PHYLUM NEMATHELMINTHES 



179 



surface near the mouth. There are no flame cells, but four large branched 

 excretory cells are present in the anterior part of the body. A circum- 

 pharyngeal nerve ring, containing nerve cells, surrounds the pharynx and 

 is connected with two larger nerve trunks, one dorsal and the other 

 ventral, and one or two smaller trunks on each side. The body cavity 

 is not strictly comparable to the coelom of higher forms, since it lies 

 between the entoderm of the alimentary canal and the mesoderm which 

 forms the muscular layers of the body wall (Fig. 89), whereas a typical 

 coelom is completely invested by mesoderm. Large cells, containing 

 enormous vacuoles, have been described as present in the body cavity. 



Dorsof/ /;he 



nerve cord 



ccty/'fy 



Irjfesf/ne 



Fxcretory 

 duct 



Larferaf 

 Z/'ne 



McJ^c/e 

 ceJ/ 



Ovary 



Ver7¥^rcr/ ^ Ventral 



nerve cord //he 



Fig. 89. — Semidiagrammatic cross section of an ascaris. {Based upon Leuckart, wall 

 chart.) Processes of the muscle cells are seen running across to the dorsal or ventral nerve 

 cords. 



207. Characteristics and Advances. — The nemathelminths are bilater- 

 ally symmetrical and triploblastic. The greatest advance is seen in the 

 development of an alimentary canal to replace the gastrovascular cavity, 

 the disadvantages of which are easily made apparent. An animal which 

 has a gastrovascular cavity, the one opening into which serves as both 

 mouth and anus, is manifestly at a great disadvantage when it comes to 

 taking in food and passing out waste. Attention has been called to the 

 fact that a hydra which has fed will not again take food until the food it 

 already has is digested and the waste matter is passed out. Should an 

 animal with such a digestive cavity take additional food, the mixing of 

 food at various stages of digestion would inevitably occur, which would 

 be clearly a disadvantage. Furthermore, although the digested food 



