TYPE TB ACHE AT A. 469 



CHAPTEE XV. 



TYPE TKAOHEATA. 



The Tracheata are, like the Arachnida, essentially terres- 

 trial forms, for, though a few Insects have adapted themselves 

 to an aquatic mode of life, they are nevertheless air-breathers, 

 living either at the surface of the water or coming to the sur- 

 face from time to time to renew the air contained in the 

 tracheae which ramify through the body and serve as respira- 

 tory organs. However, a few Insect-larvse have acquired the 

 power of extracting oxygen from the water by branchia-like 

 processes of the body, but, even in these cases, tracheae form 

 the organs by which the respiration is carried-on, the branchiae 

 being richly supplied with them. 



The body is distinctly segmented (except in Peripatus,) 

 and is covered by a chitinous cuticle secreted by the ecto- 

 dermal cells, which constitute the so-called hypodermis. The 

 appendages are usually uniramous, and with few exceptions 

 (Peripatus) are jointed. The anterior pair in all cases are 

 more or less elongated multiarticulate structures provided 

 with sense-hairs, and are situated prseorally, while of the 

 remaining pairs, varying in number in different groups, the 

 most anterior pair is specialized to serve as mandibles, while 

 the succeeding one or two pairs usually form maxillae. Numer- 

 ous glands of varying function are developed in the hypo- 

 dermis, the most interesting of which are the crural glands, 

 well developed in Peripatus, and represented more or less 

 perfectly in certain other forms. In addition to these, glands 

 which secrete an acrid or offensive fluid (repugnatorial glands) 

 are frequently present, as well as others which secrete waxy 

 substances, or even in some cases silk. 



The coelom except in Peripatus is lacunar throughout, pos- 

 sessing no definite walls, and is traversed in various directions 

 by muscles, serving to flex or extend the body and to move 



