454 THE BIOLOGY OF MARINE ANIMALS 



chaete, showing rhythmic behaviour concerned with irrigating and cleaning 

 its burrow, and with feeding. The animal feeds by protruding its proboscis 

 at regular intervals and taking in sand (p. 232). Vigorous muscular con- 

 tractions of the proboscis alternate with periods of rest, a whole cycle 

 occupying some 7-8 min. Contractions of the proboscis of Arenicola are 

 controlled by a plexus in the oesophageal wall; from the oesophagus the 

 intermittent rhythm invades the proboscis and may also spread into the 

 anterior three segments of the body wall (Fig. 10.10). Impulses from the 

 oesophageal plexus reach the c.n.s. via proboscidial nerves, and excitation 

 of the body wall will not extend past a point where the ventral nerve cord 

 is cut. 



In addition to the feeding cycle the behaviour of Arenicola is character- 

 ized by an irrigation cycle showing a periodicity of about 40 min. This is 

 three-phasic, consisting of periods of headward creeping, headward 

 irrigation and tailward irrigation. Water is pumped through the burrow by 

 the development of peristaltic waves along the wall of the trunk. The phas- 

 ing and intermittence of activity are due to a pacemaker located in the 

 nerve cord, the activity of which is spontaneous and not dependent on 

 external stimuli. Pacemaker activity, however, is influenced by external 

 conditions — for example, the vigour of pumping is reduced in stagnant 

 water. 



In Arenicola neither pacemaker — in oesophagus or cord — directly 

 affects the rhythm of the other. The integration of activity depends on the 

 extent to which their influences spread through the neuromuscular system. 

 In brief, the two pacemakers compete for activity, the oesophageal pace- 

 maker predominantly controlling the proboscis and buccal region, whereas 

 the irrigation pacemaker is dominant over most of the body wall (118, 

 120, 123). 



Spontaneous activity cycles, controlled by visceral pacemakers, occur 

 in other polychaetes, e.g. the isolated proboscides of Nereis and Glycera. 

 Sabella, a tubicolous polychaete, drives water through its tube by pumping 

 movements of the body wall. Control of periodic irrigation in this animal 

 depends on pacemaker activity of the ventral nerve cord. In Chaetopterus 

 irrigation is carried out by the regular, sequential beating of three para- 

 podial fans. The rhythm is spontaneous and originates in ganglia of the 

 ventral nerve cord; it is sensitive, however, to environmental influences 

 (119, 121). 



Electrical Evidence of Spontaneous Central Activity. The 

 ganglia and nerve centres of many animals are in a state of continuous 

 electrical activity, which appears as rhythmical oscillations of potential 

 or periodic discharges of impulses (brain waves). The brain waves seen in 

 electro-encephalograms of vertebrates find their counterpart in recordings 

 made from invertebrate nerve ganglia. It is rather striking that many 

 nerve centres continue to exhibit rhythmic electrical activity even when 

 isolated from all incoming sensory messages, e.g. in isolated abdominal 

 ganglia from the crustacean nerve cord. 



