PLAT'YHELMIN'THES (Neo-Lat. nom. pl., from Gk. 7rXcrth. platys, flat, broad + 6.).Los, helmins, worm), or PLATODA. A phylum of in vertebrate animals (flatworms), easily recognized by the absence of a body-cavity and anus. There is no blood system, the results of digestion being carried to the different parts of the body by branches of the intestine. The body is flattened dorso-ventrally, so that it is often thin and leaf like. The surface is covered with cilia in most of the free-living forms, but in parasitic forms is usually smooth and somewhat chitinous. The muscular system is well developed and remark ably complex, so that almost any sort of move ments are possible to these worms. The digestive system is usually very well developed and highly varied, but in some forms is wholly wanting. The nervous system consists of a ganglion above the and two lateral nerve cords, con nected by transverse commissures. Other longi tudinal trunks are sometimes present, and rarely the transverse commissures are wanting. A very characteristic and remarkable set of fine tubes ramifies through the body and apparently serves as an excretory system. Sense organs of vari
ous kinds occur in the free-living forms, but are usually wanting in those which are parasitic. Occupying a large part of the body, and often nearly the whole space inside the skin, are the reproductive organs, and the fertilization of the egg is in almost all cases internal. The Tur bellaria are hermaphroditic. The eggs are very numerous, provided with considerable yolk and covered by a shell, but some planarians multiply by fission, and when cut in pieces each piece may eventually become a well-formed planaria a. The young undergo a metamorphosis and develop ment sometimes very complex. The phy lum is usually divided into three classes: (1) Turbellaria. free-living flatworms, with the sur face ciliated ; (2) Trematoda, parasitic, usually leaf-like, flatworms having a well-developed di gestive system ; (3) Cestoda, the tapeworms, usually elongated, jointed, and without a diges tive system. Consult Parker and Haswell, Text book of Zoology (New York, 1897). See FLAT WORM : CESTODA ; TREMATODA.