xxvii NERVOUS SYSTEM, ETC. 313 



the intestine by an elongated network or plexus of vessels. 

 From the circular vessel five radiating trunks (Rad. B. V] 

 pass to the arms. 



Parallel with and above the circular blood-vessel is a 

 similar but larger structure, the ambulacral ring ( C. Amb. V) 

 which also sends off five radiating vessels (Rad. Amb. V] to 

 the arms. These give off a branchlet to each tube-foot 

 (B, T.F.\ the branchlet having a sac or ampulla (Amp) at 

 its base. From the ambulacral ring a tube with calcareous 

 walls, the stone-canal (St. C} passes upwards and ends 

 in the madreporite (Mdpr\ by the apertures in which the 

 fluid filling the whole of the ambtilacral system of vessels is 

 placed in communication with the surrounding water. 



The function of the ambulacral system is mainly locomo- 

 tive. By the contraction of the ampullae fluid is forced into 

 the tube feet, and by the action of the muscles of the tube- 

 feet it is sent back into the ampullae, and in this way the 

 tube-feet are protruded and retracted at the will of the 

 animal. The system, which is peculiar to the Echinodermata, 

 is lined with epithelium, continuous, in the larva, with the 

 ccelomic epithelium. It has been compared to a gigantic 

 and greatly modified nephridium. 



The nervous system is very simple. It consists of a 

 pentagonal ring (A, Nv." R] round the mouth giving off 

 five radial nerves (A and B, Rad. Nv) which pass along the 

 ambulacral grooves, below the blood-vessels, to the ex- 

 tremities of the arms, where each is connected with an eye- 

 spot. Both nerve-ring and radial nerves are mere thicken- 

 ings of the deric epithelium. 



The gonads (A, Ovy) are branched organs, five in num- 

 ber, which lie inter-radially near the bases of the arms, and 

 open by gonaducts (Ovd) on the dorsal surface of the disc. 

 The sexes are lodged in distinct individuals. 



