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.
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.)
Planaria alpina (Fig. 14, B) is characteristic of cold mountain streams, but occurs down to 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 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]]
Occurrence and Distribution of Land Planarians.—The terricolous Triclads or Land Planarians are the most interesting division of the group. Some forms, such as Bipalium kewense, attain large dimensions, being usually 6 to 9 inches in length, and specimens fully extended have measured 18 inches. Their bodies are frequently banded or striped with brilliant colours. Geoplana coerulea Mos. has a blue ventral surface and is olive green or dark Prussian blue above. G. splendens Dendy, is marked dorsally by three stripes of emerald green alternating with four dark brown longitudinal bands. The mode of coloration, though somewhat variable, is an important specific character. Its significance, however, is not clearly understood. The colours may be a warning signal, as some Geoplana at least are disagreeable to the taste of man and some birds[[46]]; but since Land Planarians are largely nocturnal animals, living by day under logs, banana leaves, and in other moist and dark situations, this explanation is clearly insufficient. Two Geoplana have been noticed by Mr. Dendy which seem to be protectively coloured. G. triangulata var. australis occurs abundantly in the beech forest in the South Island of New Zealand, and its brown back and yellow or orange ventral surface match the leaves around its haunts. G. gelatinosa again looks like a mere slimy patch on the rotten bark where it is found. In arid districts, during the dry season, Land Planarians burrow in the soil and form a cyst, in which they lie coiled up, after the manner of earthworms.[[47]] The glutinous investment of their delicate bodies forms a moist medium in which the cilia covering the body (and especially the ventral surface) may constantly and evenly vibrate, and by which they adhere firmly to their prey. In some tropical Planarians, in addition to possessing offensive properties, the mucus is so copious in amount and hardens with such rapidity, that these Triclads may creep over bridges of it, and may even be blown from one stem or branch of a plant to another, hanging at the ends of their threads.[[48]]
Fig. 15.—Some Land Planarians found in Europe. A, Bipalium kewense Mos. × ⅓ (after Bergendal); B, Rhynchodemus terrestris O. F. M., × 2; C, Geodesmus bilineatus Metsch., × 2½ (after Metschnikoff). mr, Region of mouth; gp, region of genital pore.
In Europe there are only two or three indigenous Land Planarians, of which Rhynchodemus terrestris O. F. M. (Fig. 15, B) is the most widely distributed, and has been found in moist situations for the most part wherever it has been carefully looked for. It measures about ¾ inch in length, and is dark grey above, whitish below, and bears a pair of eyes near the anterior extremity (Fig. 15, B). Bipalium kewense (Fig. 15, A), which has been found in the forests of Upolu, Samoa, by Mr. J. J. Lister, has been accidentally imported, from the (unknown) districts where it is indigenous, with plants and soil to various parts of the world—England, Germany, the Cape, and also to Sydney, where it appears to have established itself. In these Bipalia living in hothouses, the genitalia never appear to attain maturity, and apparently multiple fission and subsequent reparation of the missing parts is the only mode of reproduction. Geodesmus bilineatus (Fig. 15, C), which has occurred at Giessen, Würzburg, and Dresden, has, in all probability, been introduced with ferns from the West or East Indies. Microplana humicola, described by Vejdovsky from dunghills in Bohemia, is doubtfully indigenous.
In marked contrast with the poverty of the temperate zones in Land Planarians, is the abundance and great variety of this group in Southern Asia, South America, and especially in Australasia, where the rich Land Planarian fauna has been carefully investigated by Spencer, Dendy, Fletcher, and others, in certain parts of Victoria, New South Wales, and New Zealand.[[49]] About forty species of Planarians have been discovered on the Australian continent, thirty-five of which belong to the predominant genus Geoplana, distinguished by the presence of numerous eyes along the border of the simple anterior extremity. Of the remaining five, four belong to the genus Rhynchodemus, with, lastly, the introduced Bipalium kewense. The distribution of any one species, however, is so limited that only three forms are common to the two former colonies; and although some of the twenty known New Zealand Planarians (chiefly species of Geoplana), are identical with Australian species, yet only one, or possibly two, varieties of these species are Australian also. In addition to their prevalence in Australasia, the Geoplanidae also occur in South America, South Africa, Japan, and the East Indies. The Bipaliidae are characteristic of the Oriental region, being found in China, Borneo, Bengal, and Ceylon. The Rhynchodemidae are a cosmopolitan family, occurring in Europe, North and South America, the Cape of Good Hope, Ceylon, the East Indies, Australia (particularly Lord Howe Island), and Samoa.[[50]]