Polyclads are delicate, unsegmented, leaf-like worms that are bilaterally symmetrical and have no body cavity. The outline of the dorso-ventrally flattened body varies from discoid to elongate or ribbon-like, but it is more often oval. Generally, the worms measure between 10 mm and 40 mm in length but may occasionally be much shorter or much longer. The creeping or ventral surface is flattened, usually without pigment, whereas the slightly convex upper or dorsal surface is often greyish or brownish, but may be brilliantly coloured and strikingly patterned. A few species are pellucid or translucent.
In the anterior region of the body, there may be a pair of tentacles, these being ear-like projections or mere folds of the anterior margin, or dorsal appendages, known as nuchal tentacles, situated posteriorly to the anterior margin. The dorsal surface is generally smooth, but in a few forms it is raised into numerous tubercles, papillae or slender processes. Eyes, as small dark specks lying beneath the dorsal epidermis, may be seen variably distributed in the cephalic region and sometimes along the marginal or submarginal zones of the body. The mouth is situated in the mid-ventral line. It lies towards the middle of the body. In the region of the mouth, the median line of the dorsal surface is often raised into a longitudinal ridge by an underlying pharynx. In pellucid or translucent forms the median intestinal trunk and its typical dendriform lateral branches are plainly visible, and it is from this feature that polyclads derive their. On the ventral surface of the body, the male and female copulatory complexes appear as whitish areas, placed one behind the other, and almost invariably occurring posteriorly to the pharynx. The male is usually situated anteriorly to the female. Each complex opens independently on the ventral surface through a small pore lying in the median line. Rarely, the pores unite to form a common aperture, similarly situated. In mature worms, a pair of uterine canals may be seen as a dark band on either side of the pharynx. Some forms possess a glandulo-muscular ventral sucker or a shallow adhesive depression or pad on the ventral surface. The sucker lies more or less centrally in the median line, posteriorly to the genital pores, whereas the adhesive organ occurs at or near the posterior end of the body, or rarely between the genital pores.
The body wall consists of a superficial epithelium or epidermis overlying a basement membrane, which is supported by layers of muscle fibres investing a connective tissue or parenchyma. The epidermis comprises a single layer of ciliated cells, among which are sensory cells and gland cells of several kinds. Each of the ciliated cells possesses several cilia, which are better developed ventrally than dorsally, being longer and thicker. Gland cells are especially numerous dorsally, particularly in the anterior region of the body. They do not, however, occur in the tentacles. Although similar in appearance, these cells differ in the nature of their products and are of two main types: (1) those producing viscous substances; and (2) those whose secretions are formed into bodies of definite shape. According to the nature of their products, these gland cells are known as granular or slime cells, pigment cells, rhabdite cells and so on, but of whose functions little or nothing is definitely known. The commonest of these gland cells is the rhabdite cell, which is present in all polyclads, often in large numbers, particularly in the dorsal epidermis. These cells produce solid, rod-like, refractive bodies known as rhabdites. A single cell may contain one or more rhabdites. It is generally assumed that they are used defensively, for when a polyclad is disturbed or irritated, rhabdites are instantly discharged in large numbers, coalesce to form a crust over the body and presumably protect the worm from an adverse condition. Moreover, potential predators might find rhabdites noxious, or even toxic. While pigment cells may be present in the epidermis or sunk into the parenchyma, coloration of the body is not always due to the products of pigment cells, but may be due to pigment in the intestinal system obtained from ingested prey, or to pigment granules lying on either side of the basement membrane.
The basement membrane is a lamellated non-cellular layer of hyaline tissue serving to support the epidermis and for the attachment of muscles lying beneath.
The musculature consists of three or more subepidermal layers of longitudinal and circular muscles, between which two sets of diagonally-disposed fibres intersect to form a layer of criss-cross muscles. There are also bundles of muscle fibres in the parenchyma connecting the dorsal and ventral muscle layers or attaching themselves to the organ systems.
The parenchyma is a network of nucleated cells secreting fibres that unite to form a reticulate syncytium, filling the area enclosed by the subepidermal musculature and investing the various organ systems. This form of connective tissue supports the internal organs and endows the body with a high degree of flexibility. The alveoli of the reticulum are filled with a granular matrix in which lie sensory cells, sunken epidermal cells and gland cells associated with the reproductive organs.
