MULTICELLULAR ANLMALS 215 



while the fibres of the segmental ganglia control the slower crawl- 

 ing movements. 



A similar arrangement consisting of a ventral chain of paired 

 segmental ganglia, with larger ganglia dorsal to the pharynx, con- 

 stitutes the nervous system of the Arthropoda (Fig. 145). The 

 ganglia above the pharynx are relatively larger and more complex 

 than in the Annelida; fibres from these ganglia communicate with 

 the antennae and eyes, thus establishing an elaborate sensory system. 

 The dorsal anterior ganglia in both annelids and arthropods con- 

 stitute primitive brains. 



The Vertebrate Nervous System. In vertebrates the nerv- 

 ous systems all represent modifications of a single basic plan. The 

 vertebrate nerve plan may be divided for study into three rather 

 distinct systems, the central, consisting of the brain and spinal 

 CORD, the PERIPHERAL, cousistiug of the nerves from the central sys- 

 tem that extend to the muscles and sense organs, and the sympa- 

 thetic, or autonomic system, consisting of ganglia and nerves that 

 are more or less independent of the central and peripheral systems. 

 The sympathetic system is concerned in the automatic control of 

 the visceral organs, the circulatory system, and various other func- 

 tions, glands for instance, that are not directly under control of the 

 central system. 



The central and peripheral systems in reality constitute an operat- 

 ing unit and are the elements that receive, transmit, and effect re- 

 sponses to stimuli received from the environment. 



Developmental Origin of the Vertebrate Nervous Sys- 

 tem. In connection with the description of the ectoderm of 

 coelenterates (p. 132) it was stated that the ectoderm of all animals 

 is the layer of origin of all purely nervous elements of the metazoon 

 body. To accomplish this so that all the deeper tissues of the com- 

 plex adult vertebrate body are in contact with nerve fibres, requires 

 an invasion by that portion of the ectoderm destined to form nerves, 

 at some time during development. This is brought about early in 



