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, {41}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. [Illustration: 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] {42}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._ } Doubtful _Procotyla._ } genera. _Sorocelis._ _Dicotylus._ MARICOLA. PROCERODIDAE _Procerodes_ (= _Gunda_) _ulvae_ Oersted, _P. littoralis_ van Beneden. (= Gundidae). _Cercyra._ _Uteriporus._