Pomona College, Claremoiit, California 53 



4. Drawing of the upper part of an aboral radial nerve. The 

 eye-spot reg-ion is up in the figure. X'J. 



5. Camera lucida drawing of a part of an aboral nerve showing 

 position of cell areas. X70. 



6. Drawing of a section of an oral radial nerve. X300. 



7. Drawing of a section of circumoral nerve. X300. 



8. Drawing of a section of aboral nerve. X300. 



9. Nerve cell.s from central regions of a radial nerve. The ar- 

 rangement is as shown in the drawing, cells of various levels shown 

 as one layer. Some of the processes possibly relate nearby cells, 

 but most fibers run into the general fibrous mass. All fibres or 

 fibrils are small. There is some indication of tigroid substance in 

 some of the cells. X450. 



10. Nerve cells from near a lateral branch from the radial band. 

 X450. 



shaded area. Uexkull, 1897, was of the opinion that the sea-urchin 

 possessed a special set of nerve fibers concerned with photic 



responses. 



If a bit of the test with one or more spines be separated from 

 the rest of the animal, the spine or spines may be stimulated to 

 react much as before. In the sea-urchins there are several kinds 

 of motile organs. There are the jaw-like organs or pedicellariae, 

 borne on movable stalks ; there are the tube-feet and the long mov- 

 able spines. All these sets of organs are controlled by nerves, and 

 a nerve network connects all these motile organs. One general 

 network of nerves is within the shell and one without, and these are 

 connected with the five radial nerves and the circumoral nerve ring. 

 Each of these motile organs has a definite number of reaction or 

 responses and in these each group may act independently and each 

 organ may react as an independent individual. Each sea-urchin 

 then seems according to Uexkull to be made up of a colony of 

 almost independent structures yet all these are connected by the 

 nerve network and when one carries out a reaction others may 

 receive a stimulus to carry out its special activity. 



The independence of these systems of spines, pedicellariae and 

 tube-feet, and the definite character of their reflexes has been 

 clearly expressed l)y Von Uexkull He considers the sea-urchins as 

 made up of a "republic of reflexes." Each reflex is of the same 

 value and is independent of the others except for the nerve-net 

 connections between the systems. This group of chiefly independ- 

 ent systems has nothing like a central unity controlling them as a 

 whole and it is only by the synchronous course of different reflexes 

 that a unified action is simulated. The action is not unified but 

 the movements are ordered. Separate reflexes are so constituted 

 and so combined that the simultaneous but independent course of 

 reflexes in response to outer stimulus produces a definite general 

 action similar to the condition in animals with a common center. 



