178 



COLLEGE ZOOLOGY 



worm is inefEcient as an oxygen-transporting 

 substance compared to the hemoglobin of 

 man. 



Excretory system 



Most of the excretory matter is carried 

 out of the body by a number of coiled tubes 

 termed nephridia (Figs, 91 and 92), a pair 

 of which are present in every segment ex- 

 cept the first three and the last. A nephri- 

 dium occupies part of two successive seg- 

 ments; in one is a ciliated funnel, the 

 nephrostome, which is connected by a thin 

 ciliated tube with the major portion of the 

 structure in the segment posterior to it. 

 Three loops make up the coiled portion of 

 the nephridium. The cilia on the nephro- 

 stome and in the nephridium create a cur- 

 rent which draws in waste material from 

 the coelomic fluid; other waste is received 

 directly from blood vessels surrounding the 

 nephridium. These excretory products (am- 

 monia, urea, creatine) are eventually car- 

 ried out through the nephridiopore. Chlora- 

 gogue cells may store excretory matter 

 temporarily before releasing it into the 

 coelomic fluid. The nephridia serve the 



same function in the earthworm that 

 the kidneys do in man. 



Nervous system 



The nervous system is concentrated (Figs. 

 91, 94, and 95). There is a bilobed mass 

 of nervous tissue, the "brain" or cerebral 

 ganglia, on the dorsal surface of the pharynx 

 in segment 3. This is connected by 2 cir- 

 cumpharyngeal connectives with a pair of 

 subpharyngeal ganglia which lie beneath 

 the pharynx. From the latter, the ventral 

 nerve cord extends posteriorly near the 

 ventral body wall. The ventral nerve cord 

 enlarges into a ganglion in each segment 

 and gives off 3 pairs of nerves in every seg- 

 ment posterior to segment 4. Each ganglion 

 really consists of 2 ganglia fused together. 

 Near the dorsal surface of the ventral nerve 

 cord are 3 longitudinal giant fibers. The 

 brain and nerve cord constitute the central 

 nervous system; the nerves which pass 

 from and to them represent the peripheral 

 nervous system. 



The nerves of the peripheral nervous sys- 

 tem are either motor or sensory. Motor 

 nerve fibers (Fig. 95) are extensions from 



Sensory cell (receptor) 

 Sensory fiber 



-Dorsal giant fiber 



Association neuron 

 Lateral nerve 



Epidermis 



Longitudinal muscle (effector) 



Figure 95. Diagram of sensory and motor neurons of the ventral nerve cord of an earthworm, 

 showing their connections with the skin and the muscles to form a reflex arc. 



Ventral giant cells 

 Motor neuron cell body 



