170 



Aquatic Organisms 



The free-living flatworms abound in most shoal fresh 

 waters. Some live in shallow pools; others in lakes 

 and rivers, others in spring-fed brooks. They gather 

 on the under sides of stones, sticks and trash, and con- 

 ceal themselves amid vegetation, usually shunning 

 the light. They are often collected unnoticed, and 

 crawl at night from cover and lie outspread upon the 



Fig. 79. Flatworms. 



A, diagram of a planarian, showing food cavity; M, mouth at end of cylindric pharynx, directed 

 downward underneath the body;. B, Dendroccelum; C, a chain of five individuals of Slenos- 

 tomum formed by automatic division of the body, (after Keller). Note the anterior position 

 of the mouth and the unbranched condition of the alimentary canal in this Rhabdocale type. 



sides of our aquaria. We may usually find the larger 

 species by lifting stones from a stream bed or a lake 

 shore, and searching the under side of them. 



Flatworms are covered with vibratile cilia and travel 

 from place to place with a slow gliding motion. They 

 range in length from less than a millimeter to several 

 centimeters. The smaller among them are easily mis- 