The nervous system consists of a 'brain' or cerebral organ and several pairs of nerve cords. Typically, the cerebral organ lies in the median line, deep in the parenchyma, between the pharynx and the anterior margin of the body. It is more or less globular or transversely elliptical, with an indentation on the anterior and posterior margins to give it a bilobed appearance. On the anterior margin of each lobe lies a mass of ganglion cells from which sensory nerves pass to the eyes, to the tentacles, if present, and to the anterior margin of the body. From a central core of nerve fibres in the 'brain' several pairs of nerve cords arise and extend anteriorly, laterally and posteriorly in the dorsal and ventral parenchyma. These cords are linked by branches that anastomose here and there to form a synpatic nerve plexus, which is attached to the dorsal and ventral subepidermal musculatures by nerve fibres and is more strongly developed ventrally than dorsally.
Sense organs in polyclads are represented by a ciliated anterior marginal groove, eyes, tactile and chemosensory receptors. The marginal or cephalic groove occurs subventrally on the anterior margin of the body. It is innervated and chemosensory. Tentacles are muscular and normally retractile. They are generally free of pigment and gland cells, but are well provided with long cilia, nerve fibres and nerve cells and appear sensitive to touch and particularly water movement.
With very few exceptions, polyclads have pigmented eyes or photoreceptors. They are seen as numerous small dark spots, usually disposed in groups, as follows: (1) cerebral eyes in two elongate groups overlying or bordering the 'brain'; (2) tentacular eyes clustered within or beneath the tentacles, or alongside the cerebral eyes or merging with them when tentacles are not apparent; (3) frontal eyes scattered anteriorly, often fanwise, from the region of the 'brain'; and (4) marginal or submarginal eyes, sometimes forming a complete series round the body. Eyes lie in the parenchyma, the tentacular clusters more dorsally than the cerebral, and have the structure of pigment-cup ocelli. Each eye is a cup or bowl formed of pigment cells and filled with refractive rods connected to retinal or photosensitive cells, which lie at the opening of the pigment cup and communicate with the cerebral organ by means of sensory or optic nerves. The epidermis in the ocular regions usually has no pigment, thus allowing light to penetrate to the sensory cells. Eyes are sensitive only to light admitted through the open end of the pigment cup. The direction of the eye cup in each cluster is variable; the cerebral eyes may be disposed to perceive light from above and below, the tentacular eyes from the front and behind, as well as from the sides. It appears that species living at depth have smaller eyes than those living in shallow water.
Special cells sensitive to touch, water movement, temperature and chemical stimuli lie immediately below the subepidermal musculature, more especially on or near the margins of the body. Their endings project through the epidermis as stiff tactile bristles or as flagella, often disposed in groups or tufts.
The digestive system is essentially similar in all polyclads and is characteristic of the order. It may be differentiated into three parts - a mouth, a plicate pharynx and a ramified intestine.
The mouth is a simple aperture in the ventral wall of the body and leads into a spacious and elongate chamber containing a protrusible pharynx. This chamber - the pharyngeal chamber or buccal cavity - lies in the median line and may possess a number of lateral pockets of variable depth, arranged more or less symmetrically in pairs. Within the pharyngeal chamber, the pharynx may appear crinkled or ruffled, bell-shaped or tubular.
The intestinal trunk possesses several pairs of lateral branches and often a forwardly-directed branch reaching to the cerebral organ or beyond. The branches ramify towards the periphery of the body, where they generally end blindly. Sometimes the intestinal ramifications anastomose to form an intricate network of canals. Sphincter muscles are disposed at regular intervals along the branches, which thereby often have a beaded appearance. The epithelial lining of the intestinal system is ciliated and contains gland cells, mainly of two kinds, phagocytic cells and granular Minot's gland cells, the latter often being more abundant in the main intestinal trunk than in its branches, and their secretions have been regarded as precursors of digestive enzymes.
With few exceptions, polyclads are without any form of anal opening. Food is digested partly extracellularly in the intestinal lumen and finally intracellularly, and unassimilated material from the gut branches is accumulated in the intestinal trunk and discharged forcibly through the mouth. In Oligocladus sanguinolentus , however, there is a dorsal anus at the hinder end of the intestinal trunk, and in Cycloportis papillosus the branches terminate in small vesicles situated marginally and opening dorsally.
The excretory system in polyclads is little known, for it is exceedingly difficult to trace. The system probably has an osmoregulatory as well as an excretory function and appears to be composed of three parts, namely, (1) bulbous flame cells opening into (2) capillary vessels leading to (3) large longitudinal canals. The flame cells lie in the parenchyma and contain a lumen filled with a fluid into which long cilia project and oscillate. They open into the proximal ends and along the course of the capillaries, which lead into several sinuous longitudinal canals running the length of the body in the lateral fields. These canals give off branches connecting them with the dorsal and ventral walls of the body, where they open to the exterior through tiny pores.
