NERVOUS SYSTEM AND BEHAVIOUR 431 



of the pharyngeal retractor muscles have been analysed. Here, also, the 

 motor fibres from one radius are distributed to each of the other radii. The 

 synapses of this system are non-frequency sensitive, and conduction around 

 the ring is decremental. The latter condition is apparently due to the way 

 the fibre tracts are organized : the number of motor fibres going from any 

 one radius to another radius decreases with distance. 



The central nervous system of echinoderms lies at a low level. Note- 

 worthy features are : presence of association and motor centres ; organiza- 

 tion of nerve tracts into restricted conducting pathways which feed distri- 

 butory nerves into motor centres; and the existence of patterns of neuronal 

 organization serving to co-ordinate activities of the whole organism. 

 Fundamentally, these animals are provided with a nerve-net for localized 

 responses, and a simple c.n.s. for control and co-ordination of widespread 

 activities (64, 65, 87, 88, 103, 104, 105, 106). 



Stomatogastric or Sympathetic Nervous System. The visceral 

 (stomatogastric) nervous systems of invertebrates are imperfectly known. 

 The gut contains several kinds of effectors, namely glands, ciliated cells, 

 muscles, of which the muscles certainly, and some glands probably, are 

 under nervous control. In polychaetes and crustaceans nerves proceed 

 from anterior ganglia and from the nerve cord to the alimentary canal. In 

 addition, various parts of the gut are frequently provided with a nerve-net 

 and display autonomous activity. Innervation of the hearts of invertebrates 

 is described in Chapter 3. Accounts of the visceral nervous systems of 

 lower chordates are available (61a, 75). 



Many polychaetes possess stomatogastric nerves which arise anteriorly 

 from brain, connectives and suboesophageal ganglia, and which proceed to 

 the anterior gut (pharynx, oesophagus, stomach). Visceral ganglia are dis- 

 tributed along the course of these nerves, which terminate in plexuses in the 

 gut wall (aphroditids, eunicids, etc.). The stomatogastric system of Areni- 

 cola takes the form of a plexus of nerve bundles connecting the supra- 

 oesophageal ganglia and circumoesophageal commissures with a ring 

 ganglion at the anterior end of the oesophagus. This visceral plexus con- 

 tinues along the length of the oesophagus (123). Petta possesses sympa- 

 thetic nerves which run from the abdominal nerve cord to visceral ganglia 

 and a plexus in the intestine. 



The alimentary canal of various polychaetes shows rhythmic activity, 

 originating in an intrinsic nerve-net. In intact specimens of Tomopteris 

 (a transparent animal), rhythmic pulsations can be seen which take the 

 form of peristaltic waves along the gut. Isolated extrovert preparations 

 (proboscis and oesophagus) of Arenicola marina show bursts of vigorous 

 rhythmic contractions, alternating with periods of relative rest. The 

 rhythmic activity, with a periodicity of 6-7 minutes, forms part of the feed- 

 ing cycle (Fig. 10.10, cf. Fig. 4.7, p. 148). Excitation originates in a diffuse 

 oesophageal nerve plexus, from which it radiates to the proboscis and 

 anterior body wall through proboscidial nerves, and evokes rhythmic 

 muscular contractions. This behaviour pattern is responsible for the dig- 



