

1890.] Excavations by Sea-Urchins. 21 
keinen Einfluss auf die chemische Beschaffenheit der Oberflache 
desselben, kOGmmen daher auch nicht mit dem Entstehen der 
Echinushohlen in Zusammenhang gebracht werden.” 
The above conclusions are substantially those to which the 
author has been led by a study of the sea-urchin excavations at 
Grand Manan, except that he would also adduce the motion of 
the sea as an additional factor in the excavation of these cavities. 
By this wave action the sea-urchin covered with spines is moved 
about, rubbing against the wall of the cavity in which it rests, A 
continued action of the spines and other hard parts of the animal 
on the rock thus deepens the depression at the same time that it 
files the surface smooth. 
SUMMARY. 
The results of the preceding pages may be briefly stated as 
follows : 
1. Strongylocentrotus drobachiensis on the coast of Grand Manan 
sometimes makes excavations in solid rock. 
2. The excavations are made by the sea-urchin by means of 
- its teeth, spines, combined with motions of the animal produced 
by waves and tide. The object is primarily for protection, but 
secondarily a sufficient amount of water is in that way retained 
by the animal during half tide, or when otherwise uncovered. 
3. The coralline accompanying the sea-urchin, generally found 
at the rim’of the excavation, takes no part in the formation of 
the recess, although its presence may be necessary to the animal 
for some unknown reason. | 
3. Sea-urchins are found at Biarritz, France, which live in 
stories on the walls of peculiar pot-holes, some of which have a 
central style also tenanted by the same animals in small depres- 
sions. 
5. The sea-urchins play some part in the formation of stylated 
pot-holes, although they are assisted by the movements of stones, 
produced by the action of tides and waves. 
Boston, January rst, 1890. 

