168 Papers from the Department of Marine Biology. 



area of y4^ its original extent. A piece of subumbrella tissue 0.0001 

 the original area of the disk is still capable of responding to each 

 induction shock when applied at the rate of 126 per minute. Appar- 

 ently, therefore, the necessity for a latent period on the part of the 

 muscles is not an important factor in the decline in rate of pulsation. 

 It appears more probable (as indicated by the manner in which each 

 of the 16 sense-organs, usually present on a medusa, take up in turn the 

 initiation of the stimulus for pulsation) that some chemical interchange 

 between the sense-organs and the surrounding tissue is necessary in 

 order that the activity of these structures shall be maintained at the 

 highest state of efficiency. As the tissue area is reduced this readjust- 

 ment takes place at a correspondingly lower rate, because the material 

 essential for this adjustment is less readily available to the sense-organs. 

 (15) The evidence from the foregoing experiments establishes the 

 fact that some sort of trophic influence is exerted in general metabolic 

 activities by the sense-organs. The structure of the nervous system of 

 Cassiopea makes it impossible, however, to prove the existence of 

 trophic nerve-fibers as distinct from those of sensory or motor function. 

 On the other hand, it is shown that in this lowly organized form there 

 are transmitted from the nerve-centers certain influences that are abso- 

 lutely essential for maintaining general metabolic activities at their 

 normal rate. 



