604 Comparative Animal Physiology 



influence on tonus. Similarly in holothurians cutting of the radial nerves 

 decreases resistance to stretch. ^'^^ At low temperatures resistance to stretch 

 is greater than it is at high temperatures (Anodonta,^*^-^ He/ix^-^*'). If it is 

 stimulated during the gradual loss of tension while stretched, a Helix foot 

 contracts and then relaxes to the level it would have reached had the release 

 of tension continued uninterrupted. 



Increase in "viscosity" as judged by resistance to stretch during contraction 

 and during chemically induced tonus has frequently been described. A strik- 

 ing alteration in "viscosity" is found in the anterior byssus retractor of My- 

 tiliis.-'-^'^- -*- This muscle lengthens linearly when stretched. When stimu- 

 lated with alternating current (AC) the "viscosity" is reduced, whereas 

 after stimulation by direct current (DC) the "viscosity" is raised and may 

 remain high for an hour or two (Fig. 233). Intermittent DC lowers the 

 "viscosity" when the current flows for 3/ small part of a cycle, but the usual 

 DC value is approached as the current flows a larger part of a cycle. AC 

 causes a strong contraction followed by quick relaxation, and brief DC pulses 

 applied every minute or two maintain tension with practically no fatigue 

 (Fig. 233). It is suggested that the maintained tension results from high 

 "viscosity." 



In most bivalves the adductor contains a translucent soft striated portion 

 which contracts and relaxes rapidly and fatigues after brief contraction. In 

 Pecten the valve hinge is elastic, and by repeated brief contractions the 

 scallop swims through the water. A smaller portion of the adductor is opaque, 

 tough, smooth muscle which can remain contracted for hours or days with 

 little or no fatigue. This tonus muscle can maintain high tensions, as shown 

 by Marceau^'^^ in the following table: 



kg./cm.- 

 Anodonta cygnea 5.2 



Ostrea edulis 12.0 



Venus verrucosa 35.4 



Mytilus edulis 11.3 



Pecten maximus 8.5 



It was claimed by Parnas^'*''^ that the adductors could hold great weights for 

 hours without measurable increase in O^ consumption, but Ritchie-*^'^ cal- 

 culated that the methods used could not have detected an increase in res- 

 piration of this muscle. Lactic acid does increase during 27-84 hours of 

 contraction."'* If the visceral nerve of Pecten niaxivms is cut while the ad- 

 ductor is contracted the muscle remains short and cannot readily be 

 stretched;--*'' •^" if the nerve is cut while the muscle is relaxed, stimulation 

 of the slow muscle is diflicult but causes contraction, after which relaxation 

 may be more rapid than when the nerve is intact. If Pecten contracts against 

 a block of wood between the valves, the animal holds this position when the 

 wood is withdrawn; the muscle resists stretching although the valves can be 

 pushed together. Stimulation of some afferent fibers to the visceral ganglion 

 causes contraction; stimulation of others causes relaxation. Without nerve 

 stimulation relaxation requires many hours. A stimulus every 30 seconds 

 keeps Pecten contracted, an economy many times greater than that in frog 



