274 



General Biology 



outside on the ventral wall of the somite. All of the turns are well 

 supplied with blood vessels. 



An excellent way of demonstrating 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- 



Wall of Digestive Tract. 

 Ciliated Nephrostome. 



Septum. 



Duct of Nephridium. 



Connective Tissue 



Containing Blood Vessels. 

 Glandular or Secreting Portion. 

 Body-Wall Composed of 

 Longitudinal and Circular 

 Muscle Fibers and 

 Epithelial Layer. 

 External Opening. 



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. 



THE NERVOUS SYSTEM 



Notwithstanding the nerve cells scattered about in 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. 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. The knowledge of this fact is quite important and will 

 be of use in the later study of evolutionary theories. 



Nerves are sensory, motor, or mixed as we saw in the study of the 

 frog. Both sensory and motor nerves run to the muscles of the earth- 

 worm, 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 a sensory fiber need not first pass to the brain before 

 meeting the motor fiber, it is called a reflex. 



