18 
THE - CONCHOLOGISTS’ - EXCHANGE, 
THE SHELL-BEARING MOLLUSCA OF 
RHODE ISLAND. 
BY HORACE F. CARPENTER. 
Chapter XX XVIII. 
Class Pelecypoda or Acephala. 
Synonyms: Conchifera, Lamarck; Lamelli- 
branchiata, Blainville; Bivalves, common 
name. 
Acephala means headless, and the animals of 
this class have no head, and are the lowest in 
the scale‘of being. There is no sexual union 
between the animals of this class, fertilization 
being accomplished by the surrounding water, 
containing the male element. They breathe by 
means of gills only, and are therefore inhabi- 
tants of water, mostly marine, though there are 
a few genera which live in fresh water. 
organs of the animal are enclosed in a mem- 
braneous sac, called the mantle, one fold of 
which covers each side, and is in turn pro- 
tected by a shelly valve. In many species: the 
mantle is prolonged into a tube called the si- 
phon. It may seem strange to be told that our 
clam has no head, but that which is popularly 
called the head, is in fact, the siphons of the 
animal united in one tube, which projects two 
inches or more beyond the shell. At the .ex- 
treme end are two orifices, one of which serves 
to convey the currents of water (caused by the 
agitation of the fringed cilia at this point), to 
the gills, where it is filtered, and the particles 
contained in it carried to the stomach; the other 
siphon serving to expel the purified water. It 
is said that if clams are placed in a basin of sea 
water containing indigo, they will, in a short | 
time, render it perfectly clear, by collecting the 
minute particles of the impurity and condensing 
them into a solid form; and not only indigo, 
but whatever particies may be contained in the 
water, organic or inorganic, animal, vegetable 
or mineral, are thus. removed, and the water 
purified. The thousands who visit our shores 
every summer to partake of the luscious clam- 
bake of Rhode Island, may not be aware that 
they are filling up on the sewage of the city, but | 
ithe | 
as no one was ever known to be injured by eat- 
ing any amount of them, concentrated and re- 
fined sewage, obtained in this way, must be 
healthy. The orders and sub-orders of the 
class, Pelecypoda are named from the peculiar- 
ities of the animal, and the families, genera and 
species, from the form of the shell, &c. 
The shells of this class have two valves, thus 
distinguishing them from all those heretofore 
described in these papers. ‘These valves are 
equal sided as well as equivalve, thus distin- 
guishing them from the Brachiopoda, which 
are inequivalve although equal sided. ‘The 
valves in Brachiopoda are termed upper and 
lower, but in Pelecypoda they are called right 
and left, the animal living and moving in an 
| upright position, resting on the thin edges of 
the valves. These edges are called the ventral 
edges, and the opposite ones the dorsal edges. 
The two valves are united at their dorsal edges 
by a ligament, and articulated by a hinge, gen- 
erally furnished with interlocking teeth. The 
valves open spontaneously by the action of this 
elastic ligament, and are closed at the will of 
the animal, by the powerful adductor muscles 
which pass through the animal from side to 
side, and are inserted upon the middle or side 
of each valve, leaving a scar or impression upon 
the shell. As I said before, bivalves live and 
move in an upright position. There are excep- 
tions to every rule, and this statement is not 
correct as applied to all bivalves. Oysters, 
scallops and some others, live on one side, and 
the lower valve in these cases, is deeper and 
more capacious than the upper. 
A specimen of a common Unio, or fresh- 
water clam, will serve to illustrate the meaning 
of the terms used in descriptions of bivalve 
shells. The apex is the point from which the 
growth of the valves commences, and is called 
the beak or umbo; these are near the “hinge, 
that part of the shell growing least rapidly. As 
the animal plows along through the sand or 
mud, with the shell standing erect, and the 
sharp edges of the valves down, and the shorter 
portion of the shell nearest the apex forwards, 
the valve which corresponds to your right side 
is the right valve, and the opposite the left. 
The whole of the upper length of the shell is 
called the dorsal margin, and the opposite length 
the ventral margin or base. The beaks are 
i 
Pe 
