CoNCHIFKKA. MOLLUSCA. CoSCHIFEKA. 
(Pecten), the foot is either very small or is nearly 
wanting; in Nucula and some others it is deeply cleft, 
and is capable of being expanded into a disc, like that 
of the snails ; and in the burrowing species the foot is 
strong and stout, and adapted for boring vertically into 
the sand or mud. 
The body of the Conchifers is entirely covered by a 
two lobed mantle, which envelopes it as a book is in- 
closed within its boards, and is protected on each side 
by a shelly valve. It generally incloses a pair of gills 
which are placed on the sides of the body, and are each 
composed of two large membranous laminse which, when 
examined by the microscope, appear to be a complete 
network of bloodvessels fringed with vibratile cilia. 
The water in which they live is filtered through these 
gills, and thus whatever particles the cuiTent brings, 
are collected on their surface and conveyed to the 
mouth. The mantle which covers the body of the 
animal is usually more or less united at its margins ; 
sometimes being produced into long siphons or tubes 
(fig. 228, Venus verrucosa), at others merely having two 
holes (fig. 229, Anodon anatinus) for the passage of 
the current of water. The various modifications which 
the end of the mantle assumes, afford some of the best 
characteristics for dividing the species into orders. The 
mantle secretes the shell (which, as we have stated, 
consists of two valves), and lines the interior, to which 
it is attached by a pair of adductor muscles. The 
valves are always placed one on each side of the animal, 
and are united together on their dorsal edges by a liga- 
ment of greater or less strength. Within this ligament, 
in a cavity, there is placed an elastic cartilage ; and the 
359 
two valves are articulated together by a hinge furnished 
with interlocking teeth. The use of the ligament is to 
keep the two valves together, whilst that of the carti- 
lage is to keep them open without any exertion to the 
animal while waiting for its prey, or moving from one 
place to another by means of its foot. The valves are 
shut, or brought together, by means oftwo muscleswhich 
are attached to their internal surface, and are hence 
called adductor muscles. In the greater number of 
species these two muscles are quite distinct, one placed 
near the anterior, the other near the posterior apertures, 
whilst in others there is apparently only one ; the two 
being so approximated, as to make it appear as if there 
were only one. Lamarck and other conchologists 
adopted this disposition of these muscles as characters 
for the systematic ariangement of the bivalve shells; 
but Cuvier and other later authors have preferred to 
base their systems upon different characters altogether. 
The majority of the bivalve mollusca are believed 
to be monoecious or unisexual; but as Dr. Johnston 
observes, the exceptions are yearly increasing under 
the dissections of the comparative anatomist, who has 
proved several of them to be bisexual, “although the 
distinction between the male and female does not appear 
in any external character either of the animal or shell.” 
A few of the genera in this class are viviparous, the 
ova being carried from the ovary to be deposited in 
the interstices of the external plates of the gills, where 
they are retained till the young are hatched and 
arrive nearly to maturity, the ova remaining there in 
some of the genera for several months after leaving 
the ovaria. 
SYSTEMATIC AERANGEMENT OF CONCHIFEEA. 
The principal character adopted in the present arrange- 
ment, that of Dr. Gray, is the structure of the mantle. 
We have stated above that the margins of the mantle 
are either produced into siphons, or that there are only 
two holes in it, for the passage of the water to the gills 
for aerating the blood. Tlie species of this class have 
Fig. 228. 
in consequence been divided into two large groups or 
sub-classes — I. Siphonophora, in which the mantle 
leaves or lobes are connected, and produced into 
two elongated siphons or tubes, through which the 
water is admitted (fig. 228, Venus verrucosa). II. 
Asiphonophora, in which the mantle lobes are mostly 
free, or when united, are pierced with one or two holes, 
Fig. 229. 
instead of being produced into siphons, for the admission 
of water (fig. 229, Anodon anatinus). 
