178 GENERAL ZOOLOGY 



what little power it has of directing its attention must be 

 divided between the eight or more marginal sense organs. 

 A planarian has only one chief nerve center in the head; 

 the eyes and other sense organs are localized at the anterior 

 end. The jellyfish can perceive stimuli from different 

 directions; the planarian always moves with a certain part 

 of the body ahead and this part receives most of the stimuli 

 from the outside world. Planaria is better able to devote 

 its very limited powers of attention to one thing at a time 

 because the chief perceiving and modifying organs are 

 gathered together to some extent, and from a psychological 

 point of view this is an important point. The ability to 

 pay attention comes before the power to draw conclusions, 

 or to reason. Though Planaria, on account of its bilateral 

 symmetry and cephalization, is on the road which leads 

 toward the higher types of mental development, it has made 

 little progress. Its mental powers are at best extremely 

 limited, and its activities consist largely of a few rather 

 simple, and more or less stereotyped, reactions. 



There are a number of different kinds of turbellarians. 

 Some are much more complicated structurally than Planaria, 

 and others are far simpler. The rhabdoco3les usually have a 

 straight unbranched digestive cavity; some even have no 

 definite enteron and the food simply enters irregular spaces 

 in the parenchyma after passing down the pharynx. The 

 polyclads have many intestinal branches leading out from 

 the pharynx. Planaria, like most freshwater turbellarians, 

 belongs with the triclads because its enteron has three chief 

 branches. 



CLASS 2. TREMATODA 



The trematodes are all parasitic flatworms with a cutic- 

 ular covering and a rather simple enteron. They may be 

 divided into two groups, according to habits. The ecto- 

 parasitic trematodes are monogenetic, that is, they live on 

 the outside of one host. They have a simple life history, 

 the adults laying eggs which hatch into small worms like 



