416 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



to start eating when its stomach is empty, to go on till the stomach is 

 full, and then stop ; and to make the backward trip to the surface, to 

 shoot out a sand cylinder, whenever its rectum fills up to a certain 

 degree. But the facts are quite otherwise. Under natural conditions 

 it feeds in little bursts, each lasting for a few minutes, with rests of a 

 minute or so in between; and if you watch it in a glass tube of sea 

 water, without any sand to eat, you often see a similar alternation of 

 feeding movements and rest. The important point here is that each 

 of the outbursts subsides although there has been no satisfaction by 

 eating. Again, under natural conditions a lugworm moves backward 

 to the surface and shoots out a sand cylinder at regular intervals which 

 vary somewhat with the temperature and the size of the worm — com- 

 monly about once every 40 minutes — and the fasting worm in a glass 

 tube can often be seen to make similar backward trips, as before, at 

 regular intervals of about the normal length, even though it has no 

 urge in the form of residues to discharge. It looks as if the worm had 

 "physiological alarm clocks" in its organization which go off spon- 

 taneously every so often, irrespective of its needs, and compel it to 

 make a burst of feeding movements, or a backward trip. 2 



In the case of the feeding rhythm, the analysis has been pressed fur- 

 ther. The "alarm clock" has been located. If you remove the front 

 part of the gut — the esophagus — put it in a dish of sea water and watch 

 it carefully, you see that this little fragment of the worm has a compli- 

 cated automatic rhythm of its own. For a few minutes it is vigorously 

 active, with waves of contraction running along it in regular sequence 

 from the front end to the back, then it becomes quiet for a couple of 

 minutes, and so on. We can distinguish two rhythms here, of different 

 levels — the first is that of the contraction waves themselves, and the 

 second, superimposed upon it, the alternating appearance and dis- 

 appearance of the first. It can be shown quite convincingly, by appro- 

 priate experiments, that this behavior of the esophagus is the cause of 

 the intermittent feeding of the intact worm. When the esophagus is 

 active, its activity spreads through the nervous system to most of the 

 muscles of the body, affecting them in various ways and producing 

 periodic feeding movements. 



In writing these opening paragraphs, I tried above all to be clear, 

 and the resulting picture is rather oversimplified. The behavior of 

 the isolated esophagus is very regular and mechanical, but when it is 

 part of the worm its rhythm and the extent to which its activity 



2 Under the conditions of the lecture, it was necessary to leave out a number of 

 interesting points — such as the fact that each backward excursion to the surface 

 is followed by a spell of vigorous water pumping, or the way in which the excur- 

 sions can be exploited at low tide as a method of aerial respiration. Some of 

 these points are mentioned in the subsequently added footnotes. 



