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Rh one extremity, and the absence of any morphologically distinct anterior extremity, are adaptations to the wholly parasitic life of this class. Their structure is similar to that of Trematodes, from which in the opinion of most zoologists they have been derived.

Affinities.—As the Turbellaria (Planarians) are the most primitive division of the Platyelmia, the problem of the affinities of this phylum resolves itself into that of the relationships of the Turbellaria. With regard to the origin of this class two divergent views are still held. On the one hand the Turbellaria are considered to be an offshoot of the early Coelomate stock, on the other they are held to be descendants of a simpler two-layered stock. The former hypothesis with its variants may be called the Trochosphere-hypothesis, the latter the Gastraea-hypothesis. The Trochosphere-hypothesis (2), (3) is based chiefly on the occurrence in certain Polyclad Turbellaria, of a larval form (Müller's larva) which is comparable to a certain stage (pro-trochula) in the development of the Trochosphere-larva. This Trochosphere is the characteristic larva of Mollusca, Annelida and some Gephyrea; and the Rotifera appear to remain throughout life as modified Trochospheres. It is a top-shaped, free-swimming organism provided with a preoral band of cilia, an apical sense-organ, a simple gut, protonephridia and schizocoele. The importance of this resemblance between the Polyclad larva and the Trochosphere-larva of higher invertebrates is increased if the widely adopted

view (held on other grounds) that the Polyclads are the most primitive of the Turbellaria, is soundly based. The grounds for this view are the radial symmetry of several Polyclads and the supposed origin of gonads and excretory flame-cells from the walls of gut, the occurrence of nematocysts in Anonymus, one of the most radially constructed Polyclads, and lastly the presence of two peculiar animals Ctenoplana and Coeloplana, which suggests a transition from Ctenophora to Polyclads. At the present time, however, none of these grounds can be said to possess so much force as they did some years ago (4). The argument has come to rest on the agreement between the cell-lineage of Polyclads and that of certain Mollusca and Annelids. This resemblance is considered by Hubrecht (5) to give reason for concluding that the Polyclads are an offshoot, and possibly a degenerate offshoot, from the early Coelomate stock.

The Gastraea-hypothesis is founded on quite other considerations. In effect (6) it traces the Turbellaria to small two-layered organisms consisting of an outer ciliated epidermis and a central syncytial tissue. Such an organism is found in the peculiar Trichoplax, Lohmanniella, &c. The early stages of most animals pass through such a stage, which is known as a “planula.” From such beginnings the evolution of the Turbellaria leads first through the Acoelous forms in which the central syncytium is partly differentiated into digestive, muscular and skeletotrophic tissue, then to the more specialized Rhabdocoela, and so through the Alloeocoela to the Triclads and finally to the Polyclads. The careful study of the development of one coelous form and of certain Rhabdocoels has strengthened this hypothesis by showing that no definite enteron or gut is at first laid down, but that certain embryonic syncytial tracts become digestive tracts, others excretory, others again muscular. The study of Rhabdocoels (7) has led to the important discovery that the rudiment of the gonads and that of the pharynx are the first organs to appear, and that the alimentary sac arises independently of them. This segregation of the germ cells and their independence of the intestinal sac is an indication that the origin of these cells is not coelomic nor enteric, and until we possess further information as to the evolution of the complex genitalia of the higher Turbellaria we cannot hope to understand the presence of such highly modified structures in animals of an otherwise low grade or organization.

.—Recent discussions of the affinities of the Platyelmia will be found in (1) A. Sedgwick, Textbook of Zoology (1898), i. 212; (2) Hatschek, Lehrbuch der Zoologie (1891), pp. 316-326; (3) A. Lang, Die Trophocoel-Theorie (Jena, 1903); (4) E. Ray Lankester, Treatise on Zoology (1900), pt. ii. Introduction and ch. vii. pp. 15-19; (5) A. A. W. Hubrecht, Jenaische Zeitschrift für Naturwissenschaft (1905), pp. 151-176; (6) Von Graff, Die Acoela, p. 519 (Leipzig, 1891). For the development of Rhabdocoelida see (7) Bresslau, Zeitschrift für wissenschaftliche Zoologie (1904), vol. 76.

 PLATYPUS. The duck-billed platypus (Platypus anatinus) was the name assigned to one of the most remarkable of known animals by George Shaw (1751–1813), who had the good fortune to introduce it to the notice of the scientific world in the Naturalist’s Miscellany (vol. x., 1799). In the following year it was independently described by Blumenbach (Voigts Magazin, ii. 205) under the name of Ornithorhynchus paradoxus. Shaw's generic name, although having priority to that of Blumenbach, could not be retained, as it had been used at a still earlier time (1793) by Herbst for a genus of Coleoptera. Ornithorhynchus (Gr., bird, and, , bill) is therefore now universally adopted as the scientific designation, although duck-billed platypus (Gr., flat, and , foot) may be conveniently retained as a vernacular appellation. By the colonists it is called “water-mole,” but its affinities with the true moles are of the slightest and most superficial description.

The anatomical differences by which the platypus, and its only allies the echidnas, are separated from all other mammals, so as to form a distinct sub-class, are described in the article , where also will be found the main distinctive characters of the two existing representatives of the group. It is there stated that the early stages of the development of the young are not yet fully known. Sir R. Owen, and later E. B. Poulton, showed that the ovum of the platypus was large compared with that of other mammals, whilst W. H. Caldwell showed that it was filled with yolk, and finally established the fact that Platypus as well as Echidna is oviparous. Two eggs are produced at a time, each measuring about three-fourths of an inch in its long and half an inch in its short axis, and enclosed in a strong, flexible, white shell.

The platypus is pretty generally distributed in situations suitable to its aquatic habits throughout the island of Tasmania and the southern and eastern portions of Australia. 