THE EARTHWORM 



273 



posterior parts of the somites, three coils or turns in each, the third 

 ending in an enlarged portion opening to the outside on the ventral wall 

 of the somite. All of the turns are richly supplied with blood vessels." 

 An excellent way to demonstrate the action of these nephridic 

 organs is that of injecting carmine powder into the coelom. It will then 

 be observed that this foreign substance is taken up by the chlorogogen 

 cells, which then break down, freeing the carmine together with frag- 



Fig. 168. Nephridium. 



ments of the chlorogogen cells, and all are caught up by the current 

 made by the nephrostome, and carried through the nephridium to the 

 outside. From this experiment the conclusion has been drawn that 

 some, at least, of the waste matters of the tissues are brought to the 

 chlorogogen cells by the circulation and are acted upon by the fluids of 

 those cells. The products of this activity are liberated into the coelom* 

 by the fragmentation of the cells, and then excreted from the worm by 

 the nephridia. ? :V " "~'^uy 



THE NERVOUS SYSTEM 



Notwithstanding the nerve cells scattered about in the Hydra, it is 

 in the Earthworm that we meet with our first organized nervous system 

 (Fig. 169). That is, of course, excluding our study of the frog. And it 

 will be remembered that the nerve cord was on the dorsal side of the 

 frog. In the earthworm, and all animals lower than vertebrates, it lies 

 on the ventral surface. This is quite important and will be of use in our 

 later study of evolutionary theories. 



Nerves are sensory, motor, or mixed as noted in the frog. Both 

 sensory and motor nerves run to the muscles of the earthworm, causing 

 reflex action. A reflex action means that an impulse sent toward the 

 central nervous system through a sensory nerve, meets a motor nerve 

 (the meeting place being called a ganglion), and the motor impulse is 

 then returned to the place from whence the sensory impulse originated, 

 permitting an organ to move. If such ganglion lies in the lower nerve 

 centers, that is, if it lies caudad to the brain, so that an impulse from 



