NERVOUS SYSTEM. 



The nervous system of Echini has been shown by Tiedemann to consist of 

 a circular ring with five ambulacral branches. The circular ring surrounds 

 the oesophagus. It is placed above the actinostome and the extremity of the 

 pyramids in the toothed Echini ; in the Petalosticha it is placed round the 

 actinostome at the point of contact of the upper lip and of the test, where it 

 forms an irregular pentagon, from the angles of which the five ambulacral 

 branches radiate. This is the general structure in all Echini. The nervous 

 ring is intimately connected with the circular aquiferous ring placed within 

 the nervous pentagon. The ambulacral canals cover the nervous branches, 

 passing over the nervous pentagon at their origin. 



Krohn* was the first to observe accurately the course of the nervous sys- 

 tem. The pentagon is kept in its place by delicate transverse bands. The 

 ambulacral branches are somewhat broader than the pentagonal ring, and 

 send off delicate lateral branches, in its course towards the anus (PI. XXXI. 

 f. is), which extend under the ambulacral vesicles. According to Midler, 

 branches are sent to the pedicellaria* and to the muscular base of the 

 spines. 



* Archiv. f. Anat., 1841. 



