THE SKIN AND SKELETON. 181 
taceans in consisting mainly of chitine and in containing 
no lime. The head, thorax, and abdomen are distinct, 
and usually consist of thirteen visible segments—one for 
the head, three for the thorax (called prothorax, mesotho- 
vax, and metathorax), and nine for,the abdomen, The 
antenne, or feelers, legs, and wings, as well as hairs, 
spines, and scales, are appendages of the skeleton. As 
Insects grow only during the larval, or caterpillar, state, 
molting is confined to that period. 
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.” So various and characteristic 
is the microscopic structure of shells, that a fragment is 
sometimes sufficient to determine the group to which it 
belongs. A large class of shells is represented by that of 
the Oyster, which is composed of three parts: the external 
brown epidermis, of horny texture; then the prismatic 
portion, consisting of minute columns set perpendicularly 
to the surface; and the internal nacreous layer, or “ moth- 
er-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 laminze. In many cases, the 
prismatic and nacreous layers are traversed by minute 
tubes. Another typical shell-strncture 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 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- 
