162 THE WORLD OF THE SEA. 
the carapace. Each of the prickles, notwithstanding its extreme 
minuteness, is put in action by a separate muscular apparatus. 
They are porous, and are often grooved longitudinally—being 
formed of thin plates, which radiate from their centres.’ These 
are penetrated with countless holes, and are affixed to each other 
by prominences; so that, looking at the spine, we only see the 
edges of the plates which compose it. A membrane covers the 
whole, which is furnished with vibratory cils. 
The shape and dimensions of these spines are very variable. 
ECHINUS MAMILLATUS. 
In certain of the echinoderms they are three or four times longer 
than the diameter of the shell; while, in others, they are only 
three-fourths or four-fifths of that diameter; while, in others again, 
they are reduced to mere protuberances from the carapace. These | 
appendages are ordinarily awl-shaped and pointed, occasionally 
they are cylindrical and obtuse; and in some species they are 
flattened, and even have their edges truncated. 
In one species which inhabits New Holland, M. Hupé found 
a mollusk Gasteropod, belonging to the genus Szylifera, enclosed 
in one of the spines, which was hollowed and greatly changed, 
both in form and structure, by the presence of this little parasite. 
