1890. ] Excavations by Sea-Urchins. II 
affords in a way a harmonious explanation of the extreme rarity 
of these excavations on the coast of an island densely populated 
by sea-urchins. There seems no satisfactory reason why, if the 
process of excavating is simply a habit, we should find it so rarely 
exhibited. It is true that some species of mollusks sometimes 
excavate and sometimes do not, but while it is not improbable 
that there are many localities on our coast where specimens to 
illustrate this process may be found, they are certainly not as 
numerous as we might justly suspect they might be if the power 
of excavating was a voluntary one alone, and not in some way 
connected with forces acting about them. It likewise seems as 
if, knowing as we do that the habit of boring in rocks is found 
in those animals where the process is wholly voluntary, we might 
ascribe the habit in sea-urchins to the animal itself. We are at a 
loss to explain why sea-urchins make excavations only when 
they are in certain places and under certain conditions. 
This peculiarity has already been noted by Deshayes in regard 
to Echinus (Strongylocentrotus) lividus on the two shores of 
France, and needs a further study.” 
Dr. George Dimmock informs me that he has seen these ex- 
cavations at Banyuls-sur-Mer. As these excavations exist 
on the shore of the Mediterranean, the suggestion that the 
excavations are limited to rocks beaten by a heavy surf or where 
there is a large tide would seem to be unsupported. Still, in a 
general way, the excavations made by sea-urchins are greater 
and more numerous on coasts beaten by a strong sea, and where 
there is a considerable tidal variation. 
A theory that the work of excavation is in part assisted by 
acid secretions from organs of the body, feet, or mouth, does 
not seem plausible, or at all events requires more decisive ob- 
sérvations than have yet been brought to its aid. The excava- 
tions are in all kinds of rock, and it seems improbable that a 
secretion could be made by them which would act on lava, lime- 
stone, slate and granite. The same may be said also of the 
23 "piss to ne erates PE them near Marseilles, while Arthur Eloffe, 
Marce rved or recorded sea-urchins in holes near the 
lighthouse of Planier, pres on the Mediterranean. 

