TON US 537 



contraction of the other. In its contraction it follows up, as it were, that of the 

 other muscle and fixes the whole system at the point where it stops. 



Another aspect of the phenomenon is pointed out by Parnas (1910). If we 

 try to pull apart the shells of Pecten by means of weights, we find that, as already 

 mentioned, a very large weight is required to do so. If, however, we hang a 

 considerably less weight on the shells when open, they are unable to close against 

 it. Thus the muscle is able to hold up a weight which it cannot raise. 



If the nerves from the visceral ganglia to the muscles are cut through while 

 the shell is open, stimulation of the muscle ends of the nerves will produce 

 contraction but no maintenance beyond the duration of the stimulation. On the 

 other hand, a remai'kable fact shows itself if these nerves be cut while the catch 

 mechanism is at work, the shells being closed ; there is no relaxation, neither can 

 stimulation of the nerves remove the catch. The catch muscle remains per- 

 manently at the length it had at the moment when the nerves were cut. 



But there are other nerve cords, one on each side, which connect the visceral 

 ganglia with the cerebral mass, and electrical stimulation of these nerves is able to 

 control the catch muscle in both directions. That of the right side causes its 

 inhibition, so that the shell opens. That of the left side causes shortening and 

 catch action, so that the shell is permanently closed. These various observations 

 are due to von Uexkiill, who regards them as a confirmation of his view that 

 " excitation " is not a wave of change passing along a nerve, but something which 

 flows liither and thither like a fluid. He speaks of the tonic state in which the 

 catch muscle remains, when its nerves are cut while in that contracted condition, 

 as the "tonus- or excitation-trap." This view has been criticised on a previous 

 page (page 424). 



A corresponding phenomenon is described by Veress (1908, pp. 195-196) in the 

 caterpillar of Cossus, after it has spun its cocoon. If extracted from the cocoon 

 and pinned out, it exhibits rhythmical contraction of the muscles of the body wall. 

 These can be inhibited by touching the cuticle. But the interesting point is that 

 at whatever stage of contraction the muscle may be in at the time, it is fixed at 

 that stage for a certain period. The " inhibition " affects only the rhythmical 

 movements ; it does not cause relaxation of the muscle, but merely fixation at that 

 degree of contraction which existed at the moment of the stimulus. 



Brief reference may be made to some other cases in which similar phenomena are 

 to be seen. The spines of the sea urchin are surrounded at their bases by a circle of 

 about thirty double muscles around each spine. Each of these muscles consists of 

 an inner cord, white and opaque, and an outer one, clear. If we apply a 

 momentary stimulus to the integument near one of the spines, the muscles on that 

 side contract, but only for a moment, and the spine afterwards returns to its 

 original position. If the stimulation be repeated several times, the spine is pulled 

 over and remains so for a considerable time after the stimulation ceases. In this 

 state it opposes much resistance to being displaced. The single stimulus excites 

 only the outer motor muscles ; the repeated stimulation excites the inner catch 

 muscles in addition (von Uexkull, 1909, p. 91). 



A mechanism similar to that of Pecten has been described by Jordan (1913) in 

 the case of Holothuria. 



The phenomena shown by Sipunculus are very instructive. When the body 

 wall of this tubular animal contracts, it forces out the proboscis, exerting a 

 pressure of about six centimetres of mercury. The proboscis is then cut off, the 

 central nervous system removed, and the remains of the body tube tied on to the 

 end of a glass tube. Sea water is then poured into the tube until its level is 

 about half way up the tube. Then the preparation is immersed in sea water. It 

 is found that to whatever depth the body tube is immersed, the meniscus always 

 remains at the same place ; in other words, the capacity of the sac remains 

 constant, although the actual internal pressure must be considerably greater when 

 it is in air than when the weight of the water inside is counterbalanced by that 

 outside. That is, the muscle fibres can maintain the same length in equilibrium 

 with different pressures. 



This phenomenon is also to be met with in the urinary bladder of the higher 



