206 
BULLETIN OF THE BUKEAU OF FISHERIES 
The mantle covers the animal as the flyleaves cover the body of a book, and by its 
activity secretes the valves of the shells, wliich thus come to occupy the position 
of the covers of the book. If we think of a book with limp leather covers, which 
bend over to meet each other around the edges and imagine the flyleaves doing 
the same thing, the jjicture is a very complete one. 
Shells found on the beach or in the cannery shell piles lie flat, as an open 
book on a table. They are held in this position by the black hinge at the back. 
In life the A^alves are closed, to protect the animal, by the contraction of two large 
muscles, one toward each end of the shell. The muscles are called the anterior 
and postenor addiictors^ the latter being that nearer the siphon. 
In the simplest form of mantle the edges are free except on the back, where 
the hinge is located, corresponding to the arrangement in a book, and the sea 
water may enter the cavity inclosed by the mantle at almost any place ; this is the 
condition, for instance, in the oyster. In the razor clam, however, the edges of 
the mantle are fused, not only along the back, where the valves are joined together, 
but for about one-half of the remainder, chiefly at the posterior end. Three open- 
ings are left. The largest of these extends from the back around the anterior end 
and about halfway along the lower margin. Through this large slit the foot can 
be extended, and the hole is guarded by finely branched papillae extending like 
ruffles along the margin (see figs. 4 and 5). When these are brought together, 
the sand is prevented from entering. At the posterior end the mantle is developed 
into a long double tube surrounding the two other openings of the mantle cavity. 
These form the siphon or "neck", which, contrary to the implication of the com- 
mon name, is not at the anterior or head end but at the posterior or hinder end. 
We thus have, when the clam is feeding quietly in the sand, a fairly large cavity, 
in which hangs the main body of the clam, closed except for the double tube reach- 
ing up through the sand to the water above. 
The mantle is lined with cells, most of which are covered with cilia — fine, 
microscopic, hairlike projections, but, unlike hair, always in vigorous motion. 
They lash sharply in one direction and return slowly, thus driving along any 
water that touches the mantle, like a myriad of tiny oars. The cilia on the mantle 
and gills all beat according to a plan which causes a current to flow out of the tube 
nearer the back, which is thus called the exhalent siphon^ and in at the other, or 
inhalent siphon. 
The water that passes through the mantle cavity supplies the clam with 
food and oxygen. The food consists of a great many microscopic plants and 
animals that live in the sea Avater. Tlaey are passed by the cilia across the gills 
(commonly called "livers" by those engaged in the industry) and the palps to 
the mouth, which lies just above and anterior to the foot near the anterior 
adductor. As the Avater passes through the gills it comes in close contact with 
the blood. An interchange takes place, similar to that in our own lungs in which 
oxygen is taken up by the blood and carbon dioxide given off. The waste 
products from the intestine are carried out by the Avater as it leaves the body 
through the exhalent siphon. 
