NERVOUS SYSTEM AND BEHAVIOUR 443 



Evidence pertaining to Cholinergic Systems. As an initial approach 

 we observe that acetylcholine is not confined uniquely to nervous systems, 

 and occurs in bacteria, protozoa, non-innervated vertebrate tissue (pla- 

 centa) and even in plants. The function of acetylcholine, therefore, may 

 transcend that of chemical transmission, even in metazoan animals pro- 

 vided with nervous tissue. Acetylcholine and cholinesterase are lacking in 

 sponges. In primitive nerve-nets of coelenterates there is no evidence for 

 cholinergic and adrenergic systems. Acetylcholine has not been detected in 

 coelenterates, but cholinesterase sometimes occurs in significant amounts 

 (hydroids, anemones). Cholinesterase content is very low or absent in 

 Scyphomedusae and ctenophores. In apparent agreement with these 

 analyses it is known that neuromuscular systems of Scyphomedusae and 

 actinians are insensitive to acetylcholine, eserine and curare (6, 7, 17, 81, 

 90, 93, 94). 



In the lower triploblastic phyla, platyhelminthes, nemertines and anne- 

 lids, there is some evidence for cholinergic systems. Cholinesterase has 

 been found in turbellarians (Procerodes) and nemertines (Cerebratulus, 

 Lineus). More data are available for annelids. Acetylcholine and cholin- 

 esterase occur in the body wall, especially muscles, of polychaetes (Areni- 

 cola, Spirographis), and Sipunculus. Sensitivity to acetylcholine is shown by 

 the body wall of nemertine (Cerebratulus), polychaetes (Branchiomma), 

 leeches (Pontobdella) and sipunculoids {Sipunculus), pharyngeal retractor 

 of Golfingia and extrovert of Arenicola. Eserine potentiates the action 

 of acetylcholine in some of these animals (7, 27, 76). 



The highest sensitivity to acetylcholine thus far discovered in any 

 invertebrate is shown by the heart of a lamellibranch Mercenaria, which is 

 inhibited in concentrations of 10 -10 to 10~ 12 (Chapter 3). Acetylcholine 

 has been detected in various molluscan tissues: foot, oesophageal ganglia 

 of Aplysia ; purple gland of Murex ; mantle muscle and cerebral ganglia of 

 Octopus, etc. Cholinesterase occurs in gastropods, lamellibranchs and 

 cephalopods; the cholinesterase content of squid ganglia is relatively high. 

 Sensitivity to acetylcholine varies greatly in different molluscs: positive 

 responses are given by the foot of Buccinum, siphons of Hiatella, mantle 

 muscles of Sepia, etc. Eserine potentiates the action of acetylcholine, and 

 curarine inhibits acetylcholine-induced contractions in muscle of Sepia. 

 The eserinized perfusate of central nervous and mantle tissues of Eledone 

 shows acetylcholine activity, but this is not increased by stimulation; 

 neither are acetylcholine and eserine stimulants for squid giant synapses 

 (7, 146, 134). 



Relatively high levels of acetylcholine and cholinesterase have been 

 found in ganglia and nerve cord of various crustacean species (Homarus, 

 Cancer, Callinectes). Crustacean muscle is insensitive to acetylcholine, even 

 at high levels (10 -3 ); it is without effect on intestinal muscle, but acceler- 

 ates the heart. Eserine and curare have no effect on neuromuscular trans- 

 mission in crustaceans. The nervous system of crustaceans is equally 

 insensitive to acetylcholine, except in very high concentrations (10 -3 ), and 



