SEA UECHIN8 



167 



two rows of flat angular plates united at their edges. Five of these 

 segments, arranged alternately with the others, are perforated by 

 numerous holes, through which the tube-feet of the urchin are pro- 

 truded, while the remainder are imperforate : and all ten plates bear 

 the little hemispherical processes to which the spines are jointed. 



One of the most interesting features of this urchin is un- 

 doubtedly its complex and wonderful masticating system. There 

 are five teeth, symmetrically 

 arranged, and all pointing to- 

 wards the centre of the mouth. 

 Each is attached to a wedge- 

 shape jaw, made up of several 

 pieces, and the whole apparatus 

 is attached by ligaments to loops 

 in the interior of the shell, and 

 is moved by no less than thirty 

 distinct muscles. The complete 

 system may be readily dissected 

 out, and is well worthy of study 

 and preservation. (The harder portions of the system may often 

 be found in the interior of the empty shell of an urchin after the 

 softer structures of the body have decayed away.) 



An interesting dissection of the globular urchin may also be 

 made by cutting completely round the shell with a pair of sharp- 



FIG. 111. SHELL OF SEA URCHIN 

 WITH TEETH PROTRUDING 



FIG. 112. INTERIOR OF SHELL or 

 SEA URCHIN 



FIG. 113. MASTICATORY 

 APPARATUS OP SEA URCHIN 



pointed scissors midway between the mouth and the apex, and 

 then separating the upper and lower halves, as shown in tig. 114. 

 In this way the whole of the digestive tube, with its numerous 

 curves, may be traced from the mouth to the anus at the opposite 

 pole. The water-vessels that supply the tube-feet in the regions 



