io6 ELEMENTS OF BIOLOGY 



Planaria and related genera which are very common in both fresh 

 and salt water. 



Class n. TREMATODA (Fig. 38). Parasitic forms resembling in 

 general form the Turbellaria, but the ectoderm is entirely free from 

 cilia. Common examples are the Liver Fluke and the Blood Fluke. 



Class in. CESTODA (Fig. 39). Also parasitic. Have no digestive 

 canal. The body forms a chain of segments as it increases in length. 

 The tapeworms are the most common examples. 



A fourth class of Platyhelminthes is sometimes included, namely, 

 the class nemertinea (Fig. 40). These are flat ribbon-shaped forms, 

 mostly marine and all free living. Some varieties attain a length of 

 several feet. In some important respects they resemble the other 

 free-living flatworms. 



Phylum Nemathelminthes 



The animal is cylindrical and smooth (Fig. 41). The body is 

 bilaterally symmetrical, derived from three primary layers, and con- 

 sists of a tube within a tube, the outer tube being the body wall 

 and the inner tube the digestive system. The digestive tube has both 

 an anterior and a posterior opening; this is the simplest type of 

 animal that has an alimentary tract with both mouth and anus. Be- 

 tween the inner, digestive tube and the body wall is a body cavity, 

 probably the same sort of body cavity that occurs in the higher ani- 

 mals. The intermediate layer, mesoderm, forms the lining of the 

 body wall. None of the tissues ever has ciliated cells. The phylum 

 contains both free-living and parasitic varieties. The parasitic forms 

 are very important from the standpoint of public health and 

 economics, for the Hookworm, the Pin Worms, the Muscle Worms 

 or Trichina, and other serious parasitic pests are members of this 

 phylum. 



Phylum Trochelminthes 

 The animals in this group are commonly called rotifers (Fig. 42) 

 and because they are exceedingly small are often mistaken for 



