i84 



GENERAL BIOLOGY 



tubular body, a tube within a tube, and a coelom or body 

 cavity between ; the inner tube is the alimentary canal and 

 the outer one is the body wall as before, but the alimen- 

 tary canal differs in tw^o important particulars; it is not 

 straight, but greatly coiled and twisted; and it is not simple, 

 but bears conspicuous appendages. And the body wall 

 differs conspicuously in that it bears a differentiated head, 

 and is extended laterally into limbs and backw^ard into a 



tail. 



On comparing the internal organs there are strong con- 

 trasts. The central part of the circulatory system is not a 



a ^ 



Fig. 113. Diagrams illustrating the plan of body in worm 

 and in salamander {b) in cross section; c, enteron; c, 

 coelom ; n, central nervous apparatus ; v, central circula- 

 tory apparatus. 



long pulsating tube lying on the dorsal side, but a heart, 

 lying upon the ventral side. The central part of the ner- 

 vous system does not lie within the coelom on the ventral 

 side, but in the body wall upon the dorsal side. The 

 nephridia are not scattered segmentally in single pairs the 

 whole length of the body, but are aggregated into special 

 paired organs, the kidneys, and in the salamander there are 

 special respiratory organs the lungs, and special supporting 

 structures, the bones. 

 The food tube, alimentary canal, or enteron, is differentiated 

 into parts the anterior of which bear the same names as 

 parts of like function in the earth worm: mouth, pharynx 



