VERMES 171 



surfaces. There is no internal skeleton and the appen- 

 dages are not jointed. They are divided into four 

 classes, the flat worms, the round worms, the segmented 

 worms, and the molluscoidea. These groups differ from 

 each other so much that it is probable that their asso- 

 ciation is an artificial grouping made for convenience. 



Flat Worms. — The flat worms may be free, living in 

 water or moist earth, or parasitic. The body is flattened 

 and has no appendages. There is no body cavity sepa- 

 rate from the digestive system. In lower forms the 

 alimentary canal has only one opening. In parasitic 

 forms the alimentary tract may be completely lost. 

 There is a small dorsal brain, and two nerve cords run 

 backward parallel to each other. Eyes may be present 

 on the dorsal surface near the brain. Some of these 

 worms reproduce by fission. A new mouth appears, the 

 body contracts in front of it and finally divides into two 

 worms. Before one division is complete new divisions 

 may begin, until a chain of as many as eight worms may 

 be hanging together. There is also a sexual reproduc- 

 tion. The parasitic forms are often responsible for 

 disease in man and other animals. They enter the 

 body in the food or water. The tapeworm is of these 

 perhaps the best known. 



Round Worms. — The round worms are long, and as 

 the name indicates, cylindrical, and the surface is cov- 

 ered by a tough cuticle. The alimentary canal opens to 

 the outside at both ends. Some forms live in water and 

 some are parasitic in plants and in animals. Trichina 

 is particularly dangerous. It enters the body in un- 

 cooked pork. 



