624 ECHINODERMATA 



aboral surface, wliicli are of importance in respiration and excretion. A 

 red eye spot is present at the tip of each arm, but with this exception 

 special sense organs do not occur. 



Internal Structure.— The calcareous plates which support the body- 

 are of various sizes and joined with one another by muscles and con- 

 nective tissue fibres. The most regularly arranged of them are the 

 ambulacral plates, two rows of narrow calcareous rods which form the 

 roof of the ambulacral groove; between these plates are the ambulacral 

 pores. 



The mouth (1) is a small opening in the middle of the peristome. 

 From it a short oesophagus leads to the large stomach (6), which, with 

 its folded walls, fills the disc. Joining the aboral portion of the stomach 

 are the five pairs of extensively branched liver sacs which lie in the 

 arms, A short, slender rectum passes from the stomach to the anus (2) 

 and may send off a small branched caecum; anus, rectum, and caecum are 

 apparently all rudimentary structures, and the anus and caecum may 

 be absent. The ring canal (4) and the radial canals of the ambulacral 

 system are extra-skeletal in position, the former lying at the margin of 

 the peristome and the latter in the middle line of the ambulacral grooves. 

 Paired branches go from the radial canals to the ambulacral append- 

 ages, which, except in three or four families, possess terminal sucker 

 discs and are used for locomotion. One or two rows of these are present 

 on each side of the radial canal; from each foot a small sac called an 

 ampulla (8) extends through the ambulacral pore into the body cavity, 

 the function of which is to aid in extending the foot. The walls of the 

 ampulla, as those of the foot, are muscular and by their contraction the 

 ambulacral fluid is forced into the foot. The terminal end of each radial 

 canal projects freely into the water at the tip of the arm. Two kinds of 

 glandular sacs may extend from the ring canal through pores into the 

 body cavity, which furnish lymph cells to the ambulacral fluid; those of 

 most general occurrence are the Tiedemann's vesicles; the others and 

 larger ones are the Polian vesicles. The ring canal is joined with the 

 madreporite by the stone canal (5). 



Special respiratory and excretory organs are not present, the ciliated 

 external surface of the body with the papulae and ambulacral appendages 

 performing the functions of such organs. 



The most important circulatory vessels are a ring vessel and five 

 radial vessels which lie just beneath the main ambulacral canals; they 

 are in open connection with the body cavity. The superficial oral nerv- 

 ous system is epithelial and extra-skeletal in position; each radial nerve 

 ends at the tip of the arm with an eye. The other nerves are intra^ 

 skeletal and are not strongly developed. 



