226 ANATOMY. 



distinct system, namely, as bones, whereas beneath the grade of the 

 Vertebrata, they by degrees disappear, and only here and there, for 

 example, in the Sepia, the Echinus, and some of the Mollusca, viz. the 

 Terebratula, we observe more or less important precursory formations. 

 In general, in the Invertebrata, the exterior integument supplies the 

 place of the passive organs of motion, and this is especially the case in 

 the Articulata. In the Crustacea and Insecta, by their solidity in 

 the latter, and their quantity of calcareous matter in the former, they 

 imitate the structure of the true bones, and send off processes into the 

 cavities they form, which serve for the insertion of muscles, and in 

 every respect appear as a skeleton removed to the exterior. As such 

 we shall also consider and describe them. But it must nevertheless 

 not be overlooked, that the integument, as a continuation of the 

 intestinal canal, and, as it were, a re-fold of it, belongs properly to 

 the vegetative organs, and will in its structure present us with many 

 accordances with it. 



I. OF THE HORNY SKELETON. 

 159. 



The exterior of insects displays itself to us as a horny case, which is 



sometimes firm and brittle, and sometimes soft and flexible, and in this 



last consistence it takes the appearance of a leathery skin. This case 



acquires its greatest consistency and strength in the beetles, especially 



in their elytra, which wholly consist of it : we find it very soft and 



thoroughly membranoxis in many of the Diptera, in most of the parasitic 



insects, and in almost all larvae, particularly in the orders with an 



imperfect metamorphosis. Also at first, when the developed head 



quits the pupa case, the horny integument is in all equally soft, flexible, 



thicker and more fleshy, and even colourless ; but after a few hours it 



attains firmness, and gradually hardens in the course of a few days to 



a rigid coat of mail, in which the insect is clothed. This change of the 



integument takes place chiefly under the influence of the solar light; 



the colours particularly are brought out by its impulse. For as plants 



which grow in the dark take a pale or light yellow colour, insects also 



retain this their original colour as long as they are withheld from the 



effects of the light of the sun. Thence also is it that the majority of 



larvae which live in the earth, or in dark shady places to which the 



light of day cannot approach, are generally pale or colourless, and it is 



