544 



COLLEGE ZOOLOGY 



serve for reception of stimuli and are in di- 

 rect continuity with fibers from nerve cells 

 (conductors) which send out processes to 

 the contractile fibrils (effectors) in the 

 epitheliomuscular cells. All together, these 

 constitute a sort of nerve net or plexus. The 

 coelenterates have a receptor-effector nerv- 

 ous mechanism. 



Coordination in 

 higher invertebrates 



A definite nervous system is present in the 

 flatworms and round worms. Flatworms 

 have long nerve cords connected by trans- 

 verse nerves, a concentration of nervous 

 tissue (the brain) near the anterior end of 

 the body, and sense organs such as "eyes." 

 More complex are the nervous systems of 

 the earthworm, crayfish, grasshopper, and 

 frog; in them and in other lower animals, 

 sense organs of many kinds are present. In 

 all of these, as well as in man, the mechan- 

 ism of reaction is the same: a stimulus 

 is received by a sense organ, the impulse is 

 conducted by the nervous system, and the 

 response is carried out by muscles or glands, 

 which are, therefore, effectors. 



The chief advances shown by the annelid 

 type of nervous system over that of the 

 coelenterates involves the concentration of 

 ganglion cells to develop a central nervous 

 system, and the addition of association neu- 

 rons within the central nervous system to 

 mediate impulses between afferent and 

 efferent neurons. These advances made pos- 

 sible the one-way conduction and reflex arcs, 

 instead of the diffuse nerve pathways typical 

 of the coelenterates. 



Nervous coordination 



Nervous coordination is brought about by 

 the activities of neurons as shown in Fig. 

 236. The reflex arc as described in the earth- 

 worm (Fig. 95), is the functional unit of 

 nervous action. With the help of association 

 neurons, which are located within the cen- 



tral nervous system, more complex activities 

 are performed. The same basic mechanisms 

 of nervous coordination exist in vertebrates. 

 Thus the differences, from earthworm to 

 man, are due to an increased number and 

 specialization of receptors, association neu- 

 rons, and effectors. It is a receptor-associa- 

 tion neuron-effector system. 



Coordination by means of the central 

 nervous system is largely responsible for be- 

 havior as far as voluntary muscles are con- 

 cerned, but coordination of the activities of 

 involuntary muscles and glands is also neces- 

 sary. This is accomplished by the autonomic 

 nervous system. 



In some cases hormones take part in re- 

 flexes. For example, in a blind catfish, bright 

 light stimulates photoreceptors of the skin 

 which send nervous impulses to the pituitary 

 gland; here a hormone (intermedin) is se- 

 creted which is carried by the circulatory 

 system to chromatophores in the skin, caus- 

 ing them to enlarge and darken the skin 

 color. This type of reflex is known as a 

 neurohumoral reflex. 



It is interesting to note that in some in- 

 vertebrates as well as vertebrates the passage 

 of a nerve impulse from one neuron to 

 another, or to a motor end organ, appears to 

 be due to the liberation of a chemical media- 

 tor. 



Nervous systems and 

 sense organs of vertebrates 



The nervous system and sense organs of 

 the frog and certain other vertebrates have 

 been described. 



Brains of vertebrates 



The differences in the brains of the vari- 

 ous classes of vertebrates are probably more 

 striking than those exhibited by any of the 

 other organs in the body, as the diagrams in 

 Fig. 392 indicate. In the cyclostomes both 

 the cerebral lobes and cerebellum are quite 

 small. In the fish the optic lobes and 

 medulla are especially large; the size of the 



