ANNULOIDA : ECHINODERMATA. 



145 



reporiform tubercle," which is situated near the angle between 

 two rays. The inferior or ventral surface corresponds to the 

 ambulacral areas of an Echinus, and exhibits the mouth and 

 ambulacral grooves. 



Fig. ^o.Cribella oculata. (After Forbes.) 



The mouth is central in position, and is not provided with 

 teeth ; it leads, by a short gullet, into a large stomach, from 

 which a pair of sacculated diverticula are prolonged into each 

 ray. A distinct intestine and anus may, or may not, be pre- 

 sent ; but the anus is sometimes wanting (in the genera, Astro- 

 pecten, Ctenodiscus, and Luidia). 



The ambulacral system is essentially the same as in the 

 Echinoidea, and is connected with the exterior by means of the 

 " madreporiform tubercle," or " nucleus," two, three, or more 

 of these being occasionally present. The ambulacral tube-feet 

 are arranged in two or four rows, along grooves in the under 

 surface of the arms. 



The blood-vascular system consists, as in the Echinus, of 

 two circular vessels, one round the intestine, and one round 

 the gullet, with a dilated tube, or heart, intervening between 

 them. There are no distinct respiratory organs, but the sur- 

 faces of the viscera are abundantly supplied with cilia, and 

 doubtless subserve respiration ; the sea-water being freely ad- 

 mitted into the general body-cavity by means of numerous 

 contractile ciliated tubes, which project from the dorsal surface 

 of the body, and are perforated at their free extremities 

 (Owen). 



The nervous system consists of a gangliated cord, surround- 

 ing the mouth, and sending filaments to each of the rays. At 



VOL. I. K 



