BRANCH PLATYHELMINTHES 



PlatyhSlmln'thes, or Flat Worms, have three germ layers, 

 the ectoderm, the mesoderm, and the endoderm. They are 

 flattened dorsoventrally and are bilaterally symmetric. They 

 have no skeleton, no circulatory system, and no coelom or body 

 cavity. They have an anterior and a posterior end, but rarely a 

 distinct head. 



The nervous system is composed of superesophageal ganglia 

 and lateral nerve-trunks. 



The excretory system consists of water-vascular tubes. 

 There is no anal opening. 



Development is sometimes with and sometimes without a 

 metamorphosis. 



Habitat. Some, as the liver-fluke and the tapeworm, are 

 parasitic; others, as Planaria, live in fresh water. Some live 

 in moist places or in the mud at the bottom of ponds and 

 streams; while others, as Leptoplana, are marine. 



Size. The parasitic forms are sometimes 30 or 40 feet in 

 length, while the free forms are but 2 or 3 inches in length. 

 These are often found under stones, and are exceedingly deli- 

 cate. 



Protective resemblance is very great in some species, while a 

 few are nearly transparent. 



CLASS I. TURBELLARIA 



The class Turbella'ria consists principally of non-parasitic 

 forms which are ciliated externally. There is usually a diges- 

 tive cavity. The prevailing shape is leaf-form, like that of 

 Plana'ria. Some marine forms, however, are shaped like " a 

 thin ribbon with puckered edges," others may be thickened and 

 band-like, as in the land planarians, while others approach the 

 shape of a cylinder. Locomotion is performed by the fine 

 vibratile cilia which cover the surface. The ectoderm contains 

 sensory and gland-cells. 



34 



