PHYSIOLOGY OF THE INVERTEBRATA. 319 



Nervous System. 



External nerve, 

 plexus. 



Internal nerve- 

 plexus. 



Nerve-centre. 



Situation. 



External to shell. 



Over internal sur- 

 face of shell and 

 is in communica- 

 tion with external 

 plexus. 



Mainly round mouth. 



Function. 



Unites feet, spines, and 

 pedicellarise together, so 

 that they all move over 

 to a seat of irritation in 

 that plexus. 



Brings feet, spines, and 

 pedicellarise into rela- 

 tion with co-ordinating 

 nerve-centre. 



Presides over co-ordinated 

 action of spines and feet. 

 It gives rise to nerve- 

 trunks. 



pentagonal nerve-ring sends off, in addition to the am- 

 bulacral trunks, the nerve-cords to the intestine. 



The physiological experiments of Fredericq (see p. 436 of 

 his paper, loc. cit.) are almost entirely in accordance with 

 those of Romanes and Ewart. 



Dr. H. Prouho* has investigated the nature of the external 

 nerve-plexus in Echinus acutus; and Ur. 0. Hamannf has found 

 and traced nerves in the various pedicellarise of the Uchinidea, 

 and he finds that from the main nerves branches are given off 

 to sense organs and glandular sacs. All the pedicellari^ are 

 tactile organs, as the nerve-terminations indicate; the tri- 

 foliate ones seem to remove sand, Protozoa, &c. The large 

 pedicellarise serve to keep off layers of living bodies — e.g.^ 

 worms, and therefore act as weapons, as well as for organs 

 of attachment when the animal is moving about. There is 

 no doubt that the latter function is the most important ; in 

 other words, the pedicellariEe aid locomotion. 



In Echinus microtuhemdatus the gemmiform gland-bearing . 



pedicellariEe hold fast sea-weeds, &c., when the animal is at 



* Comptes Bendus, tome 102, p. 444. 



+ Sitzungsherichte Jenaiich. Gesell.fur Med, und Naturwiss. 1886. 



