154 INVERTEBRATE PHYSIOLOGY 



portance to compare the annelid and the arthropod neuromuscular 

 systems. Prosser and Melton (1954) have analyzed the proboscis re- 

 tractor muscle of Phascolosoma (Sipunculoidea) with electrophysiologi- 

 cal and histological methods. They found both fast and slow contrac- 

 tions present and two types of action potentials. The muscle fibers are all 

 smooth and short. Many parallel nerve fibers innervate the muscle and the 

 axons can be divided into two classes, thicker (2/x) and thinner (below 

 IjLi). Both types of action potentials fail to be conducted after nerve de- 

 generation. It is therefore concluded that conduction is always by nerve 

 fibers and not by protoplasmic bridges between muscle fibers. Whether 

 the muscle fibers also are of two types or whether dineuronic motor inner- 

 vation occurs has not yet been decided. Multiterminal endings may well be 

 present. The fast potentials have the properties of spikes and do not facili- 

 tate, the slow ones those of junctional potentials with facilitation. On fa- 

 tigue the spike shows a prespike potential, but a similar phenomenon was 

 observed for the slow potentials. It seems therefore premature to identify 

 these phenomena with the spike and junctional potentials of the Crustacea. 

 There is, however, a similarity in the shape of the action potentials, which 

 are different in both cases from the conducted potentials of vertebrate 

 muscle. 



Thus in Phascolosoma the evidence is that relatively small muscle areas 

 are governed by specific nerve fibers ; but it should be kept in mind that 

 in the sabellid annelids M^JjfzVo/a (Nicol, 1948) smd Branchiomma (Nicol, 

 1951 ) the giant fibers in the central nervous system innervate a very large 

 part of the longitudinal musculature directly. These giant fibers consist of 

 a conglomeration of many cells, and one may, therefore, consider that in 

 principle the muscles would still be innervated by many axons forming a 

 single unit. If this were the only innervation, these muscles, which form 

 the greatest part of the whole musculature, could function only in the with- 

 drawal reflex. For the earthworm, preparations have been described in 

 the older literature which, with modern techniques, might yield significant 

 results concerning this problem (e.g., Garrey and Moore, 1915). 



Investigations in Coelenterates 



In the "lowest" phylum in which neuromuscular transmission is pres- 

 ent, investigation might well be very difficult. It is therefore gratifying that 

 certain facts have gradually come to light. A most welcome contribution 

 has been the observation of Horridge (1953, 1954) that motor-nerve im- 

 pulses can be obtained from single axons, a single impulse accompanying 

 each beat of the subumbrella in the jellyfish Aurellia. These results make it 

 much more likely that the through-conducting system of Pantin (1952) in 

 sea anemones functions indeed as he pictured it, namely by axons con- 



