PHISIOLOGV OF THE INVERTEBRATA. 



119 



Nervous Systi-m. 



Situation. 



External nerve.) 

 plexus. j 



Internal nerve- 

 plexus. 



Nerve-centre. 



External to shell. 



Over internal sur- 

 face of shell and 

 i.s in communica- 

 tion with external 

 plexus. 



Mainly round mouth. 



Function. 



Unites feet, spines, and 

 pedicellarias together, so 

 that they all move over 

 to a seat of irritation in 

 that plexus. 



Brings feet, spines, and 

 pedicellarias 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 ani- 

 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 witli 

 those of Romanes and Ewart, 



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

 nerve-plexus in Echinus acntus; and Dr. 0. Hamannf has found 

 and traced nerves in the various pedicellaria? of the Uchinidea, 

 and he finds that from the main nerves branches are given oft* 

 to sense organs and glandular sacs. All the pedicellaria3 are 

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

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

 pedicellari^e serve to keep off layers of living bodies — v.[/., 

 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 pedicellaria3 aid locomotion. 



In Echinus microtuherculatus the gemmiform gland-bearing 



pedicellaria3 hold fast sea- weeds, &c., when the animal is at 



* Comptes liendux, tome 102, p. 444. 



t Sitzuvfj all tr kilt t Jenaisch, GescI'.fUr JJeiK unci Xaturir.'ss. iS86. 



