PLATYHELMINTHES or PLATODARIA, a phylum of invertebrate animals, containing soft-bodied creatures which are bilaterally symmetrical and usually somewhat flattened in shape, and in which there is no true "coelom" or perivisceral cavity and no true (metameric) segmentation. The animals con tained in this group (flatworms) are the simplest and probably the most primitive of those in which the tissues and organs of the body are developed from three, instead of two, original embryonic layers.
The external covering of the body is typically an epidermis provided with vibratile cilia These serve partly for locomotion and partly for creating a respira tory current In the adults of certain parasitic groups, the epi dermis is replaced by a smooth or spiny cuticle Pigment is com monly present beneath the epi dermis in free-living forms, but is usually absent in those which are parasitic. The spaces between the internal organs are for the most part filled up with a kind of loose connective tissue called the parenchyma. The coelom is represented, perhaps, by the cavi ties of the gonads and of the ex cretory organs. The musculature of the body is mainly peripheral, consisting of layers of trans verse, longitudinal and sometimes also oblique muscle-fibres run ning through the outer, or 'cortical," portion of the parenchyma. These muscles render the body capable of extreme elongation and contraction, and often of surprising variability in shape.
Anterior and posterior ends of the body can usually be dis tinguished, the animals having a definite direction of locomotion, accompanied by a greater degree of specialization of the anterior extremity. Dorsal and ventral surfaces are also generally dis tinguishable, the latter being that on which the animal creeps, and on which the oral and genital apertures are commonly situ ated. In the parasitic flatworms special clinging organs are gen erally developed, in the form of muscular suckers, often supple mented by chitinoid hooks or spines.
An alimentary canal may or may not be differentiated. When present, it may either be a simple sac-like organ or may be variously branched. With very few exceptions, its only aper ture is the mouth, which may be situated subterminally, near the anterior end, or much further back, sometimes even behind the middle of the body. The mouth may be surrounded by an oral sucker, or developed into a protrusible and highly muscular pharynx.
The main ganglia of the nervous system (the "brain") and the chief sense-organs are generally concentrated towards the anterior end. In addition to tactile papillae or special sensory cilia, "eye-spots" or ocelli are frequently present in free-living forms, and sometimes in the free-living larval stages of para sitic forms. In certain free-living flatworms statocysts (sac-like organs containing minute calcareous nodules or statoliths) and ciliated pits, probably sensory in function, also occur.
There is no blood-vascular system or haemocoele. The ex cretory organs consist of a branching system of canals ending internally in "flame-cells." These are minute pyriform structures containing cilia which keep up a constant flickering movement.
The main collecting vessels of the system open to the exterior by one, two or many pores.
The Platyhelminthes are, with rare exceptions, hermaphroditic animals, each individual being potentially both male and female. The reproductive system is usually very complex. The male or gans consist essentially of one or more (often very numerous) testes, whose ducts are usually connected with a protrusible intro mittent organ (penis or cirrus). The essential organs of the female apparatus are an ovary (sometimes multiple ovaries) and a tubular duct communicating with the exterior. The arrange ment of the parts of the female system is subject to great vari ation in different groups. Sometimes the same duct functions in turn as a vagina or fertilization canal, as a uterus or reservoir for fertilized eggs, and as an oviduct by which the eggs reach the exterior. In some cases (most Cestodes) the only communica tion with the exterior is a vagina, which is connected internally with an oviduct leading from the ovary to a sac-like uterus; but does not serve for the expulsion of eggs. In such forms the eggs are either shed only by the dehiscence of the wall of the uterus and of the body-wall, or by a special (temporary or per manent) birth-pore. In almost all flatworms there is a yolk gland or vitellarium (of ten multiple vitellaria), producing yolk cells which form nutritive material for the developing embryos. The vitellarium, in the most primitive forms, appears to be developed as a sterile portion of the ovary. There is also usually a "shell-gland," possibly concerned in the secretion of the outer covering of the eggs. The ducts of these glands open into the oviduct or into a specialized portion of it called the ootype. The external apertures of the male and female ducts are sometimes separate, but frequently both ducts open into a common "genital atrium," which is often muscular.
The Platyhelminthes are extremely widely distributed, f ree living forms occurring in almost every kind of environment—in shallow or deep water, both fresh and marine, and on land— while parasitic forms occur on or in animals of almost every class. The free-living forms usually feed actively on small animals or plants. The parasitic forms show various degrees of modifica tion in habits, some being external parasites and feeding on mucus or other matter derived from the skin of their hosts, while others are internal parasites and feed on partly digested food or on body fluids. Among the latter class some are without special digestive organs and can only feed by the absorption of liquid nourishment.
The phylum is usually considered to include three main divi sions or classes : (I ) Turbellaria (including Temnocephalidea), the majority of which are free-living, but some parasitic. This group is probably nearest to the primitive ancestral form. (2) Trematoda (flukes), all of which are wholly or partly parasitic either upon or within other animals. (3) Cestoda (tapeworms), all of which are wholly endoparasitic. See NEMERTINEA, TAPE