OCCURRENCE OF THE PALUDICOLA.—The Planarians of our ponds and streams are the most familiar and accessible Turbellaria. Their elongated, flattened bodies, and gliding movements, render them conspicuous objects on the under surface of stones and on the leaves of aquatic plants, where they live gregariously. The variable _Polycelis nigra_ (Fig. 14, H) is very abundant in stagnant water and slowly-moving streams, whereas its ally, _P. cornuta_ (Fig. 14, G), distinguished by a pair of tentacles, is more local. _Planaria_ (_Dendrocoelum_) _lactea_ (A), _P. polychroa_ (I), _P. torva_, and _P. punctata_ are not infrequently found together, but the last is at once the largest and rarest. {31}[Illustration: FIG. 14.—Forms of Triclads, with the distinguishing specific characters of certain British forms. A, _Planaria lactea_ O. F. M., × 2; B, _Planaria alpina_ Dana, × 4 (after Kennel); C, _Phagocata gracilis_ Leidy (after Woodworth), × 6; C', the same with the pharynges (_ph_) extruded; D, _Gunda ulvae_ Oer., × 4; E, _Planaria gonocephala_ Dug. (after Schmidt), × 4; F, genitalia of _Gunda ulvae_ (after Wendt); G, head of _Polycelis cornuta_ Schm.; H, head of _Polycelis nigra_ Ehr.; I, head of _Planaria polychroa_ Schm. K to N show the distinctive characters of the genital ducts in K, _Polycelis nigra_; L, _Planaria polychroa_; M, _Planaria alpina_; N, _Planaria torva_ Schultze (after Iijima and v. Kennel). _ga_, Genital atrium; _go_, common genital opening; _mgr_, "musculo-glandular organ"; _mo_, "mouth"; _ovd_, oviduct; _pe_, penis; _ph_, pharynx; _pyr_, pyriform organs of unknown significance; _sc_, sucker; _sp_, spermatophore lying in (_ut_) uterus; _vd_, vesicula seminalis. (All except C and E are found in England.)]