8 The American Naturalist. [January, 
the cavities. In sea-urchins taken from rock cavities the spines 
immediately about the mouth and those on the circumference are 
somewhat worn in specimens from Grand Manan. Moreover, 
the surface of the depression is perfectly smoothed in such a sym- 
metrical manner that it seems necessary to suppose a rotary’? 
motion of the sea-urchin to effect it. This smoothness, already 
pointed out in sea-urchin excavations by Trevelyan, could not, 
I think, have resulted from the teeth alone. 
The major part of the work in the excavation of rock by sea- 
urchins must, however, fall upon the teeth and the dental appa- 
ratus. Upon this point the writings of Cailliaud are conclusive. 
These teeth are probable chisels, which pry into the rock or 
gouge out fragments, and in that way eventually remove con- 
siderable quantities of rock from its bed. A. Agassiz adopts the 
theory that sea-urchin excavations are chiseled out by the teeth, 
but offers no evidence bearing on Cailliaud’s theory. 
John has, it seems to me, furnished important data, indicating 
that the teeth play a part in the wearing out of the rock cavities 
in which the urchins live, for he was able to find in the aliment- 
ary canal of the sea-urchin fragments of stone similar to that in 
which the borings were made. The simple fact, however, that 
grains of rock similar to those of the locality of the sea-urchin 
are found, is not in itself an evidence that they were chiseled out of 
the rock by the teeth. We know that toothless Echinoidea, as for 
instance genera of Spatangoids, often have their alimentary canal 
full of sand, which has apparently been swallowed by the animal. 
There seems no reason to refuse to believe that any of the so- 
called round sea-urchins are not able to swallow small pebbles in 
the same way, but while it is true that the one (spatangoid) gener- 
ally lives on the sand, and the other (echinoid) on rocky bottom, 
one can readily see how small fragments might be picked up by 
the latter and swallowed.” 
19 This rotary motion is yet to be observed, but it may be said that if it exists it must 
be very slow, as the animals never move rapidly. 
20 Sea-urchins are at times scavengers, or at all events a carcass of a seal sunk in the 
water will be found to be covered with these animals. Possibly they feed on the sea 
fleas and other animals which do the work of scavengers, 
