THE NERVOUS SYSTEM OF CCELENTERA. 103 



nervous impulses. In the higher animals it is not difficult 

 to obtain such physiological proofs of the functions of the 

 nervous tissues but in most Ccelenterates where these 

 tissues are extremely minute and diffused over a wide area 

 the proof is not so easy to obtain. 



A series of observations and experiments made last year 

 by Hesse seem to afford the necessary proof of the nervous 

 character of the ectodermic nerve plexus in certain Scypho- 

 medusse, and consequently mark a very important step in 

 advance. 



At the margin of the umbrella in these animals there 

 may be seen a small number of very minute and extremely 

 modified tentacles called the tentaculocysts. 



These organs have been shown by Romanes and by 

 Eimer to be the centres from which the impulses controlling 

 the movements of the bell proceed, and from this fact, and 

 others which it is not necessary for our purpose to refer to, 

 they are frequently spoken of as the " sense organs " of the 

 medusa. When all eight of the sense organs of an Aurelia, 

 for example, are removed, the rhythmic contractions of the 

 bell cease altogether, if one or any greater number are left 

 intact the contractions continue. If the margin of the bell 

 with one or more tentaculocysts be removed it continues to 

 contract, whereas the remainder of the bell from which the 

 margin has been removed ceases to contract. These and 

 other experiments together prove that the nerve centres 

 originating and controlling these contractions reside in the 

 sense organs. Now on the inner, and in some genera on 

 the outer, surface as well of the base of the tentaculocyst 

 there is situated a pit, and it is a well-known fact that at the 

 base of these pits there is a considerable thickening of the 

 subectodermic layer of cells and fibres which is commonly 

 called the " nerve sheath ". 



Hesse shows that the thickening of the subectodermic 

 nerve plexus in each inner pit gives origin to several very 

 much elongated bipolar ganglion cells, which run in very 

 definite tracts. Passing from the sense organ immediately 

 below the subumbrellar epithelium they form a bundle of 

 fibres running in a radial direction towards the centre of the 



