64 PHYLUM VERMES 



masses united by a broad commissure. From its anterior and 

 lateral sides numerous sensory nerves pass to the anterior body 

 surface, which render this extremity a highly sensitive tactile 

 organ. From its posterior side a pair of large longitudinal nerve 

 cords passes to the hinder end of the body, being united at 

 intervals by transverse nerves. 



The Excretory System. This is difficult to see ; it consists of 

 minute tubes which extend throughout the body and collect the 

 excreted matters from the tissues. There are two main longitudi- 

 nal tubes extending the length of the body, which open to the 

 outside through minute pores on the dorsal surface of the animal. 

 These tubes are not straight but coiled and give off numerous 

 branches, at the termination of each of which is a peculiar cell 

 with a vibratory process at its base called a flame cell ; they are 

 joined by a transverse tube at the anterior end of the animal. 

 Portions of the excretory system can often be seen in the com- 

 pressed animal, where they appear as fine lines. 



Exercise 4. Draw an outHne of the animal and place in it as much of 

 the nervous and the excretory system as you have observed. 



No special respiratory system is present in the Turbellaria, the 

 ciliated outer surface of the body performing this function. A 

 circulatory system and a blood fluid are also wanting. The 

 branching of the digestive and excretory systems is correlated 

 with this feature. Can you explain how ? As in other flatworms, 

 the turbellarians possess no body cavity, the primitive body 

 cavity being filled secondarily by a peculiar vesicular connective 

 tissue called parenchyma. The muscular system consists of a 

 layer of strong circular and longitudinal muscles just beneath the 

 surface of the body and of oblique muscles passing through the 

 parenchyma. 