Respiration and circulation:
In the absence of respiratory and circulatory systems, oxygen is probably passed to the tissues in two ways. Firstly, the movement of the epidermal cilia supplies a constant change in the surrounding water, from which a worm may obtain part of its oxygen requirement and give off carbon dioxide by diffusion through the epidermis. Secondly, the peristaltic motion of the digestive system produces a rough circulation of water entering with the food, and dissolved oxygen is taken from the water to supply the internal organs.
There is no evidence of sexual dimorphism among polyclads, which are essentially protandric hermaphrodites; protogyny occurs very rarely. Typically, the testes are small compact follicles widely distributed in the ventral parenchyma, sometimes extending between the gut branches dorsally. They are numerous in the young adult, but with the development of the female organs they decrease in number. The testes give rise to fine sperm ducts or vasa efferentia, which unite to form a delicate network opening at several places into a pair of seminal canals or vasa deferentia. These latter are symmetrically disposed laterally to the pharynx or to the median line posteriorly to the pharynx, and at maturity they become convoluted and swollen with sperm. The distal regions of the vasa deferentia are sometimes
bulbous and muscular and are regarded as spermiducal vesicles or accessory
seminal vesicles. The vasa deferentia unite in the median line to form
a long ejaculatory duct, which is generally modified successively into a seminal vesicle, a prostatic organ, a penis-papilla or intromittent organ and an antrum masculinum, and these modifications together constitute the male copulatory complex.
The seminal vesicle has relatively thick muscular walls. In many forms, the ejaculatory duct runs directly from the seminal vesicle to the penis-papilla, and in such instances the prostatic organ is either absent or 'free'. When 'free' or independent, the prostatic organ lies in the parenchyma dorsally to the ejaculatory duct, into which it opens by a short efferent or prostatic duct. When the organ lies directly between the seminal vesicle and the penis-papilla, and spermatozoa pass through its lumen during mating, it is said to be interpolated or intercalated. Often the prostatic organ is a large muscular structure lined with a glandular epithelium secreting a granular material that mingles with sperm during mating. From this organ or from the union of the prostatic organ and the ejaculatory duct, the latter extends to open into the male antrum, usually through a penis-papilla disposed in the roof of the antrum. The penis-papilla is muscular and protrusible, but when small it is often provided with a cuticular stylet. The male antrum varies from a short narrow canal to a large spacious chamber and is lined with a ciliated epithelium. It opens to the exterior by a simple ventral pore placed in the median line. In some species, the male antrum is divided into a narrow inner or proximal chamber and a wider outer or distal one. The inner chamber or penis-pocket, encloses the penis papilla or the penial stylet and opens into the outer chamber through a conical prominence, the penis-sheath, which acts as a guide for the papilla or stylet. In planocerid polyciads, the intromittent organ is a spacious cavity or cirrus-sac lined with spines, and able to turn itself inside out to form a spiny protuberance or cirrus for copulation. A single mate complex usually lies close behind the pharynx and anteriorly to that of the female. In two or three families, however, there are instances of a duplication or multiplication of the male copulatory complex.
The ovaries are rounded follicles, less numerous than the testes, and lie scattered principally in the dorsal parenchyma. Each ovary possesses a germinative zone and a sterile zone; in the germinative zone oocytes develop and in the sterile zone yolk is formed, independent yolk glands are absent. The ova are endolecithal, i.e., with yolk contained in the egg cytoplasm. The ovaries give rise to oviducts, which form a reticulum in the dorsal parenchyma and open into a pair of longitudinal uterine canals lying laterally to the median line and ventrally to the intestinal branches. The uterine canals act as repositories for eggs, and their posterior ends open individually, or by a common canal, into the inner region of the vagina. The latter is divided into three sections: the vagina interna, the vagina media and the vagina externa. The vagina interna may end by receiving the uterine canals, or following this union continue as a genito-intestinal canal, or as a ductus vaginalis opening on the ventral surface of the body, or terminate in a bulbous or elongate vesicle, known as [t]Lang's vesicle[/t], which serves as a sperm receptacle. Later, the vesicle also becomes a receptacle for the disposal of unused reproductive materials that are digested by the epithelium lining the vesicle. The vagina media is that section of the vagina into which open innumerable 'shell'-glands lying in the surrounding parenchyma. This region of the vagina is also known as the 'shell'-chamber, since it is here that fertilized eggs are coated with a gelatinous material emitted by the 'shell'-glands when egg laying occurs. The vagina externa or female antrum extends from the 'shell'-chamber to the female pore lying ventrally in the median line posteriorly to the male pore, only occasionally merging with the latter. In Cryptocelides loveni , however, the male pore lies posteriorly to the female one. The vagina externa may be provided with thick muscular walls and is then known as a vagina bulhosa. Multiple copies of the female copulatory complex are rarely found.
Source: Prudhoe, 1982.