1879.] System of the Crayfish (Astacns Fluviatilis). 



381 



touching the ground with the tail-fin. In this position the animal 

 will remain for a minute or so, one or more cf the chelate legs 

 engaging in feeding movements, while the last pair are doing their 

 best to preen the abdomen. At length there is an attempt at locomo- 

 tion, the limbs being moved slowly and in a tottering fashion, though 

 with fair co-ordination, till after a few steps, having no power to 

 recover its equilibrium, the animal rolls over helplessly on to its back. 

 In some cases the chelae were folded rigidly across each other so as to 

 render locomotion impossible. 



III. When loth commissures are divided behind the suh -oesophageal 

 ganglion, the antennas are moved more frequently and more vigorously 

 than in the last case : the eye-stalks too 'are oftener in motion. The 

 rhythmic swing is not infrequent in the posterior maxillipedes, but 

 very exceptional and of very short duration elsewhere Preening 

 movements are more common than under the last head, aud in these 

 all four pairs oftener take part ; but feeding movements, save after 

 external excitation, are quite exceptional. Then, however, they are 

 vigorous enough, but the chelate legs are very uncertain in their aims 

 at the mouth, do not loose their hold of the food when they get it there, 

 and all of them attempt to crowd food into the mouth together. But 

 the food is frequently rejected : in two cases out of three in which the 

 experiment was tried, this " sulkiness " disappeared on dividing the 

 supra-cesophageal commissures. 



On the table these crayfish are unable to support themselves, the 

 chelae sprawl helplessly on either side and the legs are for the most 

 part doubled up under the body. The posterior maxillipedes alone 

 retain their wonted strength, and by means of these the cephalothorax 

 is raised from the ground two or three times a minute till they are 

 exhausted ; the antennas too being waved vigorously all the time. 



IV. In three cases in which a longitudinal division of the supra- 

 cesophageal ganglion was accomplished fairly satisfactorily, the animal 

 assumed the stilted position above described, but the abdomen, instead 

 of being bent sharply downwards, was alternately elevated to the 

 utmost and then depressed and sometimes curved rigidly backwards 

 for a minute or more : at which times, owing to the rigor of the 

 chelae, it was possible to make the animal stand upon its head. These 

 animals had considerable power of maintaining equilibrium and were 

 active in the water, making, however, very pronounced " circus-move- 

 ments." Their ambulatory legs were always obedient to the impulse 

 to walk, and never betook themselves to feeding or preening move- 

 ments at such times. 



From the foregoing it may perhaps, with more or less probability, 

 be inferred : — 



(a.) That there is no decussation of the longitudinal fibres in the 

 nervous system of the crayfish. 



