52 THE CRAYFISH [CH. II 



sense of touch (taste?). It is probable that the fine 

 bristles with which they are fringed are sensory in function. 

 If inanimate food is found by these appendages they at 

 once rake it back to the mandibles. Should the food be 

 some living animal it is seized and held firmly by one or 

 both nippers, and by them and the small nippers torn up 

 and ultimately conveyed to the mouth, where the powerful 

 mandibles cut off pieces which are swallowed and pass up 

 into the 'stomach' for further mastication. When alarmed 

 the Crayfish moves with alacrity: bringing into play 

 its broad tail-fin and telson; bending the abdomen re- 

 peatedly downwards and forwards by a few vigorous 

 contractions of the powerful flexor muscles which occupy 

 the greater part of the abdominal cavity, it darts swiftly 

 backward into some sheltering recess. It is however un- 

 able to travel more than a few yards at a time in this way, 

 the muscles soon becoming exhausted. Experiments 1 have 

 shown that under electrical stimulus the contraction of 

 the abdominal flexor muscles only lasts one-tenth of the 

 time occupied by that of the muscles of the great chelae ; 

 from 80 to 100 stimulations per second are required to 

 produce tetanus of the former, whereas from two to four 

 suffice to bring about this condition in the latter. The 

 tetanus of the flexors only lasts about half-a-minute, but 

 that of the chelae muscles nearly half-an-hour, and during 

 the first five minutes of the period steadily increases in 

 strength. The physiological properties of these muscles 

 are thus exquisitely suited to the nature of the work 

 performed by them. A crayfish will yield its life rather 

 1 Bichet, vide Nature, xx. p. 106, 1879. 



