moniez ('Les Cestodes,' p. 194 et seq.), this organ in the Taeniae named and their allies, e. g. T. serrata, is an ovary or germarium.

1 In Ligula successive germaria are connected, according to Moniez. In both Schistocephalus and Ligula the vitellarian follicles and testes do not appear, according to Kiessling, to be delimited from one another in adjoining proglottides; the vitellarian ductules are continuous throughout the body; A. N. 48, (I), 1882, p, 276.

2In T. lineata the ova accumulate finally in the cavity of the exhausted shell-gland which marks the commencement of the uterus. A resistent calcareous shell is then formed round them.

3T. lineata is an example. Hamann proposes the generic name Ptychophysa for these Taeniae. Pt. lineata has (I) the aperture of the sinus on the surface of the joints; (2) the vagina opening

Self-impregnation appears to be common if not universal; indeed, there does not appear to be the slightest evidence for the fertilisation of one joint by another; and if such an occurrence happens, it must take place by the accidental contact of individuals inhabiting side by side the same host2. The egg is composed of two sets of elements - the germ-cell derived from the germarium, and the vitellogenous cells or the secretion derived from the vitellarium. The germ-cell is nucleated and hyaline as a rule. In Leuckartia and Abothrium it is said to absorb the vitelline granules set free by degeneration of the vitellogenous cells, but in other instances the latter appear either to retain their integrity or else break down as in the Taeniae into a more or less granular albumen. The germ is impregnated, and together with the vitelline cells or secretion enters the uterus where it is surrounded by a shell, the material of which appears to be derived from Mehlis' gland (supra), inasmuch as the latter reaches its greatest development at the time when germs enter the uterus.

The shell is delicate in the Taeniae and where the embryo is formed during the sojourn of the ova in utero, but resistent and furnished with an operculum where as in some species of Bothriocephalus, in Schistocephalus, Triaenophorus, and Ligula, it lies in water for a shorter or longer period. The shell is sometimes furnished with processes, e. g. in Calliobothrium Esch-richti, which however appear in Taeniae to be accidental formations. The ova of certain fish tapeworms turn green or black on exposure even for a few moments to light. The embryo is formed by the fission of the germanteriorly to the cirrus; (3) the uterus at a certain period convoluted; (4) a peculiar shell-gland (supra); egg-shell hyaline, ovate as in Pseudophyllidea. In all four points it differs from a typical Taenia. Cf. Zschokke on sexual organs of T. litterata from the Fox, Z. A. viii. 1885.

1 Van Beneden in his Vers Cestoides sometimes figures the cirrus of fish tapeworms as posterior to the vagina. Leuckart states that this is an error so far, at least, as concerns the genus Tetrarhynchus.

2 The cirrus has been observed in the vagina of the same joint in Taenia Echinococcus by Leuckart; in Phyllobothrium Lactuca by P. J. van Beneden, who witnessed the emission of sperm. 'Depuis,' adds the latter, 'j'ai vu ce phénoméne se reproduire dans d'autres espéces' (Vers Cestoides, p. 64). Sommer states that in T. mediocanellata the sperm passes into the sinus genitalis, and regurgitates into the vagina owing to the closure of the pore. The protrusion of the cirrus hinders impregnation in this case (Z. W. Z. xxiv. 1874, pp. 520-21). See also Moniez on T Giardi, C. R. lxxxviii. 1879.

Cell alone. It has been found in those instances which have been accurately investigated, that the superficial cells of the embryo give origin successively to two envelopes - (1) an outer, which is very delicate in Ligula and other Pseudophyllidea, but in Taeniae constitutes an albuminogenous layer; (2) an inner, which loses its cellular character and is ciliated in Ligula, Triaenophorus, Sehistocephalus, and some species of Bothriocephalus, but is non-ciliated and capable of creeping movements in other species and in certain Taeniae inhabiting water birds and fresh-water fish; whilst in most other Taeniae it constitutes the chitinogenous layer and is transformed into a firm chitinoid embryonic shell (cf. p. 227-8 ante). When the shell is non-resistent it increases in size during the evolution of the embryo and eventually disappears.

The chitinoid shell is lost when the Taenian embryo enters the stomach of its first host. The soft ciliated or non-ciliated coat of the Pseudophyllidea, etc. is stripped off after a time, a gap previously appearing between it and the contained embryo; it is possible, however, that this occurrence takes place normally only after the entry of the embryo into its first host. The embryo itself is cellular, and provided as a rule with six hooks arranged in pairs close together at one pole. The number of hooks may be increased 1. The muscles which move these hooks are sometimes visible as delicate lines. The embryo itself is composed of cells, among which a superficial set may be distinguished from a more central, the former appearing to grow round the latter2. It is possible that the former may represent an epiblast, the latter a hypoblast. After its entry into the first host (p. 656) the embryo or proscolex grows and its tissues become differentiated. The account given of T. serrata p. 230 may be considered as typical. The adventitious connective tissue cyst formed by the metamorphosed lymph cells of the host is not (?) present when the first host is a non-vertebrate. The size attained by the proscolex varies much.

When its central cells liquefy, as in many Taeniae, and it is large, it is more or less globular in shape; when they do not liquefy and it is large, it assumes an elongated shape, e. g. in Tetrarhynchus. It sometimes grows into an irregular or branched form, due apparently to the influence of the tissues surrounding, e. g. the racemose form of Cysticercus cellulosae from the base of the human brain. It may multiply asexually as in the formation of brood capsules or daughter vesicles in Echinococcus, in Staphylocystis and Urocystis which inhabit the Myriapod Glomeris limbatus. It gives origin to a head and neck = scolex, except in Echinococcus, in which the scolices originate from brood-capsules only, or the proscolex and its daughtervesicles sometimes grow and multiply without giving origin to scolices at all. The scolex developes from a small mass of cells: see p. 230. The majority of non-sexual Cestoda have the Cysticercus-form, i. e. a single scolex in connection with each proscolex. The Coenurus-form, i. e. many scolices in connection with a single proscolex, occurs in T. Coenurus 1, and perhaps in Triaenophorus. Brood-capsules which give origin to several scolices are distinctive of T. Echinococcus; or to one scolex, of an Echinococcoid inhabiting the Earthworm 2.

1Cf. Leuckart, Parasiten (ed. 2), pp. 417-18; Hamann, Z. W. Z. xlii. p. 728. The entire absence of hooks in some cases, asserted by J. P. van Beneden (Vers Intestinaux, p. 237), is extremely doubtful, and in certain instances, e. g. Ligula, Bothriocephalus, is known to be incorrect.

2Cf. E. van Beneden, Archives de Biol. ii. p. 198; Hamann, Z. W. Z. xlii. p. 728.