Fertilisation appears to occur in the uterus, where ova, yolk, and spermatozoa, or (in P. torva) spermatophores (Fig. 14, N, sp), are found. The formation of the cocoon in Planaria lactea is probably begun in the "uterus," but is undoubtedly completed in the genital atrium. In P. polychroa, however, the stalked cocoon is formed wholly in the "uterus." Thus we find two types of cocoons in different species of the genus Planaria associated with two types of reproductive organs (Hallez):—
I. Planariae in which the two oviducts open separately into the posterior part of the duct of the uterus. A musculo-glandular organ is absent. The cocoons are spherical and stalked. Examples—Planaria polychroa (Fig. 14, L), P. albissima, P. gonocephala.
II. Planariae in which the two oviducts open by means of an unpaired duct into the genital atrium. A musculo-glandular organ present (Planaria torva (Fig. 14, N), P. mrazekii, P. lactea, P. cavatica), or absent (P. alpina, Fig. 14, M). The cocoons are sessile.
The genitalia of the Maricola (Fig. 14, F) and Terricola do not differ very much from those of Planaria. The uterus (greatly reduced in the Land Planarians) lies behind the genital pore, and several ova, together with much milky yolk, are enclosed in a capsule which is formed in the genital atrium.
Asexual Reproduction.[[58]]—It has long been known that fresh-water Planarians have not only great powers of repairing injuries, but that they use this faculty in order to multiply by transverse fission. Planaria alpina and Polycelis cornuta, in summer, separate off the posterior part of the body, and this ultimately becomes an entire individual. P. albissima, and especially P. subtentaculata, anticipate matters so far, that before fission is complete, the new individual has a head nearly fully formed. The new organs are largely regenerated in both parent and young, apparently by the division and specialisation of scattered embryonic cells in the parenchyma. The asexual reproduction of Land Planarians is not fully proved, though it is known that they repair injuries to the body completely, and that Bipalium kewense is often found in hothouses, divided into fragments which regenerate all the organs of the parent, but like the latter, do not mature their sexual organs.
Fig. 18.—Semi-diagrammatic view of the excretory system of Planaria lactea. (Partly after Chickoff.) can, Capillary network on both dorsal and ventral surfaces; g.br, branches of the intestine; lg, lateral branches of the digestive system; ln, longitudinal nerve; ph, pharynx, with intermuscular capillary excretory network arising from the point marked pht; tp, principal vessels of the excretory system, the external opening of which is not certainly known; vs, vesicula seminalis.
Excretion.—The excretory organs of Triclads consist of flame-cells, canaliculi, and a pair of longitudinal canals, the external openings of which, have not been satisfactorily ascertained. The flame-cells are difficult to detect in Planaria lactea, and the latest observer, Chickoff,[[59]] was unable to see them, although to him we are indebted for figures of this system in P. lactea (Fig. 18) and P. alpina (P. montana). In the latter, the flame-cells are distinct, and may open directly into the two main canals or indirectly through unbranched canaliculi. The pharynx possesses a special supply of excretory tubules communicating with the main canals. A similar system has been described and figured in Gunda segmentata by Lang.[[60]]
Classification of Tricladida.
| PALUDICOLA. | ||
| Family. | Genus and British Species. | |
| Planariidae | Planaria lactea O. F. M., P. punctata Pall., P. polychroa Schm., P. torva M. Sch., P. alpina Dana. Polycelis nigra Ehr., P. cornuta Schm. Anocelis. Oligocelis, Procotyla. (Doubtful genera.) Sorocelis. Dicotylus. | |
| MARICOLA. | ||
| Procerodidae (= Gundidae). | Procerodes (= Gunda) ulvae Oersted, P. littoralis van Beneden. Cercyra. Uteriporus. | |
| Bdellouridae | Bdelloura. Syncoelidium. | |
| TERRICOLA. | ||
| Bipaliidae | Bipalium kewense Moseley (introduced). | |
| Geoplanidae | Geoplana. Geodesmus. | |
| Rhynchodemidae | Rhynchodemus terrestris O. F. M. | |
| Belonging to undetermined Families |  | Dolichoplana. Polycladus. Microplana. Leimacopsis. |