GENERAL ZOOLOGY 



nerve cord; lesser processes of these cells may connect with the nerve cells of a 

 subepidermal nerve net or plexus. Within the nerve cord the fibers of the 

 neurosensorv cells may make synaptic contact directly with eflferent neurons,- 

 or their svnapses may be with cells called adjuster or association neurons, 

 whose chief function is the distribution of incoming impulses among 

 many efferent neurons. The branching processes of adjustor neurons lie en- 

 tirely within the nerve cord, but the principal fibers or axons of eff^erent 

 neurons pass outward from the cord to make contact with muscle cells or 

 other effectors, as shown diagrammatically in Figure 17.7. The resemblance 

 of this type of system to that found in vertebrates is apparent. Like the 

 vertebrate nervous system, the sensory-neuro-motor mechanism of the annelid 

 consists of two interconnected divisions, the central system, the "brain" 

 and the nerve cord, and the peripheral system, the nerves and their com- 

 ponent nerve fibers. The relationship to these divisions of the neuro- 

 sensory cells and sense organs, and of the muscles and glands which are the 

 effectors, is similar in the annelid and in the vertebrate. 



In the annelid, stimulation of peripheral neurosensory cells sets up nerve 

 impulses which travel to the subepidermal plexus and also along the axons 

 of the sensory cells extending into the ventral nerve cord. Within the nerve 

 net the impulses seem to pass in all directions from their point of origin, as 

 in the coelenterate nerve net, but not for any great distance. Within the 

 nerve cord the pathway of impulses may be a short cut to an efferent neuron 

 and thence outward to a nearby effector; or it may involve the mediation of 

 one or more adjustor neurons and so extend to any part of the animal. In 

 addition to the more typical adjustor neurons, the nerve cord of the annelid 

 contains what are called giant fibers; these function for the rapid conduction 

 of impulses over long distances within the cord. 



Like the vertebrate nervous system, that of the annelid is a receptor-adjustor- 

 eflPector system. The differences between the two systems lie not in fundamen- 

 tal relationships of parts but in degree of complexity. In the vertebrate there 

 are more adjustor or association neurons, and hence more complicated con- 

 nections and pathways within the central nervous system. The complexity is 

 notably greater in the vertebrate brain, a region which is relatively much 

 larger in the vertebrate than in the annelid, and in which the number of nerve 

 pathways is infinitely multiplied. The brain of the vertebrate is also the 

 region which is most intimately related to the highly specialized sense 

 organs of the head. Similarly, in the annelid, the concentration of gan- 

 glion cells in the anterior end, forming the so-called "brain," lies close 

 to, and mediates the functions of, the most sensitive parts of the earthworm 

 and the specialized eyes and antennae of the clamworm and other polychaetes. 



The main advances of the annelid type of nervous system over that of the 

 coelenterates involve two interrelated lines of specialization: the concentra- 

 tion of ganglion cells to form a central nervous system, and the development 

 of adjustor neurons within the central system to mediate impulses between 

 afferent and efferent neurons. These advances made possible the develop- 



524 



