_Planaria alpina_ (Fig. 14, B) is characteristic of cold mountain streams, but occurs down to {32}sea-level in England, the Isle of Man, and Ireland, and from its abundance in spring water, probably enjoys a wide distribution underground. In the Swiss Alps it has been found at altitudes of over 6000 feet, at lower levels in the Rhone, and also in the Lake of Geneva. This wide distribution may perhaps be accounted for, partly, by its faculty for asexual reproduction in summer, and also, by the production, later in the year, of hard-shelled eggs which are laid loosely, not attached to stones or plants.[40] But we have no really direct evidence of the means of dispersal of this or of any of the foregoing species, although they all have a wide distribution in Europe. Of extra-European forms the accounts that exist are very fragmentary. The only indubitable diagnostic character of a Triclad is the structure of its genital ducts, and this is accurately known in only a few cases. Several species such as _Dicotylus pulvinar_ (Fig. 16, B), at present known only from Lake Baikal,[41] and others (_Planaria mrazekii_, _P. albissima_) from Bohemia,[42] will doubtless be found elsewhere when they are carefully looked for. _Phagocata gracilis_ is a remarkable North American form, possessing several pharynges (Fig. 14, C and C'), recalling the independent movement of the pharyngeal lobes of _Discocelis lichenoides_ (Fig. 9).[43] OCCURRENCE OF THE MARICOLA.—Little as we accurately know of the distribution of the fresh-water Planariae, our knowledge of the occurrence of the marine forms is still more limited. _Gunda_ (_Procerodes_) _ulvae_ (Fig. 14, D) is the commonest European form, occurring abundantly in the upper part of the littoral zone, on the shores of the Baltic. _G. segmentata_ from Messina has been carefully described by Lang,[44] but these are almost the only species of Maricola which can be accurately determined. They differ from the Paludicola in the position of the "uterus" behind the genital pore and in the absence of a "musculo-glandular organ" (Fig. 14, F). A special interest attaches to the Bdellouridae, a family containing three species, all parasitic on _Limulus_ from the east coast of America. These remarkable Triclads usually have a sucker at the hinder end of the body, by which they attach themselves firmly to the cephalo-thoracic appendages and to the {33}gill-plates, upon which the eggs may be found in considerable numbers. One species, _Syncoelidium pellucidum_, possesses a pair of problematical organs in the hinder part of the body, opening to the exterior ventro-laterally by a couple of chitinous mouth-pieces, but having no connexion with the genital ducts.[45]