CHAPTER XXXI 

 PHYLUM PLATYHELMINTHES 



The phylum Platyhelminthes (plat i hel min' thez; G., platys, broad, 

 and helminthos, worm), of which a planarian has been taken as a type, 

 represents in many respects a marked advance over previous phyla. 

 Bilateral symmetry is here fully established. The animal possesses a 

 definite head and tail. The head represents not only the part that goes 

 ahead in locomotion but also the most sensitive part of the body and the 

 portion in which is located a nerve center. This results in a definiteness 

 in the movements of the animal which is in marked contrast to the 

 haphazard locomotion of the coelenterates or the weak but more definitely 

 directed swimming of the ctenophores. The presence of bilateral 

 symmetry is not here accompanied, however, by the presence of metamer- 

 ism. These animals have begun, too, to reap the advantages derived 

 From the development of the mesodermal layer, and there is shown, in 

 its inception, the plan of organization of all higher animals. This organi- 

 zation involves the development of tissues, organs, and systems and 

 greatly increases the effectiveness of the organism. In the Turbellaria 

 the gastrovascular cavity reaches its highest degree of development and 

 of efficiency. By means of numerous complexly branched canals the 

 food is distributed throughout the body of the animal, in spite of 

 the fact that the development of the mesoderm has greatly increased the 

 bulk and the number of cells to be reached. The nervous system in 

 the free-living flat worms includes a pair of ganglia below the eyespots, 

 which have by some been called a brain and which is a definite evidence 

 of centralization. 



191. Classification. — This phylum is divided into three classes 

 as follows: 



1. Turbellaria (ter bel la' ri a; L., turhella, a little stirring). — Consists 

 of soft-bodied forms, usually free-living, with a ciliated epidermis which 

 contains an abundance of secreting cells and which also produces rodlike 

 bodies called rhabdites. The mouth is on the ventral surface but in 

 different species it varies in location from near the anterior end to even 

 behind the middle of the body. 



2. Trematoda (tre ma to' da; G., trematodes, having pores). — Forms 

 in which the soft, ciliated epidermis is replaced by a thick, firm cuticle, 

 without cilia, in which the mouth is situated at or near the anterior end 

 of the body and surrounded by a sucker, and in which there may also 



164 



