FLATWORMS 



197 



from the dorsal to the ventral side, whence the common name 

 " flatworms.'^ Unlike nearly all other many-celled animals 

 they have no body cavity, the organs being embedded in a sort 

 of spongy " packing " tissue. The digestive tract has only a 

 single opening which serves both for mouth and vent (Fig. 60A), 

 and in the tapeworms the entire alimentary canal is absent. 

 The nervous system is very simple. Performing the function 

 of kidneys is a system of tubes, the terminal branches of which 

 are closed by " flame cells," so called from the flamelike flickering 

 of a brush of cilia which keeps up a flow of fluid toward the 



Fig. 60. Types of digestive tracts in worms; A, fluke, — note branching and 

 absence of anus; B, roundworm, — note simple form, with only pharynx differen- 

 tiated, and presence of anus; C, leech, — note extensive pouches or cceca which 

 serve as reservoirs for surplus food. 



larger branches of the system and ultimately to the excretory 

 pore, thus conducting the waste products out of the body. The 

 absence of any kind of blood system or other apparatus for trans- 

 porting food or waste products in the body necessitates a branched 

 condition of the digestive and excretory systems. The most 

 highly developed system of organs and one which occupies a 

 large portion of the l^ody is that concerned with reproduction. 

 Usually there is a complete male and female system in each 

 worm and in some tapeworms there is a double system of each 

 kind. 



The flatworms are usually divided into three classes, the Tur- 

 bellaria, the Trematoda and the Cestoda. The Turbellaria are 

 for the most part free-living animals and include the " pla- 

 narians " which can be found creeping on the under side of stones 

 in ponds. The Trematoda include the flukes, all of which are 

 parasitic, some externally on aquatic animals, others internally 



