88 THE MAIN CURRENTS OF ZOOLOGY 



heading Vermes present diverse and often puzzling 

 relations. It is now universally recognized that there 

 is no natural group or phylum "vermes' : but the 

 title is often retained on account of its convenience. 

 The title is justifiable only as a popular name for 

 shape. The animals included under it do not form a 

 natural group of related species and the general 

 tendency is to split the group into at least three 

 phyla, the flatworms, the roundworms and the 

 segmented worms or annelida. The brachiopod, 

 which are so abundant as fossils (5000 species) are 

 frequently classified as shelled worms; at other 

 times, they are separated into an independent phy- 

 lum, or are united with the bryozoa into a phylum 

 mollmcoida. 



IV. Platyhelminthes. This phylum of flatworms 

 includes fresh water, salt water and land planarians 

 and parsitic forms. The parasitic flatworms, such as 

 tapeworms, liver-flukes, etc., are responsible for 

 certain diseases of man and other animals. Four 

 thousand six hundred species are known. 



V. Nemathelminthes. The roundworms or thread- 

 worms are chiefly small parasitic worms, living in 

 the alimentary canal, the blood or the tissues of 

 man and other animals. The hookworm, respon- 



