TEGUMENTARY ORGANS— SKIN STRUCTURES. 463 



covered with epidermal horn. In both the carapace and 

 the plastron the epidermal horny plates do not corre- 

 spond with the bony plates beneath, but break joints with 

 them. 



Mammalian Shell. — At the present time shells are 

 found only in the armadillos and pangolins, but in early 

 times — Quaternary — armored mammals were numerous 

 and of great size. 



Endoskeleton and Exoskeleton. — Thus, even in mam- 

 mals and much more in reptiles, we begin to have the 

 distinction between an exterior shell and interior skele- 

 ton, or exoskeleton and endoskeleton. In vertebrates the exo- 

 skeleton is purely protective, but in invertebrates, which 

 have no endoskeleton, it becomes locomotive as well as 

 protective. This brings us naturally to the invertebrates. 



SECTION II. 

 Invertebrates. 

 ARTHROPODS. 



Insects. — Arthropods are all covered with an effi- 

 cient exoskeleton, both protective and locomotive. In 

 insects it is composed of chitin (a partly calcified horny 

 substance). As this is usually rigid and unyielding and 

 is not shed, insects can not grow after they have once 

 put on this coat of mail. Therefore the whole growth 

 must take place in the soft larval condition. Neither 

 butterflies, nor beetles, nor flies, nor bees and ants, etc., 

 grow. They finish their growth in the form of cater- 

 pillar or grub. Many extremely beautiful and curious 

 appendages are found on insects in the form of elab- 

 orately sculptured scales, and the splendid colors of 

 insects are mainly due to these. Such are the scales 

 which give color to butterflies, and the gorgeous iri- 

 descent green-gold hues to some beetles. 

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