THE SEA-URCHIN 103 
which is capped by a calcareous plate. A system of water- 
vessels with which it is provided enables the animal to force 
water into these tubes (tube-feet), and utilize them for 
purposes of locomotion. The five longitudinal areas bearing 
perforations are raised above 
the fine intermediate areas. It 
the shell be broken it will be 
found to consist of a number of 
calcareous plates usually five- 
sided immovably connected 
together. 
eb 
8 
Fig. 68.—Test of Echinus viewed from above. 
a, perforate areas; b, imperforate areas ; c, plates of which test is composed. 
