THE NERVOUS SYSTEM OF THE 

 EARTHWORM 



Such a diffuse and uncentralized nervous 

 system is not characteristic of invertebrate 

 animals generally. Many invertebrate animals 

 have highly centralized nervous systems, es- 

 pecially among the annelids and arthropods. 



In higher animals, the nerve cells are not 

 scattered throughout the body, but are ag- 

 gregated mainly in a central nervous system. 

 The central nervous system of the earthworm 

 has the form of an elongate nerve cord that 

 extends lengthwise through the body. In the 

 earthworm and most other invertebrates, 

 the nerve cord lies ventral to the digestive 

 tract, except at the anterior end, where it 

 loops around the pharynx (Fig. 25-5). An- 

 teriorly the nerve cord terminates in two 

 distinct swellings, the cerebral ganglia, which 

 lie dorsal to the digestive tract (Fig. 25-5); 

 other swellings, or ganglia, occur in the ven- 

 tral part of the cord, typically one pair in 

 each segment of the body. 



The ganglia are very important because 

 these swellings mark the location of the cell 

 bodies, or centrons, of the many nerve cells 

 that compose the whole nervous system. Thus 

 the interconnecting strands between the 

 ganglia (Fig. 25-5) consist of nerve fibers 



Responses of Higher Animals: The Nervous System - 451 



(mainly axons), derived from the centrons in 

 the ganglia; and likewise the nerves, which 

 are associated with each ganglion (Fig. 25-5), 

 consist of bundles of nerve fibers passing to 

 and from the ganglia. 



Central vs. Peripheral Parts of the Nervous 

 System. In such a highly organized system, it 

 is possible to recognize two main functional 

 parts: (1) the peripheral nervous system, 

 which consists of the many nerves extending 

 out from the nerve cord; and (2) the central 

 nervous system, which is the ganglionated 

 nerve cord itself. Each peripheral nerve sends 

 fibers to both the receptors and effectors in 

 a certain segment of the body; and the cen- 

 tral nervous system serves as an interconnec- 

 tion among all the peripheral nerves. Thus 

 the nerves present in each segment of the 

 earthworm (Fig. 25-5) are brought into com- 

 munication with each other, and with all 

 other nerves in the body, by way of the nerve 

 cells of the central nervous system (Fig. 25-6). 



The nerves of the earthworm are all mixed 

 nerves, in that each nerve is made up partly 

 of sensory (afferent) fibers, which convey im- 

 pulses from the receptors toward the central 

 nervous system; and partly of motor (efferent) 

 fibers, which always transmit impulses from 

 the CNS to the effectors. Thus each nerve 

 consists of fibers supplying both the receptors 



Prostomium 



Buccal cavity 



Circumpharyngeal 

 Cerebral ganglion connective 



Mouth 



Segmental nerve 



Septal nerve 



Subpharyngeal ganglion 



Fig. 25-5. Side view of anterior end of an earthworm, showing the cerebral 

 ganglion and larger nerves. (After Hess. From Hegner, College Zoology. Per- 

 mission of The Macmillan Co.) 



