68 SEA URCHINS 



to stop beating." 1 (3) Very small Trifoliate pedicellaria, with 

 leaf-shaped jaws, the broad ends projecting outwards. (4) Ophi- 

 cephalous pedicellaricz, the jaws of which have broad, rounded 

 distal ends, fringed with teeth, and bearing, as the name denotes, 

 a resemblance to a snake's head. These highly specialised forms 

 of pedicellarise all have their special functions, either as cleaning 

 organs for the removal of particles of dirt or small organisms which 

 may fall upon the body, for the capture and paralysing of prey, 

 and as weapons of defence against larger foes. 



The five narrow, pierced zones through which the tube-feet 

 are extended are known as the ambulacral zones ; while the 

 five broader zones which alternate with them and bear the larger 

 or primary spines, are the interambulacral zones. All these 

 zones converge towards the summit of the test, where in the 

 rounded Urchins the anal opening is situated, and occupies a 

 more or less excentric position within a space called the peri- 

 proct. 2 This space is separated from the apices of the ambu- 

 lacral and interambulacral zones by an outer and an inner ring of 

 large plates alternately arranged. Those forming the inner ring, 

 which terminate the interambulacral zones, are pierced by the 

 ducts of the genital glands, and one is also pierced by the water- 

 pores, thus representing the madreporite, which is the upper 

 extremity of the water-tube of the starfish ; while the plates of 

 the outer ring are pierced by unpaired tentacles which terminate 

 the water-vascular trunks, and are comparable to the ocular 

 tentacles terminating the arms of the starfish. 



In the rounded Urchins, and also the flat Cake Urchins, the 

 mouth is provided with a very complicated masticating appara- 

 tus, consisting of twenty principal pieces arranged into a five- 

 sided conical mass, called after the old philosopher who first 

 accurately described it, and from its general resemblance to an 

 Oriental lantern, " Aristotle's Lantern." In the centre of the 

 whole dental pyramid are the five sharp teeth, the points of which 

 fill and slightly project beyond the entrance to the mouth. They 

 work in bony sockets that are connected by muscles with one 

 another, and with the interior of the test. The gullet passes up- 

 wards through the " Aristotle's Lantern," and is continued into 

 an elongated digestive tube, which exhibits no differentiation into 



1 Prof. MacBride, M.A., F.R.S. Greek, peri, round about. 



