SEA URCHINS 67 



a foot or more in diameter, is an interesting example of these 

 branching, rayed Stars. 



The third class of the Echinozoa, the Echinoidea, comprises 

 the Sea Urchins, globular or disk-shaped animals with a skeleton 

 forming a compact cuirass of plates of carbonate of lime, except 

 for a space around the mouth and anus (the peristome and peri- 

 proct) ; the anus, generally situated at the aboral pole, in some 

 cases is displaced towards the side, or on to the ventral surface. 

 The animals have gained both their popular and scientific names 

 from the forest of long spines with which their bodies are covered. 

 These spines are jointed on to low bosses or knobs on the closely 

 fitting plates of the test or shell, and are chiefly disposed in five 

 broad zones extending from one pole of the animal to the other. 

 Alternating with these are five narrower zones, on which the 

 spines are not quite so plentiful ; they are pierced with small holes 

 through which the long, slender tube-feet, that are provided 

 with terminal suckers, are extended. The Urchin uses the tube- 

 feet for crawling and climbing over the rocks, the spines also 

 being brought into play and acting as levers in tilting the body, 

 and they are also used by the Urchin when walking over flat 

 ground, the animal progressing by a kind of tilting motion. The 

 five narrow zones bearing the tube-feet are comparable to the 

 ambulacral grooves on the surface of the arms of the starfish. 

 The spines have somewhat cup-shaped bases which are inserted 

 on the projecting bosses, or tubercles as they are called, of the 

 plates forming the test or skeleton, and they are connected to the 

 tubercles by cylindrical sheaths of muscular fibres, the contrac- 

 tion of which causes their movement in any direction. Scattered 

 throughout this forest of spines are numerous pedicellariae, much 

 more highly developed, however, than those of any starfish. 



On the Common Sea Urchin (Echinus esculentus) four vari- 

 ties of pedicellariae are present : (i) Tridactyle pedicellaria 

 which are large and conspicuous, with three pointed jaws armed 

 with two rows of teeth along their edges, and scattered over the 

 whole surface of the body ; (2) Gemmiform pedicellarice, having 

 a somewhat globular, translucent head, due to each jaw having 

 on its outer surface a sac-like gland which secretes a poisonous 

 fluid, the virulence of which " may be gauged from the fact that 

 the bite of a single Gemmiform pedicellaria caused a frog's heart 



