§ 3.12 SOMATIC MUSCLES 69 



3.12 SOMATIC MUSCLES 



Cephalopoda. Ablation of the epistellar body of the octopod, 

 Eledone moschata (§ 2.111, Fig. 2-4), is followed by general loss of 

 muscle tone, with even the tentacles hanging limply ; but recovery 

 starts about a w^eek after the operation (Young, 1936). In one case 

 the tone gradually returned to normal during the 186 days for 

 which the animal survived, although there was no trace of regene- 

 rated epistellar tissue. No attempt seems to have been made to 

 restore the tone by injecting extracts during the first post-operative 

 week; but control experiments made it clear that the effect cannot 

 have been due to shock, which is an important factor in these very 

 sensitive animals with their highly developed nervous systems. 

 The loss of muscle tone extended to the chromatophores, so that 

 animals without the epistellar body became abnormally pale (cf. 

 §3.21). 



Crustacea. Well-controlled experiments (Roberts, 1944) show 

 that exposure of the crayfish, Cambarus virilis, to relatively bright 

 light releases an unidentified eyestalk hormone that reduces loco- 

 motion. It is possible that this hormone may act either by raising 

 the stimulation threshold of the skeletal muscles of the legs or by 

 lowering the strength of the nerve impulses from the brain ; but it 

 seems more probable that the result is due to an indirect metabolic 

 efltect (§5.1). The reaction may have adaptive value because the 

 animals normally feed by night and escape from predators by 

 remaining hidden by day. 



Insecta. The normal rhythm of nocturnal activity of the cock- 

 roach, Periplaneta, is stimulated by a hormone, from the suboeso- 

 phageal ganglia, the secretion of which is controlled through the 

 ocelli. If the illumination is kept constant, or the ocelli are painted 

 over, the rhythm disappears. The action of the hormone has been 

 shown by implants and in parabiotic experiments, in which two 

 cockroaches are so joined that blood can flow from one to the 

 other. If, for instance, a specimen in which the ocelli have been 

 occluded has another specimen, with no legs, joined to its back, 

 then normal diurnal changes in illumination acting upon the 

 upper specimen cause a correlated rhythm of locomotor activity 

 in the lower (Harker, 1956). 



