1890.] Excavations by Sea-Urchins. 7 
vations suited to their purposes, and it is just to conclude that 
they continue the work of excavation which their predecessors 
began. They seem to take advantage of the depression already 
made for their protection. 
It may likewise be supposed that in the case of. death, natural 
or violent, an occupation of the former excavation by a new indi- 
vidual may take place. This is possibly true also in the case of 
boring sea-urchins from the Azores or the coast of France, where 
very much deeper recesses are sometimes hollowed in the rock. 
The fact that the sea-urchin is much larger than the entrance by 
which it entered the cavity, would thus seem to mean, not that 
the same individual had hollowed out the entire chamber, but that 
the sea-urchin which now occupies the cavity had grown larger 
since it passed through the hole excavated by its predecessor. 
An interesting question often asked, and as yet unanswered in 
a satisfactory manner, is, “ How do the sea-urchins make these 
excavations in the solid rock?” This question is a very difficult 
one to answer, and several attempts have been made to do so 
satisfactorily. It is not necessary to mention all the theories sug- 
gested, but one or two of the more reasonable may be considered. 
Robert supposed that the constant motion of the sea-urchin by 
which the spines act as files after long attrition has worn out 
these excavations. 
It has been suggested by Cailliaud, and this it must be con- 
fessed seems a more reasonable explanation, that the cavities are 
gnawed or chiselled out by the action of the teeth. Both of these 
explanations seem possible ; but the latter is more probable than 
the former, as several observations seem to point to this conclu- 
sion. Among the facts which support Cailliaud’s hypothesis are 
the following: the teeth are very hard, they have means of repair 
when their tips are worn, and their surface and cutting points are 
bare, and well adapted for cutting. It may also be added that 
the muscles which move the jaws are strong. It would seem, so 
for as their relative hardness goes, that these organs rather than 
the spines are the tools which have worn out these holes. 
The author believes, however, that Robert was partly right for 
ascribing to the spines a portion of the work done in excavating 
