
1890.] Excavations by Sea-Urchins. 17 
narily explained by geologists. It may, of course, have been 
formed from the third by repeated grinding of the rock without 
the sea-urchin contributing any help. 
The statement that the sea-urchins are arranged in stories is 
an interesting one. I am unaware that a similar arrangement has 
ever before been observed, and I have never seen similar examples 
of the arrangement of sea-urchins in a ring, as observed by Prof. 
Marcou, and described by him in the fourth instance (B, Fig. 4). 
The wearing away of the zone of rock, leaving the central 
axis (s) standing, is certainly a remarkable case of erosion, un- 
like any which has been recorded. 
The explanation advanced to explain the central style is cer- 
tainly an ingenious one, and should be investigated by those 
naturalists who visit Biarritz. The hypothesis that the sea-urchins 
bore in the solid rock a ring of excavations, the walls of which 
were broken down by the small “ cailloux ” (c), seems plausible, 
but why the sea-urchins should arrange themselves in a ring (B, 
Fig. 4) is not wholly clear to me. Itis a well known fact that 
sea-urchins have a habit of getting together bits of sticks, algze 
shell, etc., by which they cover themselves. Those that live on 
sandy sea bottom, as our common clypeastroid Echinarachnius, 
crawl under the sand in order to hide themselves. ™ The boring 
sea-urchin possibly collects the stones for this purpose, which, 
observed on the Florida Reef an interesting case of the inclusion of an 
Bchinoderm by the growth of coral about it. This phenomenon is not a rare one as far 
as other animals of various groups are ppeaspa; pa there is reason to believe that the 
inclusion of an Echinoderm by growin At all events I am not 
familiar with a printed record of this fact. oo inclusion takes: place by a ac growth 
of the ccenosarc around the urchin, leaving the animal in a cyst, which is in free commu- 

salist, only |so far as to seek the protection of the hole from the surf. Mr. Maynard 
likewise finds a small fish inhabits these sea-urchin excavations, and he tells me that 
when alarmed the fish hides among the spines or under the body of the sea-urchin. This 
is a phenomenon somewhat like the habit of hiding under stones which has long been 
known in Lepadogaster and the “ butterfish,"” and other fishes, and can hardly be 
i as an instance of commensalism. 
Am. Nat.— J. 
: 
