ECHINOIDEA 



645 



lantern consists of five composite y^w;^, each clasping a tooth, and five 

 radial pieces, known as rotulae, which unite the jaws aborally. The 

 teeth can be moved outwards and inwards by muscles running from 

 the jaws to radially placed arches, known as the auriculae, which arise 

 from the inside of the corona near the lantern. Under each auricula, 

 which perhaps represents a pair of ambulacral ossicles, runs a radial 

 nerve, with its epineural canal, and the radial perihaemal canal, 

 *' blood vessel ", and water vessel. Within the lantern is a space, known 



rrrv. 



aumpr- 



Fig. 452. An oral view of a sea urchin, with part of the shell removed to 

 show the course of the alimentary canal. After Cuvier. amp. ampullae at 

 base of tube feet; arc. arch; b.r. "blood" ring; d.b.v. "dorsal blood vessel"; 

 /.jaw; Ian. lantern of Aristotle (displaced); M. mouth surrounded by five 

 teeXh {tth.)\ oe. oesophagus, coiled intestine and rectum; ov. ovaries with 

 oviducts ; per. fold of peritoneum supporting genital rachis ; rm. rectum ; 

 sip, siphon; st. stomach; v.b.v. "ventral blood vessel". 



as the lantern coelom, which is an enlarged perihaemal ring. Muscles, 

 running from the auriculae to slender ossicles, known as compasses, 

 which overlie the rotulae, can raise and depress the roof of the lantern, 

 and thus pump the fluid of its coeldm into and out of the gills. In 

 some urchins, but not in Echinus, pouches of the lantern coelom 

 project upwards into the perivisceral cavity. These are known as 

 Stewart's organs or internal gills and when they are present external 

 gills are often lacking. 



The oesophagus enlarges into a flattened tube, the stomach (Fig. 452), 



