352 



INVEBTEBRATSB. 



CHAPTER XV. 



DIVISION II.— INVERTEBRATES. 



CLASS v. — INSECTS. 



The Invertebrates haye no such skeleton as the Verte- 

 brates, but, instead thereof, a collection of organs, more or less 

 distinct, and generally composed of a soft, yielding texture j though 

 occasionally protected by an external covering of shell, or other 

 hard substance. The animals of this division, having no skeleton, 

 are, of course, without any vertebral column, and are hence deno- 

 minated Invertebral. The brain and nervous system are less dis- 

 tinct and less important than those of the Vertebrates, and do not 

 resemble the corresponding organs of that division ; while, with a 

 few exceptions, the blood of the Invertebrates is white. 



Insects have no heart, but, in Ijeu of that organ, they arc 

 supplied with a vessel, or reservoir, situated along the back, and 

 extending from head to tail, which is filled with a transparent, 

 viscous fluid, and undergoes irregular contractions. This reservoir 

 contains the blood or nutritious fluid of the animal, which, by 

 absorption, is conveyed gradually to the various organs. The 

 bodies of insects are supplied with air by means of tubes, called 

 tracheae, which convey it to every part. These tubes communicate 

 externally by means of openings called stigmata ; and, therefore, 

 tliroughout its entire circulation the blood undergoes the changes 

 ■w rought upon it by the air. Two knotted cords, running the length 

 of their bodies, take the place of the brain, and perform the same 

 functions. No organs of hearing have yet been discovered, but 

 they are known to possess the senses of sight, smell, taste, and 

 feeling. 



A hard external covering serves to support the motions, and 

 protect the organs, of insects. In some species it is merely a 

 tough, muscular coating, which surrounds the body in the form 



