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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 See also:

FORM (Lat. forma)

form and habits of the group vary widely. The Planarians are See also:

FREE

free-living animals, the Trematodes are parasitic upon and within animals, and the Cestodes are wholly endoparasitic. Structure.—The See also:

CHIEF (from Fr. chef, head, Lat. caput)

chief features which Platyelmia possess in See also:

COMMON

common are the following. The body is not metamerically segmented and is composed of a See also:

MUSCULAR

muscular See also:

TUNIC (0. Eng. tunice, tunical, taken, before the Norman con-quest, directly from Lat. tunica, of which the origin is unknown)

tunic covered externally by a more or less modified cellular layer. Within this muscular See also:

TUBE (Lat. tuba)

tube lies a parenchymatous See also:

TISSUE (Fr. tissu, tissue, participle of tisser, Lat. texere, to weave)

tissue which may be See also:

UNIFORM

uniform (Cestodes) or differentiated into a central or See also:

DIGESTIVE

digestive, and a peripheral portion (some Turbellaria), or finally the central portion becomes tubular and forms the digestive See also:

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 See also:

INGRESS (Lat. ingresses, going in)

ingress to and See also:

EGRESS (Lat. egressus, going out)

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 paren-chyma gives rise to protonephridia, that is to coiled tubes commenc• See also:

ing in pyriform cells containing a See also:

FLAME (Lat. flamma; the root flag- appears in flagrare, to burn, blaze, and Gr. d, XEryew)

flame-like bundle of See also:

CILIA (plural of Lat. cilium, eyelash)

cilia and provided with branched outgrowths, and communicating with the exterior by See also:

long convoluted canals which open at the See also:

SURFACE

surface of the body. These protonephridia are the excretory organs.

The nervous system, though centralized at one end of the body, contains diffused See also:

NERVE (Lat. nervus, Gr. vevpov, a bowstring)

nerve-cells in the course of its tracts, which are disposed in two or more See also:

LONGITUDINAL

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 protrusible penis. This See also:

ORGAN

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 See also:

BASE

base or pass separately by a See also:

SPECIAL

special duct to the exterior. There. is some See also:

REASON (Lat. ratio, through French raison)

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 See also:

PREY (0. Fr. preie, mod. proie, Lat. praeda, booty, from prae and the root hed—seen in prehendere, prendere, to grasp)

prey it has become modified for hypodermic impregnation and only gradually adapted for insertion into the bursa copulatrix. The See also:

FEMALE

female organs are no less complex. They consist of solid or tubular ovaries which may be single, See also:

double or multiple. In the See also:

MAJORITY (Fr. majorite; Med. Lat. majoritas; Lat. major, greater)

majority of Platyelmia the See also:

PRIMITIVE

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 See also:

union is distinguished by the opening of a " See also:

shell-gland " which secretes a membrane around the conjoined See also:

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 protrusible 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 See also:

INTESTINE (Lat. intestinus, internal, usually in neuter plural intestina. from intus, within)

intestine, in the latter See also:

case it appears to serve as means of conveying superfluous yolk to the gut, where it may serve as See also:

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 See also:

DIVISION (from Lat. dividere, to break up into parts, separate)

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 See also:

LIFE

life, but they exhibit such a See also:

close similarity of structure with the Turbellaria that their origin from Planarians can hardly be doubted, and indeed the Temnocephaloidea (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 See also:

AFFINITIES

affinities are quite problematical. The entire See also:

ABSENCE (Lat. absentia)

absence of any trace of a distinct alimentary See also:

TRACT (from Lat. tractare, to treat of a matter, through Provencal tractat and Ital. trattato)

tract, the loss of true regenerative See also:

POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power, the See also:

PECULIAR

peculiar gametic segmentation of the body Into hundreds of "proglottides" budded off from n (From See also:

Cambridge Natural See also:

HISTORY

History, vol. ii., "See also:

WORMS

Worms, &c.;" by permission of See also:

MACMILLAN

Macmillan & Co., Ltd.) Fm. i.—Free-See also:

SWIMMING (from " swim," A.S. swimman, the root being common in Teutonic languages)

swimming Larva (See also:

Muller's Larva) of a Polyclad Planarian to illustrate the trochosphere-See also:

HYPOTHESIS (from Gr. inrorcOivat, to put under; cf. Lat. suppositio, from sub-ponere)

hypothesis of the origin of Platyelmia. The larva is seen in See also:

OPTICAL

optical See also:

section, and its distinguishing feature is the ciliated lobed See also:

BAND

band (al, sl, dl), which corresponds to the pre-oral ciliated band of a trochosphere-larva. It is here See also:

DRAWN

drawn out into eight processes, of which six are shown, their continuity being expressed by the dotted See also:

LINE

line. br, See also:

BRAIN (A.S. braegen)

Brain. mg, See also:

STOMACH (Gr. vrbsaxos from arbga, a mouth)

Stomach. dr, Glands. n, Nerves. ep, Epidermis. ph, Pharynx. mo, Mouth. See also:

PAR (Lat. par, equal)

par, Parenchyma.

one extremity, and the absence of any morphologically distinct ante'iot extremity, are adaptations to the wholly parasitic life of this class. Their structure is similar to that of Trematodes, from which in the See also:

OPINION (Lat. opinio, from opinari, to think)

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 See also:

hand the Turbellaria are considered to be an offshoot of the See also:

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 (Muller's larva) which is comparable to a certain See also:

STAGE (Fr. 6/age; from Lat. stare, to stand)

stage (See also:

pro-trochula) in the development of the Trochosphere-larva. This Trochosphere is the characteristic larva of See also:

MOLLUSCA

Mollusca, See also:

ANNELIDA

Annelida T OT (After See also:

Abbott, Tdkyo Zool. Society's Annot. Zoologicae Japonensis, iv. 4, 2 and 3.) and some See also:

GEPHYREA

Gephyrea; and the See also:

ROTIFERA (or ROTATORIA)

Rotifera appear to remain throughout life as modified Trochospheres. It is a See also:

TOP (cf. Dan. top, Ger. Topf, also meaning pot)

top-shaped, free-swimming organism provided with a preoral band of cilia, an apical sense-organ, a See also:

SIMPLE

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 Ctenoplane and Coeloplana, which suggests a transition from See also:

CTENOPHORA

Ctenophora to Polyclads. At the See also:

PRESENT

present See also:

time, however, none of these grounds can be said to possess so much force as they did some years ago (4). The See also:

ARGUMENT

argument has come to rest on the agreement between the See also:

CELL (from Lat. cella, probably from an Indo-European kal —seen in Lat. celare, to hide; another suggestion connects the word with Lat. cera, wax, taking the original meaning to refer to the honeycomb)

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 qffshoot, 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 See also:

OUTER

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 See also:

EVOLUTION

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 Acoelous 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 Rhabdocoelsl7) has led to the important See also:

DISCOVERY

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 See also:

independence of the intestinal sac is an indication that the origin of these cells is not coelomic nor enteric, and until we possess further See also:

INFORMATION (from Lat. informare, to give shape or form to, to represent, describe)

information as to the evolution of the complex genitalia of the higher Turbellaria we cannot See also:

hope to understand the presence of such highly modified structures in animals of an otherwise See also:

low grade or organization.

(F. W.

End of Article: PLATYELMIA

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