458 



ARTHROPOD A. 



connect dorsal to the anus and are connected in the trunk by 

 numerous non -segmental commissures. 



The description may be completed by saying that the straight aliment- 

 ary canal (p and d) bears only salivary glands (sp) ; that it is accompanied 

 throughout by a dorsal heart ; that the gonads (the sexes are separate) 

 open just in front of the anus (#o), their ducts being modified nephridia. 

 The animals are viviparous, live in decaying wood, hide by day and hunt 

 their prey at night. The several species have a wide but discontinuous 

 distribution (South America, Cape of Good Hope, New Zealand, etc.), an 

 indication of great antiquity. Recently the forms have been divided into 

 several genera, Peripatus, Peripatopsis, Opisthopatus^ etc. 



Class IV. Insecta. 



The Insecta is a distinct group marked off from all other 

 arthropods by several important characters. 

 The appendages show no signs of a schizo- 

 podal condition. The head is always a 

 distinct region, bearing a single pair of 

 antennae, a pair of" mandibles, and two pairs 

 of maxillae, the posterior pair often being 

 fused into a lower lip or labium. 



The respiratory organs are trachea (figs. 

 479, 480), which resemble the trachea of 



FIG. 479. 



FIG. 480. 



FIG. 479 Tracheal system of Machilis. (From Lang, after Oudemans.) fc, head; 



J-JJI, thoracic somites; s, spiracles; 1-10, abdominal somites. 

 FIG. 480. Portion of trachea of caterpillar. (From Gegenbaur.) A, mam trunk; 



B, C, D, branches; a, epithelium with nuclei, b; d, air in tracheal tube. 



