HABITS OF SEA URCHINS. 199 
Crass III.—EcuinorpEa (Sea-urchins\. 
General Characters of Sea-Urchins.—A good idea of 
the general structure of the members of this class may 
be obtained by an examination of the common sea-ur- 
chin, Zchinus (Fig.141), of the eastern coast of the United 
Fig.141.—The common Sea-urchin, Zchinus (Strongylocentrotus) drdbachiensis. 
d@, frame-work of mouth and teeth seen in front; ¢, the same seen sideways; a, 0, side 
and external view of a single tooth (pyramid); all natural size.—After Morse. 
States, Northern Europe, and the Arctic Seas. It is com- 
mon among rocks, ranging from low-water mark to fifty or 
more fathoms. It eats sea-weeds, and is also a scavenger, 
feeding on dead fish, etc. We have observed great num- 
bers of them assembled in large groups, feeding on fish offal, 
a few fathoms below the sur- 
face, in a harbor on the coast 
of Labrador, where fishing- 
vessels were anchored. 
On placing an Echinus in 
sea-water the movements of 
the animal, especially its 
mode of drawing itself along 
by its numerous long tenta- 
cles or ambulacral feet, and 
how it covers itself by draw- 
ing together bits of sea- 
9 
Fig. 142.—Tooth-apparatus of the Sea- 
weed and gravel, may be urchin, showing the complicated arrange- 
observed ment of the muscles.—From Macallister. 
A habit less easily detected is that of some sea-urchins 
burrowing in limestone rocks and coral reefs until the ani- 
mal sinks quite far down. How the rock becomes thus 
worn away, unless simply by the rotary movements of the 
body, is not clearly understood. 
