6 The American Naturalist. [January, 
it may be that the object was food. New observations on this 
point are necessary, not only as far as the question of excavation 
in woody fibre is concerned, but also as to the contents of the 
stomach of those urchins which are found performin g this function. 
It does not seem to me that the coralline plays any considera- 
ble part in the excavation of the rock directly or indirectly, and 
the sections published by John appear to demonstrate that there 
is no chemical or other action due to the coralline upon the rock 
formation upon which it lies. While we would naturally suspect 
that the geodes” once tenanted by urchins might be covered with 
coralline when deserted by them, in a majority of cases the cavi- 
ties which are`still tenanted have no algous deposit. In those 
cases where the sea-urchin and the coralline still exist together 
and in which the surface of the latter is worn by the animal, we 
may suppose that in the interval during which the sea-urchin had 
vacated the geode the coralline had taken possession of the cavi- 
ty, and in due time another sea-urchin had returned to the 
recess and begun work again upon the layer of calcareous algous 
formation covering the stone. 
My attention was first called to the excavations of Strongylocen- 
trotus by Mr. Webster Cheney, of Nantucket Island, who had 
discovered them while gunning. Mr. Cheney was not aware, at 
the time of his discovery, that the fact was known that sea-urchins 
have their habit of boring in the rocks. I visited the place with 
a fellow student, Mr. Owens, of Bridgeton, New Jersey. 
The following observation was made on the. migration of the 
animal from its excavation, which seems to look as if more than 
one sea-urchin is concerned in the excavation of a single hole, or 
that some of the sea-urchins are nestlers as well as borers. It 
was found, after the sea-urchins had been removed from the ex- 
cavations which they had made, that in the course of time the 
holes were repeopled and inhabited by different individuals from 
those first found there.’* New individuals thus find the old exca- 

™ The term geode, meaning a cavity, has been used for these ations, It seems ap- 
plicable, although used in a somewhat diff se by geologists. 

18I have, however, no observations to prove that the sea-urchins ever voluntarily 
leave their cavities for food and after such an i again to them. 

