The Animal Kingdom - 637 



when animals began to move mainly in one 

 direction and began to maintain a more or 

 less constant orientation of the body. Appar- 

 ently the forward, or anterior, end of the 

 animal, which first encounters changes in 

 the environment, tended to become different 

 from the trailing posterior end; and consist- 

 ent differences in the environment above and 

 below the animal led to a differentiation be- 

 tween the upper (dorsal) and the lower 

 (ventral) aspects of the body. The right and 

 left sides, on the other hand, would encounter 

 approximately the same conditions, and 

 would not become different. Moreover, ceph- 

 alization, which is a tendency toward the de- 

 velopment of a "head" part of the body, 

 seems to have resulted from a concentration 

 of special sense organs at the anterior end. 

 Thus we find the light-sensitive eyespots and 

 the touch-sensitive auricles of the planarian 

 (Fig. 16-6) near the anterior, or leading, end 

 of the animal, where most new stimuli im- 

 pinge. 



The bulk of the body of the fiatworm con- 

 sists of tissues derived from the mesoderm. 

 The mesoderm intervenes between the ecto- 

 dermal epithelium, covering the outer sur- 

 face, and the endodermal epithelium, lining 

 of the complexly branched gastrovascular 

 cavity (Figs. 16-6 and 16-8). The mesodermal 

 tissues include a complicated set of well- 

 developed muscle fibers, as well as various 

 excretory and reproductive structures and 

 scattered germ cells (Fig. 32-15). 



The numerous germ cells in the mesoderm 

 give rise to gametes, and also they are re- 

 sponsible for the remarkable regenerative 

 powers displayed by planarians. Between the 

 other mesodermal structures, there is a syncy- 

 tial network of loosely arranged parenchyma 

 tissue; and the spaces within this network 

 are filled with body fluid, or hemolymph 

 (p. 318). 



Flatworms are perhaps the simplest ani- 

 mals in which well-defined organs and organ 

 systems can be recognized. As may be seen 

 in Figure 32-15, these include: (1) the ovaries, 

 testes, and ducts of the reproductive system; 



GANGLIA 



FLAME CELL 



ENTERON 



YOLK GLAND 



GENITAL PORE 



Fig. 32-15. Planaria, one of the free-living Platy- 

 helminthes; diagram of infernal structures. Flatworms 

 are among the simplest animals that show definite 

 organs and organ systems. 



(2) the ganglia and nerve strands of the nerv- 

 ous system; and (3) the flame cells and ducts 

 of the excretory system. 



Classes of the Plcttyhelminthes. This phy- 

 lum can be divided naturally into the follow- 

 ing three classes: (1) the Turbellaria; (2) the 

 Trematoda; and (3) the Cestoidea. 



Class 7. The Turbellaria. These are mainly 

 free-living flatworms, such as planarians, in 

 which the ectodermal epithelium of the lower 

 surface is ciliated. Generally these animals 

 range between one to three cm in length, 

 although there are a few semimicroscopic 

 species and a few that reach a length of five 

 inches. 



Class 2. The Trematoda, or Flukes. (Fig. 

 32-13). These flatworms are all parasites, with 

 adult stages usually found in various verte- 

 brate animals. Cilia are present in a larval 

 stage (the miracidium), but the adult body is 



