328 NATURAL HISTORY. 
563. There is a third order of the Gasteropods, in 
which the gills are not in a covered cavity or chamber, 
as they are in the second order, but they either stand out 
on the back, or are more or less concealed at the sides in 
’ folds of the mantle. Some of them have shells, but most 
have not. I will give but a single example, the Glaucus, 
Fig. 260, found in the Mediterranean and Indian Seas. 
: The hues of these beautiful 
— animals are azure blue and sil- 
ver. The gills form two or 
three large tufts on each side, 
which, besides being the 
breathing apparatus of the an- 
imal, are also its instruments 
for swimming. 
564. We now come to the 
second grand division of the 
Mollusks—the Acephalous or Headless Mollusks. These 
may be divided into two groups: 1. Those which have 
shells, called the Conchiferous, or shell-bearing. 2.Those 
which are covered with a leathery or membranous tunic, 
called the Tunicated. 
565. The shells of almost all the Conchifera are bivalve. 
This group includes the Oysters, Clams, Mussels, Seal- 
lops, etc. -The shell is exuded or secreted from the man- 
tle, and is in different layers, as may be seen in the shell 
of the Oyster. The outermost layer is the smallest, and 
as the animal grows, each layer is a little larger than the 
one outside of it. The two parts or valves of the shell 
are joined together by a hinge. Near this hinge is an 
elastic ligament, which allows the valves to be a little 
apart, which is their natural position, admitting the wa- 
ter freely to the mouth of the gills. When the animal 
wishes to shut the valves closely, it does so by means of 
a muscle. Sometimes there are two muscles for this 
purpose. 
566. That you may understand the plan of the organs 
Fig. 260.—Glaucus Atlanticus, 
