432 THE BIOLOGY OF MARINE ANIMALS 



ging activity of the proboscis. A. marina is a specialized burrowing species, 

 highly adapted for life in sand and mud (117, 120). 



Much variation exists in organization of the stomatogastric nervous 

 system of Crustacea, but the following are typical features, especially of 

 decapods. An anterior stomatogastric system, arising from tritocerebral 

 ganglia on the circumoesophageal connectives, sends sympathetic nerves 

 to oesophageal and gastric ganglia. These are motor centres from which 

 motor branches proceed to the upper lip, oesophagus and stomach. 

 Sensory cells in these organs are believed to be reflexly connected with the 

 effectors through the c.n.s. From the caudal ganglia other sympathetic 



Fig. 10.10. Kymograph Tracing of Activity of Body Wall and 

 Extrovert Preparation of Arenicola marina 



Extrovert above, body wall below. At X, the nerve cord was cut between the two strips. 

 Time tracing, 1/min. (From Wells, 1937.) 



nerves supply the intestine, the wall of which contains a nervous plexus. 

 Peristaltic and anti-peristaltic waves churn and propel the intestinal con- 

 tents. The intestine is capable of autonomous activity, and movements 

 continue even in an isolated preparation (37, 108). The stomatogastric 

 nervous systems of these and other groups are described by Hanstrom (41). 

 Nerve-nets — Conclusions. In the preceding section salient features of 

 the nerve-nets of several phyla have been reviewed. This somewhat ex- 

 tended phyletic treatment is warranted by their simplicity of organization 

 and their importance in providing information about possible ways in 

 which nervous systems have evolved. The diagnostic feature of nerve-net 

 organization is the existence of a pervasive plexus of unpolarized neurones. 

 Interneural transmission is by axonal contact. The nerve-net may consti- 



