NERVOUS SYSTEM AND BEHAVIOUR 



421 



brates and all vertebrates possess heavily myelinated fibres (Figs. 10.2, 

 10.12). Some examples of invertebrates having medullated nerve fibres are 

 Clymene (polychaete), My sis and Palaemon (crustaceans). 



A comparison of different fibres shows that conduction velocity is 

 influenced by sheath thickness, and is much higher in medullated than in 

 non-medullated fibres. In the giant axons of shrimp, for example, where 

 the myelin sheath forms some 20 % of fibre diameter, a fibre of 50 fi cross- 



TABLE 10.1 

 Rate of Nervous Transmission in Nerve-Nets and Neuropile 



Animal 



Coelenterata 



Aequorea forskalea 

 Geryonia proboseidalis 

 Physalia physalis 

 Mastigias papua 

 Cassiopeia xamachana 

 Renilla kollikeri 



Pennatula phosphorea 

 Leioptilus gurneyi 

 Metridium marginatum 

 Calliactis parasitica 



Nemertea 



Cerebratulus sp. 

 Polychaeta 



Aphrodite sp. 



Polynoe pulchra 



Cirratulus sp. 

 Xiphosura 



Limulus sp. 

 Echinodermata 



Cucumaria sykion 

 ditto 



Nervous tissue 



Marginal ring nerve 

 Nerve-net of circular muscle 

 Nerve-net of filaments 

 Nerve-net of bell 



ditto 

 Nerve net 



ditto 



ditto 



ditto 

 Nerve-net of column 



ditto 

 Radial net of disc 

 Net of sphincter and 



mesenteries 



Nerve cord 



ditto 

 ditto 

 ditto 



Cardiac nerve plexus 



Radial nerve 

 Circumoral nerve 



Temperature 

 °C 



20 



26 1 



25-5 



18 

 15-17 



21 



15 



20-5 



21 



18-20 

 18-20 

 18-20 



15 

 13 

 11-20 



24-27 

 20-23 



Velocity 



m/sec 



0-7-0-9 



0-50 



0121 



0-57 

 0-136,0-234 

 0065-0- 102 

 004-0065 



0.05 



0-26 

 012-015 

 004-010 



0-6 



1-1-2 



0059-009 



0-545 



2-93 



0-90 



0-40 



017, 0-28 

 011 



(Various sources) 



section has the same velocity as a squid giant axon of 650 // (about 25 

 m/sec) (53, 98, 111, 128). 



Nerve-nets 



Phyletically the nerve-net has preceded central nervous organization and 

 appears among invertebrates in the body wall of coelenterates, echino- 

 derms, balanoglossids, and in the alimentary tracts of many more highly 

 organized animals, e.g. polychaetes, crustaceans and vertebrates. We shall 

 examine, first, the nervous system of coelenterates before passing on to 

 other groups, because the former have been much studied and display the 

 nerve-net in its simplest form. 



Coelenterates. The nervous system of coelenterates consists of a diffuse 

 network of neurones lying underneath the epithelia. Most of our informa- 



