348 Mr. Osler on burrowing 
directed downward, raised, and then returned to the side, to 
be again depressed and elevated. Thus they co-operate with 
the flat bristles in loosening the sand, while they throw up 
that which has been reduced to a semi-fluid state ; and, at 
the same time, act in some degree as levers to depress the 
Spatangus with a force greater than it could have derived 
from its weight alone. As the animal sinks, more of the 
bristles are enabled to act, and its progress becomes more 
rapid ; while the semi-fluid sand which it throws up flows 
slowly back upon it and covers it, when it has sunk below 
the level of the surface. The long dorsal spines prevent the 
sand from closing completely, and preserve a small round 
hole, by which water is admitted to the animal, and which 
marks its situation. 
The burrowing of Bivalves into sand may be observed at 
all times by persons near the coast : all that is necessary is 
to place a few of them on a bed of sand, covered with sea- 
water. The animals, which from their small size or the 
little depth at which they reside, are particularly liable to be 
exposed, will generally burrow readily ; but the large spe- 
cies will scarcely attempt to bury themselves except in the 
very young state. It may be proper to observe, that those 
thrown on shore by storms will very rarely attempt to 
burrow ; and that there is a considerable difference in this 
respect, even in the animals just taken from their natural 
situations ; some burying themselves immediately, while the 
greater number will remain inactive for two or three days. 
When the animal would burrow, it projects and elongates 
the foot, distending it until every part of it, except the point, 
appears semi-transparent. Directing its point downward, it 
