176 



METAZOAN PHYLA 



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. 83), whereas a typical coelom is completely invested 

 by mesoderm. Large cells, containing enormous vacuoles, have been 

 described as present in the body cavity. 



Dorse*/ //he 



Dorsa/ nerve corc^ 



Boc/y ccry/'fy 



Ifrf-esf/ne 



Fxcrefory 

 duct 



Loffercf/ 

 //ne 



A^asc/e 

 ce// 



0\^ary 



Yeri-Zfcr/ 

 nerye core/ 



Ye/7, tro'/ 

 Jine 



Fig. 83. — 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. 



205. 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 

 seems to be effectively distributed by the much-branched gastrovascular 

 cavity of the turbellarian, the increasing development of the mesoderm 

 makes such distribution correspondingly more difficult. In contrast to 



