> 
MOLLUSCOIDA. 
and “left” as in the true Bivalves. The two valves of the shell 
are also always slightly, and sometimes greatly, 
different to one another in size, so that the 
shell is said to be “inequivalve.” The ven- 
tral valve is usually the largest, and often 
possesses a prominent curved beak, which is 
generally perforated by an aperture through 
which there passes a muscular stalk by means 
of which the shell is attached to some solid 
object. In some cases, however, asin Lingula 
(fig. 98), the stalk of attachment simply passes 
between the valves, and is not transmitted 
through a distinct aperture. In other cases 
the shell is simply attached by the substance 
of the ventral valve. 
The inner surface of the valves of the shell 
is lined by expansions of the integument, 
which are called the “mantle-lobes,” and 
which secrete the shell. The digestive organs 
and muscles occupy a small space near the 
apex or “beak” of the shell, which is parti- 
tioned off by a membranous partition, per- 
forated by the aperture of the mouth. The 
remainder of the cavity of the shell is almost 
filled by two long processes, derived from the 
sides of the mouth, fringed with lateral 
branches, and termed the “arms” (fig. 99, 1). 
These arms are usually closely coiled up, and 
191 
Fig. 98. — Lingula 
anatina, showing 
the muscular stalk 
by which the shell 
is attached. 
serve to obtain food for the animal. It is from these organs 
N 
Fig, 99.—Terebratula vitrea, 1. Showing the ciliated “arms.” 2, Showing 
the shell with its loop for the support of the arms. (After Woodward.) 
that the name of the class is derived (Gr. dvachion, arm ; and 
? 
