PHYLUM ANNELroA. SEGMENTED WORMS 



179 



cells in the ganglia of the central nervous 

 system. They pass out to the muscles or 

 other organs; and, since impulses sent along 

 them give rise to movements, the cells of 

 which they are a part are said to be motor 

 nerve cells. The sensory fibers originate 

 from nerve cells in the epidermis and carry 

 impulses into the ventral nerve cord. The 

 peripheral nervous system is composed of 

 elements which have definite connections 

 in the nerve cord. 



The functions of nervous tissue are recep- 

 tion, conduction, and stimulation. These are 

 usually performed by nerve cells called 

 neurons. The neuron theory assumes that 

 there is no nerve fiber independent of a 

 nerve cell, that the nerve cell body with 

 all of its processes is a unit, called the 

 neuron. There is no protoplasmic con- 



tinuity of one neuron with another; the 

 relation between the neurons is probably 

 contact of the terminals of one neuron with 

 those of another. 



The reflex is considered the functional 

 unit of the nervous system. The apparatus 

 required for a simple reflex in the body of 

 an earthworm is represented in Fig. 95. A 

 sensory neuron, lying at the surface of the 

 body, sends a fiber into the ventral nerve 

 cord, where it branches out; these branches 

 meet but are not continuous with branches 

 from an association neuron lying in the 

 ventral nerve cord. The association neuron 

 is in contact with a motor neuron that sends 

 fibers into a reacting organ, which in this 

 case is a muscle. These fibers extending to 

 the reacting organ are called motor fibers; 

 those leading to the ventral nerve cord are 



Mucous secretion 

 Cuticle 



Epithelial 

 cell 



Gland cell pore 

 Supporting cell 



•^ens' 



photoreceptor 

 (light sensitive cell) 



Nucleus" 



Nerve fiber 



Sensory cell of 

 sense organ 



Gland cell 



Basal membrane 

 Nerve fiber 



Figure 96. Diagram of the epidermis of the earthworm showing sense organs. 



termed sensory fibers. The first neuron or 

 receptor receives the stimulus and produces 

 the nerve impulse which is carried on to the 

 association neuron; the association neuron 

 in turn transmits the impulse to a motor 

 neuron which has processes extending to 

 an effector such as a muscle or other organ. 



Any action which takes place through 

 such a reflex arc is termed a reflex act. 



Within the ventral nerve cord are associa- 

 tion neurons whose fibers serve to connect 

 structures within one ganglion or two suc- 

 ceeding ganglia. These neurons are doubt- 

 less responsible for the muscular waves which 

 pass from the anterior to the posterior end 



of the worm during locomotion. The three 

 giant fibers, which lie in the dorsal part of 

 the ventral nerve cord throughout almost 

 its entire length, are connected by means of 

 fibrils with nerve cells in the ganglia, and 

 probably distribute the impulses that cause 

 a worm to contract its entire body when 

 stimulated. The earthworm's behavior is 

 largely a matter of reflex acts. 



Behavior due to simple reflexes are as 

 mechanical as the reflection of light from a 

 mirror, they often save animals from in- 

 jury and even from death. Fortunately, we 

 ourselves do not have to think before we 

 pull our hand away from a hot stove, or 



