4 
t 85 ] 
exhalent Jipcrture. The respiratory organs are gills, of leaf- 
like shape. There are two on either side, hanging down bet- 
ween body and mantle. The foot may be cylindrical in shape, 
or wedge-like. In some cases it is reduced or absent, e.g. in the 
Oyster, when the animal has adopted a fixed mode of life and 
requires no locomotary organs. Frequently there is a groove 
near the hinder end of the foot, lined by a gland. This gland 
discharges a secretion which hardens on contact with the water 
into tough horny threads, called the 'byssus.* By means* of 
these threads the animal anchors itself to rocks and other 
objects. This byssus (or ' beard *) is familiar to everyone in 
the Common Mussel 
The two valves of the shell are usually symmetrical. In 
some cases they are more or less asymmetrical, especially when 
the animal has become sedentarv', like the Oyster. Oysters he 
on their left side, but other asymmetrical shells lie more usually 
on their right side, like Pifina and Spondvhts. The shell consists 
of three layers, an inner or pearly, a middle or prismatic, and 
an outer or horny layer. The oldest part of each valve is 
called the umbo. It is situated close to the upper border of 
the shell, and forms the starting point of a series of 
concentric rings on the surface of the shell which are called 
* lines of growth ' and mark successive stages in the 
growth of the shell. The two valves are along their upper 
margins held together by an elastic and resilient ligament which 
causes the valves to gape if not counteracted by the so-called 
adductor muscles, which run across from valve to valve. Some 
Lamellibranchs have one, others two of these muscles, and 
their impressions are plainly visible on the inside of the shell. 
The line of attachment of the mantle which leaves an impres- 
sion near the lower margin of the shell, ]h also easily discerned. 
More than 5OOO living species of Lamellibranchs are 
known. They are all aquatic. The majority (about 80 ') are 
marine, the rest live in freshwater. 
Various classifications have been proposed for the 
Bivalves, Some divide them into StphamUa and Astphontahi, 
according to whether siphons are present or not, others cii^i- 
tinguish Monomya^ forms with one adductor muscle only, isomya, 
with two equally developed muscles, and Hctenmya, with two 
unequally developed muscles. More modern classtifications are 
based upon the structure of the gills, and four orders are recog- 
nized according to whether the filaments of which the gtU is 
composed are flat and non-reflected {Protobranchia), or reflected 
( FiUbranchia) or transversely united into a kind of meshwork 
