Chap. 25 



FLATWORMS VANGUARD OF THE HIGHER ANIMALS 



499 



-V051HJ 



Fig. 25.1. Two flatworms: the planarian, free living, and the tapeworm, a 

 parasite. The planarian (mouth and pharynx extended) shows the thorough bi- 

 lateral symmetry of the flatworms, a feature that brought great changes into 

 the evolution of animals. The tapeworm has developed extraordinary reproductive 

 capacity by means of the many sections (proglottids) of the body almost all of 

 which are capable of producing hundreds of fertilized eggs. (Courtesy, Pauli: 

 The World of Life. Boston, Houghton Mifilin Co., 1949.) 



by the animals on or in which they live. All have an outer covering of resistant 

 cuticle and no cilia. 



Cestoda — Tapeworms. All cestodes have extreme capacity to multiply. 

 All of them are parasites with a world-wide distribution assured them by the 

 far-ranging vertebrates that are their hosts. They travel by land, sea, and air, 

 in goats that scale the mountains, in salmon that swim the Pacific, in bobo- 

 links that fly from Brazil to New England. 



Class Turbellaria 



Turbellaria are free-living flatworms. The most familiar of them are the 

 fresh-water planarians, one-half to one inch long (Fig. 25.2). They are com- 

 mon in streams and lakes and in many laboratories they are among the classic 

 subjects of experimental zoology. Some are white or translucent; others are 

 sober colored, gray, brown or black, a contrast to the brilliance of the marine 

 species. 



A Representative Planarian 



The commonest planarian in the United States is Dugesia tigrina {= Pla- 

 naria maculata) (Figs. 25.2, 25.3). It glides over alga-covered stems in ponds 



