Flatworms

FLATWORMS, Platodaria (syn.: Plathelminths, Platyelmia, Platodes, Scolecida p. p., etc.), constitute a distinct phylum (or subphylum in the view of a number of authors) of the animal kingdom. Many authors classify nemerteans among the platodes in addition to the turbellarians, trematodes, and cestodes usually included in this group, so that the system of flatworms is represented (according to Poche, 1925) in the following form: the phylum Platodaria Haeckel, 1896 is divided into the subphylum Platodes Leuckart, 1851 with classes: 1. Turbellaria Ehrenberg, 1831; 2. Trematoda Rudolphi, 1808; 3. Cestoda Gegenbaur, 1859 and the subphylum Nemertarii Poche, 1911 with a single class Nemertina Ehrenberg, 1831. Of the four mentioned classes of worms, turbellarians and nemerteans are mostly free-living organisms, trematodes lead a mostly parasitic mode of life, and cestodes are all without exception parasites. General characteristics. Flatworms have a mostly dorsoventrally flattened body; its surface in nemerteans and turbellarians is covered with a ciliated epithelium, which is preserved in trematodes and some cestodes only in larval stages (miracidium, coracidium), while sexually mature worms are covered with a cuticle. The intestine in some forms is present in the form of a through digestive tube (in nemerteans), in others in the form of a blind tube (in turbellarians and trematodes), and in third ones finally (cestodes) it is absent altogether; the muscular system consists mostly of circular, longitudinal, and dorsoventral muscles; the nervous system consists of a central ganglion with longitudinal trunks extending from it; the circulatory system is present only in nemerteans. Representatives of the subphylum Platodes are mostly hermaphrodites, while Nemertarii are dioecious. Class Turbellaria Ehrenberg, 1831—turbellarians, ciliated worms (syn.: Aporocephala Blainv., Planariaea Vaillant, 1881, etc.) in the overwhelming majority have an oval or leaf-shaped body form with a ciliated epithelium and an intestine lacking an anus. The nervous system consists of a bilobed central ganglion lying mostly in the anterior part of the body; a large number of nerve fibers depart anteriorly from this ganglion, and six longitudinal trunks go backward, forming transverse commissures. Among the sense organs in turbellarians, eyes are frequently encountered, sometimes present in large numbers. In some turbellarians (e.g., Acoela), there is an unpaired static organ in the form of a vesicle with a statolith. The function of touch is performed most often by special spines located along the edge of the body. The cutaneous epithelium contains special rod-like formations (rhabdites) which are expelled and play a role, apparently, in seizing prey. The digestive tube opens with a mouth opening lying on the ventral surface, mostly in the middle of the body. The mouth leads into the esophagus, differentiated into a muscular pharynx surrounded by a special pharyngeal pocket. The intestine has the shape of a simple or branched tube; an exception is represented by Acoela, in which the digestive apparatus is represented by special cells («Fresszellen») scattered throughout the parenchyma. The excretory system consists of two lateral branched canals opening anteriorly and posteriorly by paired openings, and sometimes pouring the secretion into the pharyngeal pocket. Turbellarians are mostly hermaphrodites. The reproductive system is constructed in broad outlines in the same way as in trematodes. Turbellarians lay eggs. Development is mostly direct, only in some does metamorphosis take place and in other cases asexual reproduction by division. Turbellarians inhabit fresh or salt waters and damp soil, feed on small invertebrates (worms, insect larvae, etc.) and only a few of them lead a parasitic mode of life. Poche divides the class of turbellarians into three subclasses: Acoela, Rhabdocoeloinei, and Polyclada. Turbellarians constitute a rather large group of worms (according to Poche, they include 23 families) and have neither medical nor veterinary significance. Class Trematoda—see Trematodes, class Cestoda—see Cestodes. Subphylum Nemertarii Poche, 1911 includes the single class Nemertina Ehrenberg, 1831 (syn.: Teretularia Blainv., 1828; Nemertini Siebold, 1845; Rhynchocoela Schultze, 1849; Nemertoidea Poche, 1911, etc.). The body is elongated, slightly flattened in the dorsoventral direction. The body length ranges from a few millimeters to several meters with its very small width. The body is covered with a ciliated cutaneous epithelium; inside of it there are circular and longitudinal muscles. The mouth opening mostly opens ventrally at the anterior end of the body and leads into the esophagus and intestine, ending with an anal opening. At the anterior end of the body is the opening for the extensible proboscis lying in a special proboscis sheath. The proboscis is provided with stinging capsules or consists of two parts: an anterior muscular and a posterior glandular (with poison glands), and at the border of these parts are special stylets. The proboscis is an organ serving for seizing prey. There is a circulatory system consisting of two lateral (sometimes another median) trunks communicating anteriorly and posteriorly. The blood is colorless, less often colored red. The excretory system consists of two short lateral branched nephridia, opening laterally by a single (or multiple) opening. The nervous system consists of a rather complex central ganglion and a series of nerve trunks connected to each other by commissures. Of the sense organs there are eyes, mostly in large numbers; vesicles with statoliths are rarely encountered. At the head end there are mostly slit-like depressions leading into so-called «lateral organs» serving for touch. Nemerteans are mostly dioecious; the reproductive organs are presented in the form of multiple sac-like formations filled with reproductive products and opening outward by paired openings on the dorsal surface. The eggs being released are mostly glued by a special mass into heaps or threads; some nemerteans are viviparous. Development is direct or with metamorphosis. Nemerteans inhabit mostly in seas, under stones and in mud or freely swim in the water; less often nemerteans are found in fresh waters; some nemerteans lead a parasitic mode of life. Subphylum Nemertarii contains the class Nemertina, divided into two subclasses: Anopla and Enopla, including a total of 18 families.