Evolution 



the undigested remains passing out, by the same opening, and 

 instead of the contents of the digestive cavity being a general 

 mixture of food material in all stages of digestion, there is now a 

 regular stream of food passing through the cavity in one direction, 

 and being digested as it goes. A near relative of the Rotifers is 

 shown in Fig. 42. 



Thirdly, we must briefly allude to the Nemertines. These 



a 



FlG. 42. Chaenonotus, a lower worm. 



m, Mouth ; ^, eye ; ss, sensory hairs ; #', oesophagus ; sk, skin ; 

 d, digestive canal ; ;z, nephridia ; <?.r, excretory opening ; c, cilia ; 

 , anus ; /;, brain ; me, muscle cells ; ;/, nerves ; o, ovary. 



are a group of flattened thread-like worms of very variable size, 

 found both in fresh and salt water. The most notable advance 

 in this group is to be seen in the occurrence of a special circulatory 

 system. It has already been indicated that the gastric cavity of 



the lower forms has the double function of digestion and of the 



62 



