THE SKIN AND SKELETON 341 



thorax (called prothorax, mesothorax, metathorax), and 

 ten for the abdomen. The antennae, 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 con- 

 fined to that period. These skeletons are epidermal, 

 deposited in successive layers, from the inside, and are, 

 therefore, capable of but slight enlargement when once 

 formed. 



The shells of mollusks are well-known examples of 

 exoskeletons. The mantle, or loose skin, of these ani- 

 mals secretes calcareous earth in successive layers, con- 

 verting the epidermis into a " shell." 128 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 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 " mother-of-pearl," made up 

 of exceedingly thin plates. The pearly luster of the 

 last is due to light falling upon the outcropping edges 

 of wavy laminae. 129 In many cases,' the prismatic and 

 nacreous layers are traversed by minute tubes. Another 

 typical shell structure is seen in the common cone, a 

 section of which shows three layers, besides the epi- 

 dermis, 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, grow- 



