402 The Animal Kingdom 



The Characteristics of the Phylum. — One of the chief dis- 

 tinguishing characteristics of the members of this phykim is their pos- 

 session of elongated, dorsoventrally flattened bodies. In general, the 

 free-living forms have a cellular, one-layered, soft, ciliated epidermis. 

 In the parasitic forms, a secreted cuticle covers the body. This is at 

 times very heavy and possesses hooks and spines for attachment to the 

 host. All forms have some method of attachment to a substrate. In the 

 free-living forms, it is simply glandular patches w^hich secrete sticky 

 substances; in the parasitic species, it is the hooks and spines or well- 

 developed suckers. 



While radial symmetry has been successfully exploited by the 

 coelenterates and ctenophores, it does not allow the diversification 

 that bilateral symmetry does. The members of this phylum are the 

 first to show this latter type of symmetry; with it, they have other 

 definite advances over the radiate phyla. Accompanying bilateral 

 symmetry and associated with it is cephalization, that is, a head region 

 with sense organs and nervous tissue concentration is developed. 



Unlike many of the higher invertebrates and all the vertebrates, 

 there is no body cavity in these forms. Instead, the space between the 

 ■digestive tract and the body wall is filled with a loose mesenchyme 

 known as parenchyma; thus there is no body cavity and the flatworms 

 are included among the acoelomates. 



The digestive tract, like that of the coelenterates, is in reality a 

 gastrovascular cavity; however, it is much branched and reaches to 

 nearly all parts of the body. It consists of a mouth, a pharynx, and 

 the branched intestine. There is no definite anus. In some forms there 

 is no digestive tract, and food is either absorbed through the body wall 

 or is passed from the mouth directly into the inner cell mass. 



Among the ctenophores, the mesoderm was first utilized for the 

 formation of specialized tissue, the muscle cells. It is among the flat- 

 worms, however, that the first real exploitation of the possibilities of 

 organ formation from mesoderm appears. In contrast to the rather 

 simple musculature of the ctenophores, that of the flatworms is differ- 

 entiated into several layers : circular, longitudinal, and diagonal fibers 

 which are found below the epidermis and in the mesenchyme. Other 

 muscle fibers are important elements of the pharynx and reproductive 

 ducts. These fibers are of the smooth muscle type. 



The head region is usually well differentiated in the members of 

 the phylum. Often it is separated from the remaining portion of the 

 body by a slight constriction. Among the free-living forms, there are 



