152 INVERTEBRATE PHYSIOLOGY 



may run the length of the whole muscle, Twarog ( 1954) found that applied 

 acetylcholine depolarizes the membranes and instigates contraction. When 

 the drug is subsequently washed out, the membranes repolarize but con- 

 traction remains. Relaxation of this tonic part of the contraction can be 

 obtained by the application of 5 -hydroxy tryptamine (this is also the case 

 when contraction is caused by other means). This drug does not cause a 

 noticeable change in polarization and is inefifective in preventing the onset 

 of the contraction caused by acetylcholine. In bioassays, the normal pres- 

 ence of these substances in this muscle was observed. 



From these interesting observations it would seem that 5-hydroxytryp- 

 tamine may be considered rather a relaxing than an inhibiting substance, 

 and that active relaxation is necessary to release the "catch" mechanism, 

 provided that the substance works directly on the muscle fibers and that 

 only one type of muscle fiber is present. 



In a very recent paper (not considered in the original manuscript) 

 based on independently performed experiments, Hoyle and Lowy ( 1956) 

 come to the conclusion that inhibitory nerves are present in the anterior 

 byssus retractor muscle of Mytilus edulis. Furthermore, this muscle 

 would have a built-in system which can fire automatically, this system 

 consisting either in peripheral nerve cells or the spontaneous activity in 

 muscle fibers themselves. They find again that during prolonged tonic 

 contractions action potentials are present, although these occur only in 

 certain areas of the muscle and not everywhere. They conclude that tonic 

 contractions are based on the tetanic activity of such parts. They confirm 

 that 5-hydroxytryptamine abolishes tonic responses, and that this sub- 

 stance does not materially afifect the phasic ones. Because the muscle is 

 unable to destroy added 5-hydroxytryptamine they consider it unlikely 

 that it is a natural transmitter. This reviewer is less inclined than the 

 writers to accept on this evidence the absence of a "catch" mechanism. 

 Twarog's (1954) findings with acetylcholine, in which the fiber stays 

 contracted even after the acetylcholine is washed out, indicates to her 

 that such a mechanism may be present. One would want to correlate the 

 total electrical activity of the muscle and the contraction in order to evalu- 

 ate the significance of the action potentials present during rest. The pres- 

 ence or absence of such correlation would go far to prove or disprove the 

 contentions of this paper. It is interesting to note that, as in crustacean 

 muscle, the independent reactions of the muscle fibers making up the 

 muscle are a great hindrance to this approach. 



Investigations in Echinoderms 



The presence of phasic and tonic muscle fibers in echinoderms was con- 

 vincingly demonstrated by von Uexkiill for the muscles moving the spines 



