730 
The American Naturalist. [August, 
possibly that of the tides, combine to file away the rock under their 
soft spines, which are renewed or replaced as time goes on. 
It is instructive in connection with a mechanical explanation of the 
excavating habit of the sea-urchins to consider a geological phenome- 
non which the cavities inhabited by these animals at once suggest. 
The pot holes found between tides at certain points on Grand Manan 
are very beautiful examples of rock cavities worn by stones found 
within them. The general appearance of the sea-urchin cavities is 
much the same except in size and depth. The worn surfaces of the 
cavities are almost identical, and there seems no good reason why we 
should look for different causes in the two cases. All bowlders, even 
on apparently good positions, do not form potholes, as the majority of 
sea-urchins do not wear away a cavity for themselves, but in certain 
circumstances they do, and the result of the erosion is almost identical. 
It therefore seems as if it were far-fetched to bring in an acid secretion 
of the sea-urchin as an agent in forming these depressions in the rocks. 
It also seems as if the movement of the body did not wholly account 
for them, but that they are, in part at least, due to the erosion of the 
rock by the sea beating them against the rock surface, notwithstanding 
they are practically anchored by their feet. 
I hope to be able later to present a more extended account of these 
sea-urchin excavations, accompanied by illustrations in which it will 
be possible to show the successive growth of a typical depression. 
What is here given, while it has a bearing on the deep cavities made 
in rocks by other species of echinoids, does not necessarily apply to 
them, but only to the excavations of .S". drobachiensis found at Grand 
Manan.— J. Walter Fewkes. 
I^oulting of Spiders.— M. Wagner {Amiales des Sciences Natu- 
relles, VI. 4, 5, 6), contributes an extensive review of the moulting 
processes which take place in the Arachnida. The writer does not 
confine himself to the formation of the new integument and the re- 
jection of the old one, but treats also of the formation of the hairs, and 
the moulting of the eyes, respiratory organs, glands, intestines, and 
tendons, as well as the modifications observed in the blood-corpuscles 
during the process, and the biological phenomena which accompany 
the moulting. The interval between the old and new integument is at 
first filled with liquid, but this is absorbed before moulting. The new 
cuticle, unable to expand upon the thorax, forms folds, and the new 
hairs are held in tubes of the old cuticle. The old skin splits at the 
line of junction of the upper and lower parts of the cephalothorax, and 
