132 THE ELEMENTARY NERVOUS SYSTEM 



ture, for it disappears on applying a drug that eliminates 



nervous activity (Parker, 1917 b). 



When the nervous structure in the tentacle of an ac- 



tinian is examined with the view of seeking some condition 



upon which this form of polarity can be based, a most 



simple relation is discov- 

 erable. As Groselj (1909) 

 has pointed out, the nerve 

 fibrils that arise from the 

 sense cells in the tentacles 

 of actinians extend as a 

 rule in a direction corre- 

 sponding with the length 

 of the tentacle. In Bu- 

 nodes many of these cells 

 are bipolar and in that 



f,-, -. , -, -. 

 CaS6 OU6 nDrii CXtendS CllS- 



_ , . 



tallV alonff 1116 tentacle 

 J 



and the other proximally, 

 but there are also in this actinian a goodly number 

 of cells that are unipolar and in such instances the 

 fibrils almost invariably extend toward the base of the 

 tentacle. In Cerianthus (Fig. 39) almost all the sense 

 cells in the tentacular ectoderm are unipolar and their 

 fibrils run almost without exception toward the base 

 of the tentacle. As these fibrils transmit impulses away 

 from the receptive cell bodies with which they are asso- 

 ciated, it follows that in both these actinians nervous 

 transmission must be predominately toward the base of 

 the tentacle and that consequently the region of response 

 would be largely proximal to the region of stimulation. 

 This is what is to be seen in the neuromuscular reactions 

 of most actinian tentacles and it is, therefore, believed 



Fio. 39. Sense cells with their attached 

 fibers in a tentacle of the sea-anemone Ceri- 

 antkus. The arrow points toward the base of 

 the tentacle, a direction taken by most of the 

 fibers. (After Groselj, 1909.) 



