8 



COMPARATIVE ANATOMY 



thought by some morphologists to represent the beginnings of a circulation. 

 The nemerteans, however, have a true blood system in the form of dorsal 

 and lateral vessels with transverse connections. 



The parasitic mode of Hfe which has been adopted by many flatworms 

 has led to more or less degeneration and to a great complication of life 

 histories. 



Four thousand five hundred species are known. 



Phylum 5. NEMATHELMENTHES 



The nemathelminths or thread-worms are also well characterized by 

 their name. They differ from flatworms, not only in the shape of their 



NEMATODE. C MONH YSTERA) 



VENTRAL NERVE' 



NEMATODE - CROSS SECTION. 



Fig. 6. — Monhystera, a type of the Phylum Nemathelminths seen in lateral aspect. 



(Redrawn from Ward and Whipple after Cobb.) Like M onhystera most nemathelminths 



have an anal opening. As seen in the cross section the alimentary canal is separated 



from the body-wall by a false body-cavity (pseudocoelom). (Redrawn after Stempell.) 



bodies, but also in having a body cavity which, however, instead of lying 

 between two layers of mesoderm, separates the body wall from the 

 endodermal lining of the ahmentary canal, and is therefore a pseudocoelom. 

 Most of them have an anus. 



Threadworms are much like annelids, but have not their metamerism. 

 Muscles are hmited to the body- wall, and consist exclusively of longitudinal 

 fibers, grouped in four bundles. The nervous system is a circumesophageal 

 ring and a dorsal, a ventral, and two lateral longitudinal nerves. Most 



