PLATYHELMINTHES, a phylum of the Metazoa, commonly known as the flat worms, characterized by a bilaterally symmetri cal body, somewhat flattened dorsoventrally and usually elongated. The primary 'body cavity is filled with parenchymatous tissue, giving char acteristic firmness to the body. The alimentary system is sacculate and provided with but a single opening, and the excretory organs are of the simple protonephridial type. On the other hand the reproductive organs are unusually complex, and in all save a few species her maphroditic. The phylum is well defined, con taining three great classes, the Turbellaria or free-living flatworms, the Trematoda or flukes, and the Cestoda or tapeworms. Of the few aber rant or little known groups formerly associated with this phylum only the Nemertinea are still placed here; they are given independent rank, and usually treated as an appendix though often omitted entirely from the phylum. The body is covered in the Ttubellana by a simple ciliated epithelium, and in the Trematoda and Cestoda by a so-called cuticula which has not been clearly interpreted as yet since the epithelial cells are apparently wanting though glandular cells sunk deep in the parenchyma are often interpreted as epithelial in origin. Directly beneath the epithelium or the cuticula one finds a well developed dermo-muscular sac in which longitudinal and transverse layers are always seen and often a third layer made up of oblique bundles. Dorsoventral muscles extend through the parenchyma from surface to sur face and special muscles are also found in con nection with organs like the suckers, spines, etc. There is no skeleton and the only hard parts present are hooks and spines that develop on the surface of the body or of some invagina tion of the external layer.
Special circulatory and respiratory organs are entirely lacking, but the excretory system is well developed. It consists of so-called "flame cells*, i.e., ciliated cells, that close the inner
ends of minute tubules; these form a branch ing system that leads to one, two, or more external pores. Just within the pore one finds often a storage bladder formed by an ex pansion of the tubule. Asexual reproduction by division is common in the Turbellaria and metagmesis or alternation of generations occurs in Trematoda and some Cestoda. Although nearly all Platyhelminthes are hermaphroditic a single germ gland produces only one kind of sex cells and the complex reproductive system, which forms the conspicuous part of the body in the sexually mature individual, comprises a complete set of male and of female organs. Development is simple in the free living Turbellaria and often very greatly involved in the parasitic Trematoda and Cestoda where alternation of hosts is the rule and even three or four hosts participate in the life history of a given species. Many efforts have been made to trace relationship between the Platylielmin 'dies and the Ccelenterata on the one hand and the higher worms or simplest mollusks on the other. While one recognizes in general the intermediate character of the flatworms, it has not yet been feasible to trace more definite connections in either direction. The process of dhange within the phylum is distinctly traceable from the free living Turbellaria with ciliated epithelium and special sense organs that show adaption to independent existence through the ectoparasitic and endoparasitic Trema toda to the Cestoda. In the latter the con sistent endoparasitic habit is correlated with entire disappearance of the alimentary canal as well as with the loss of ciliary locomotor apparatus and special sense organs. Consult Lankester, (Zoology' (London 1901) and Her fiug-kingsity, of (New York 1912).