812 Comparative Animal Physiology 



ings and muscle. However, the weakness and the local nature of the response 

 and its virtual absence in anterior segments show that it can hardly be of 

 importance in behavior. 



The preceding evidence indicates that the subepidermal plexus of the 

 earthworm is not a true nerve net for general conduction, that although there 

 may be connections through it from sense cells to muscle these cannot be 

 of importance in behavior, and that there is no indication that the network 

 plays any part in initiating spontaneous movement of the body wall. The 

 peripheral plexus is probably a sensory relay area since there are many 

 more epidermal and muscle sense cells than there are segmental nerve fibers. 

 330 It is possible that among more primitive annelids the connection through 

 the peripheral plexus from sense organs to muscle was important but that in 

 earthworms the central nerve cord has now taken over complete control. Cer- 

 tainly among polychaetes (e.g., Arenicola^^^^ there is no evidence of any 

 conduction of peristaltic waves except by way of the nerve cord. 



In gastropod molluscs a subepidermal plexus has been described, particu- 

 larly in the foot.'*^- ^s Small pieces of the foot of a slug show peristaltic move- 

 ment.*^ Jordan^^^ removed the paired pedal ganglion from Aplysia and noted 

 a marked increase in tonus of the "wings"; this was arbitrarily attributed to 

 a release of the peripheral net from inhibition by the pedal ganglion. When 

 ganglionic connections to the "wings" of Aplysia were cut-''^ and one of the 

 motor nerves was stimulated, there was an immediate contraction of the area 

 served by that nerve, followed by a peristaltic wave which spread out from 

 this area. This wave was said to be reflex in nature, with the peripheral net- 

 work conducting the impulses. Excised labial palps of Anodonta show auton- 

 omous responses to stimulation by light, touch, and chemicals.^^ These palps 

 must CO .tain direct connection through the peripheral nervous network 

 from sense organs to muscle. In general, molluscan subepidermal networks 

 relay messages from central ganglia but are incapable of any independent 

 locomotor control. There is little doubt that normal coordinated locomotion 

 of the foot requires the presence of the pedal ganglia. More knowledge of 

 the conduction properties of the muscle is needed before functional import- 

 ance can be assigned to the peripheral network. 



The use of a peripheral nerve network in locomotion appears with slight 

 success in the echinoderm-chordate line, and with still less success in the 

 annelid-arthropod-mollusc line. In the higher groups— vertebrates, polychaetes, 

 cephalopod molluscs, and arthropods— there is no evidence of a peripheral 

 network for locomotor control. Yet in most of these a nerve network is re- 

 tained for regulation of movement in the digestive system. Central nervous 

 reflexes have become dominant patterns in animal behavior. 



GANGLIONIC FUNCTION IN INVERTEBRATES 



Locomotion is under reflex control in annelids, arthropods, molluscs, and 

 chordates, but ganglionic reflexes in invertebrates have not been subjected to 

 such complete analysis as spinal reflexes in mammals. 



Annelids. A number of segments of the ventral nerve cord of an annelid, 

 isolated from the body musculature, can conduct a peristaltic wave to the 

 region beyond. Normally, however, the wave is reinforced by the segmental 

 reflexes. 



