20O JAN 13. DEMBOWSKI. 



particular fixed and unchangeable nervous mechanism. As may 

 be deduced from the facts contained in the classical work of 

 Bethe on the nervous system of Carciints, this system is far too 

 simple to be able to contain such a variety of reflex arcs. It 

 does not follow from this that the activities may be controlled 

 by something besides the nervous system. But it certainly 

 follows from this that the morphology is of little help to us. 

 \Ve are compelled to admit a plasticity of the nervous centers; 

 they must possess a certain creative power which enables them 

 to become adapted to entirely new situations. The number of 

 possible nervous connections is limited, but the number of 

 possible reactions is infinite. This discrepancy may be avoided 

 only by admitting that each nervous center may perform an 

 infinity of functions. 



SUMMARY. 



The typical mode of digging a burrow is described and some 

 problems discussed which are closely connected with the bur- 

 rowing instinct. 



The choice of the spot where the digging is started is deter- 

 mined by very many factors. Phototaxis and thigmotaxis are 

 not sufficient explanations. 



The end-chamber of the burrow functions as an air-chamber 

 during the high tide. 



The fiddler crab is a true water-breathing animal, but it can 

 live in the air for several weeks without changing the water in 

 the gill-chambers. 



The length of the burrow partly depends on the degree of 

 moisture of the ground. 



There is no intrinsic periodicity in the life of Uca. 



Several modes of closing the burrow and opening it are de- 

 scribed. 



In the interpretation the plasticity of activities is strongly 

 insisted upon. 

 Woous HOLE, 



August, I02.S. 



LITERATURE, 

 i. Abbott, J. F. 



'12 '13 The Effect of Distilled Water upon the Fiddler Crab. BIOL. Bi 1,1... 

 Vol. 24. 



