120 THE ELEMENTARY NERVOUS SYSTEM 



by von Uexkiill (1909) as to the physiology of nerve- 

 nets, namely that in transmission impulses flow more 

 freely into regions where the net is stretched than into 

 those where it is not. Attention will be called to examples 

 of this kind in subsequent chapters. 



As a result of the intimate relation usually existing 

 between the nerve-net and the muscles that it controls, 

 most organs that are provided with this type of neuro- 

 muscular organization exhibit an extreme degree of au- 

 tonomy. This is perhaps one of the most striking features 

 associated with the nerve-net. It is well illustrated by 

 such an organ as the tentacle of the sea-anemone, the au- 

 tonomy of which was long ago recognized by von 

 Heider (1879). 



In most actinians the tentacles are hollow finger-like 

 projections from the oral disc. Their cavities communi- 

 cate freely with the common digestive cavity of the polyp 

 and their walls consist of an outer layer of ectoderm sep- 

 arated by a supporting lamella from an inner layer of 

 entoderm. In experimenting with tentacles it is desir- 

 able that they should be of large size, and this is the case 

 in Condylactis, whose tentacles may be from 12 to 15 cen- 

 timeters long and 1.5 centimeters in diameter. In this 

 particular sea-anemone the tentacle terminates in a blunt 

 end provided with a pore, and, as in the tentacle of 

 Metridium, there are a longitudinal ectodermic muscle and 

 a circular entodermic one. 



If an expanded quiescent tentacle of Condylactis is 

 touched near its tip witli a silver sound or a glass rod, 

 the tentacle contracts, usually bending toward the stim- 

 ulated side. The tentacle often sticks to the object with 

 which it is touched and it may in contracting thus exert 

 a considerable pull, showing that its surface is remark- 

 ably adhesive and that its musculature is vigorous. If a 



