NEUROMUSCULAR MECHANISMS 153 



of sea urchins (1900). He found that the muscles consist of two rings of 

 radial muscle fibers, the outer ring for the relatively quick movements of 

 the spine, for which relaxation is also fast, while the inner muscles can 

 lock the spine so strongly into position that, when forced, it may break 

 rather than give, or the tonic ring may be torn, so that only the phasic 

 muscle fibers remain active. Two types of contractions have recently been 

 reported for the pharyngeal retractor muscles of the sea cucumber, Cucii- 

 marm, on stimulation of the radial nerve (Pople and Ewer,. 1954, 1955). 

 The quick response did not show facilitation, but the slow one showed a 

 marked and prolonged type of facilitation. These findings are in accord 

 with the two types of action potentials previously obtained from the retrac- 

 tor muscles of the pharynx of Thyone (Prosser, Curtis, and Travis, 

 1951). Pople and Ewer argue, however, that the different contractions 

 found are not due to two types of muscle fibers but to neuro-neural facilita- 

 tion in the ganglion-cell complex, described by Smith (1950) and called 

 the "motor complex." It seems certain that this complex must play some 

 part in the reactions of these preparations but whether it can completely 

 explain the different types of contraction may be doubted. 



Prosser (1954) has made an electrophysiological and histological in- 

 vestigation of the long retractors of the body wall of the sea cucumber, 

 Thyone. This short-fibered smooth muscle gives only one type of contrac- 

 tion, which may be considered as of the fast type (Prosser, Curtis, and 

 Travis, 1951). On "direct" stimulation the spread of the muscle action 

 potentials is very restricted. The reason for failure of the propagation is 

 ascribed to the fact that many small nerve fibers pass from the radial nerve 

 to the muscle. These behave as separate units and not as branches of one 

 or a few axons. From these results the conclusions are drawn that the 

 muscle is not syncytial in structure and that many unconnected motor 

 nerve fibers are involved in the innervation of this muscle. The latter con- 

 clusion is in accord with the results of Pople and Ewer (1954) and agrees 

 too with all previous work on echinoderm musculature. 



It would seem at present that the neuromuscular systems of echinoderms 

 may show a greater similarity to those of amphibians than do those of any 

 other phylum, since two different neuromuscular systems controlled by 

 many motor neurons seem to be present. However, this resemblance may 

 still be only superficial. For it will have'fo be shown of what importance 

 the peripheral ganglion cells are in the motor chain before any conclusions 

 can be drawn. 



Investigations in Annelids and Other "Worms" 



With one notable exception no recent research seems to have been per- 

 formed on this group. This is regrettable, since it would be of special im- 



