ECHINODERMATA. 
283 
mother ! I wish for one thing only : to be — to he without these 
e xterior and compromising appendages ; to he thick-set, strong, and 
r °'md, for that is the shape in which I should he the least exposed ; 
llx short, to he a centralised being. I have very little instinct tor 
taavel. To roll sometimes from the surface to the bottom of the sea is 
Plough of travel for me. Glued firmly to my rock, I could there 
s °ive the problem, the solution of which your future favourite, man, 
^eks for in vain— that of safety. To strictly exclude enemies and 
a( lmit all friends, especially water, air, and light, would, I know, cost 
some labour and constant effort. Covered with movable spines, 
Fig. 112. Echinus mamillatus (Lamarck), natural size. 
me. 
Now, bristling like a bear, they call me an 
** 6 mies will avoid 
Urc W” 
T«t us now look a little more closely at the general structure of the 
ea -m?chins— in zoological language, Echinidse. 
of 
body of the sea-urchin is globular in form, slightly egg-shaped, 
f a disk slightly swollen. It consists essentially of an exterior shell 
° r s °Hcl carapace, clothed in a slight membrane furnished with vihratile 
This carapace is formed of an assemblage of contiguous polygonal 
eils. 
Plat 
adhering together by their edges. Their arrangement is such 
lat hie test or shell may he divided into vertical zones, each springing 
