NATURE STUDY. 
4-8 
Every such boy has seen, too, a long line, or little fur¬ 
row in the sand, under water, sometimes nearly straight, 
sometimes in graceful curves, and has often found, at one 
end, the clam that made it, upright on edge, half buried in 
the sand, but perfectly still and tightly closed. It was 
walking in the sand until the boy came splashing along ; 
then it shut up, and craftity looked like a clam. If it is left 
alone, and everything is quiet for a long time, it will begin 
to move again—very slowly of course, but it can move, 
and the way it moves is one way of walking. 
The two shells of a mussel, or fresh-water clam, are 
alike-that is, the creature is “symmetrical,” at least exter¬ 
nally. A narrow line of tough, fleshy substance, called 
the ligature, holds the shells together along the back, much 
as the leather binding of a book holds together the two 
covers. The clam can open and close his shells as he 
pleases, and when all is quiet around him, he changes 
from his unsocial, clamlike attitude towards the world, and 
begins to relax and expand a little. First, after opening 
his shell, he puts out two fringe-like pieces behind, and be¬ 
gins to take good long, full breaths again. For each fringe 
has a tube, or “orifice,” in it, and through the lower tube 
the clam draws in a current of water which he drives out 
through the upper tube. So these tubes have been named 
the incurrent orifice and the excurrent orifice. They are 
not very hard words to remember, when we understand the 
direction of the movements of water that they stand for. 
Water contains air, and the clam breathes this air, as fish¬ 
es do, and some snails, and other creatures that live in the 
water. The water, flowing in, also brings with it many 
tiny objects suitable for food which are carried along to the 
clam’s stomach, aud thus the clam is able both to breathe 
and to feed by means of a current of water. 
One might suppose that a creature whose food came to 
