THE SKIN AND SKELETON. 
133 
The shells of Mollusks are well-known examples of exo¬ 
skeletons. The mantle, or loose skin, of these animals se¬ 
cretes calcareous earth in successive layers, converting the 
epidermis into a “ shell.” 74 So various and characteristic 
is the microscopic character of shells, that a fragment is 
sometimes sufficient to determine the group to which it 
belongs. Many shells resemble that of the Fresh-water 
Mussel ( Unio), which is composed of three parts : the ex¬ 
ternal brown epidermis, of horny texture; then the pris¬ 
matic portion, consisting of minute columns set perpen¬ 
dicularly to the surface; and the internal nacreous layer, 
or “ mother-of-pearl,” made up of exceedingly thin plates. 
The pearly lustre of the last is due to light falling upon 
the outcropping edges of wavy laminae. 75 In many cases, 
the prismatic and nacreous layers are traversed by minute 
tubes. Another typical shell-structure is seen in the com¬ 
mon Cone, a section of which shows three layers, besides 
the epidermis, consisting of minute plates set at different 
angles. The Nautilus shell is composed of two distinct 
layers: the outer one having the fracture of broken china ; 
the inner one, nacreous. 
Most living shells are made of one piece, as the Snail; 
these are called “ univalves.” Others, as the Clam, con¬ 
sist of two parts, and are called “ bivalves.” In either 
case, a valve may be regarded as a hollow cone, growing 
in a spiral form. The ribs, ridges, or spines on the out¬ 
side of a shell mark the successive periods of growth, and, 
therefore, correspond to the age of the animal. The 
figures on the following page show the principal parts 
of the ordinary bivalves and univalves. The valves of a 
bivalve are generally equal, and the umbones, or beaks, a 
little in front of the centre. The valves are bound to¬ 
gether by a ligature near the umbones, and often, also, by 
means of a “hinge” formed by the “teeth” of one valve 
interlocking into cavities in the other. The aperture of 
