414 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



ventral ganglia is not only an anatomical fact, it has 

 also a physiological significance, for when that of one 

 side is removed, it is only the organs on that side of the 

 body which cease to react to stimuli, the appendages 

 on the other side alone appearing to be affected. 



The ganglia just below the O3sophagus (the sub- 

 cesophageal) appear to have a considerable function 

 as the centres of motor energy, for so long as they are 

 present the appendages move with considerable ac- 

 tivity, but when they are removed the chelae " sprawl 

 helplessly," and the legs are often found doubled up 

 under the body. As might be supposed from the re- 

 lations of their nerve fibres to the muscles of the 

 gnathites, the same ganglia appear to be the centre 

 for the feeding movements ; after their extirpation, 

 the chelae or great forceps do not always carry the 

 food to the mouth, as they do regularly in the uninjured 

 animal ; it is a curious fact that even when they do 

 carry it there they do not give it up to be swal- 

 lowed. 



With regard to the general physics of the nerve 

 fibres, we know from Fredericq that motor excitations 

 produced by electrical currents pass much more slowly 

 along the motor nerve of a lobster than that of a 

 frog, the proportion per second being as twenty-seven 

 metres in the frog to six in the lobster. 



The student of vertebrate physiology will best 

 understand the leading differences between the ac- 

 tivities of the nervous system of the frog and of the 

 crayfish, by a comparative statement : " There is much 

 less solidarity, a much less perfect consensus among 

 the nervous centres in the crayfish than in animals 

 higher in the scale. The brainless frog, for example, 

 is motionless except when stimulated, and even then 

 does nothing to suggest that its members have a life 

 on their own account ; whereas the limbs of a cray- 

 fish, deprived of its first two ganglia, are almost 



