1911 Encyclopædia Britannica/Platyelmia
PLATYELMIA, a phylum of the animal kingdom which comprises three classes, the Planarians, Trematodes (q.v.) and Cestodes. It is the group of animals in which the act of creeping has first become habitual. In association with this movement in a definite direction the body has become vermiform and bilaterally symmetrical. One end of the body, through contact, during locomotion, with fresh tracts of medium and other forms of stimuli, has become more specialized than the rest, and here the nervous system and sense-organs are more densely aggregated than elsewhere, forming a means of controlling locomotion and of correlating the activities of the inner organs with the varying stimuli that impinge upon the body. The form and habits of the group vary widely. The Planarians are free-living animals, the Trematodes are parasitic upon and within animals, and the Cestodes are wholly endoparasitic.
Structure.—The chief features which Platyelmia possess in common are the following. The body is not metamerically segmented and is composed of a muscular tunic covered externally by a more or less modified cellular layer. Within this muscular tube lies a parenchymatous tissue which may be uniform (Cestodes) or differentiated into a central or digestive, and a peripheral portion (some Turbellaria), or finally the central portion becomes tubular and forms the digestive sac (Trematodes), while the peripheral portion is separated from it by a space lined in some forms by a flattened epithelium (most Planarians). It is characteristic of the group that the mouth should be the only means of ingress to and egress from the digestive sac and that no true perivisceral space or coelom exists in the sense in which these terms are used in higher Invertebrates. The peripheral parenchyma gives rise to protonephridia, that is to coiled tubes commencing in pyriform cells containing a flame-like bundle of cilia and provided with branched outgrowths, and communicating with the exterior by long convoluted canals which open at the surface of the body. These protonephridia are the excretory organs. The nervous system, though centralized at one end of the body, contains diffused nerve-cells in the course of its tracts, which are disposed in two or more longitudinal bundles interconnected by transverse bands. The Platyelmia are hermaphrodite and the reproductive organs are complex. The male organs consist of paired testes communicating by delicate canals with a protrusile penis. This organ is generally single but sometimes paired and occasionally multiple. It is frequently armed with spines, hooks or stylets, and is further complicated by the addition of a nutritive secretion (the prostate gland) which may open at its base or pass separately by a special duct to the exterior. There is some reason to believe that this complicated and variable apparatus is used for stabbing the body of another animal and that beginning as a weapon for catching prey it has become modified for hypodermic impregnation and only gradually adapted for insertion into the bursa copulatrix. The female organs are no less complex. They consist of solid or tubular ovaries which may be single, double or multiple. In the majority of Platyelmia the primitive ovary becomes divided into fertile and sterile portions, i.e. into distinct ovarian and vitellarian regions. The yolk prepared by the latter is conducted by one or more specialized ducts to the oviduct and the point of union is distinguished by the opening of a “shell-gland” which secretes a membrane around the conjoined mass of ovum and yolk. From this junction there proceeds an oviduct or “uterus” (paired or single) which before opening to the exterior expands to form a muscular protrusile pouch—the bursa copulatrix. Frequently also from this junction of the ovaria and the vitellaria a median tube is given off which either opens to the exterior or into the intestine, in the latter case it appears to serve as means of conveying superfluous yolk to the gut, where it may serve as food.
Inter-relationships.—The inter-relationships of the three members of the Platyelmia are of a more doubtful nature than is the unity of the phylum. The Turbellaria undoubtedly form the most primitive division, as is shown by their free-living habits, ciliation and sense-organs. The Trematodes are somewhat modified in accordance with their ecto- or endoparasitic life, but they exhibit such a close similarity of structure with the Turbellaria that their origin from Planarians can hardly be doubted, and indeed the Temnocelphaloidea (see Planarians) form an almost ideal annectant group linking the ectoparasitic Trematodes and Rhabdocoel Planarians. The Cestodes, however, are connected by no such intermediate forms with the other Platyelmia. Their adaptations to parasitic life in vertebrate animals appear to have involved such modifications of structure and development that their affinities are quite problematical. The entire absence of any trace of a distinct alimentary tract, the loss of true regenerative power, the peculiar gametic segmentation of the body into hundreds of “proglottides” budded off from 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.
(After Abbott, Tōkyō Zool. Society's Annot. Zoologicae Japonensis, iv. 4, 2 and 3.)
Fig. 2.—Dorsal view of Coeloplana to illustrate the similarity between Ctenophora and Turbellaria. The branched intestine (G) is drawn on one side of the animal only; it opens to the exterior by means of a pharynx (not shown). The mouth is shown by the line surrounding the otolith (OT) in the centre. The mouth is ventral, the otolith dorsal. The two branched tentacles (TB) are seen partially extruded from their sheaths (TS); when fully extended they exceed the diameter of the animal five or six times. The short tentacles (T) are drawn on one side only. Coeloplana has been found in shallow water in the Red Sea and on the coast of Japan. Ctenophora possess two similar long branched tentacles, a ventral mouth and dorsal otolith.
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.
(After F. E. Schultze, Kgl. Preuss. Akad. der Wissenschaft, Berlin, 1891.)
Fig. 3.—Trichoplax adhaerens, an organism considered, on the Gastraea-hypothesis, to be closely allied to the progenitors of the Platyelmia. (The recent work by Krumbach [Zoolog. Anzeiger 1907, xxxi. 450], serves to show that Trichoplax is the planualarva of a Hydromedusa.)
|A,||a small specimen drawn from life. The spherical granules (G) are probably gland-secretions; the dark bodies (Z) are probably xanthellae, i.e. algal cells living in association with the animal.|
|B,||a specimen undergoing fission|
|C,||part of a vertical section.|
|D.Ep,||The dorsal epidermis.|
|V.Ep,||Ventral epidermis. The hair-like processes are cilia.|
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.
Literature.—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. (F. W. Ga.)