CHAPTER VI 



The second division of the Arthropoda. Organisation and Development of the Air- 

 breathing Articulata (Tracheata). 



Systematic Review. 

 CLASS I. Protracheata (Onychophora). 



Body vermiform. A pair of preoral feelers at the point of the head. In the 

 oral cavity a pair of horny jaws, at the side of the mouth oral papilla? (slime papillae), 

 numerous pairs of short almost truncated limbs. Eespiration through tubular 

 tracheae, whose outer apertures are scattered over the whole body. Numerous seg- 

 mentally arranged pairs of nephridia. Coxal glands on the legs. Heart a long 

 dorsal vessel with numerous pairs of ostia. 



CLASS II. Antennata (Myriapoda and Hexapoda). 



One pair of preoral feelers, 3 pairs of oral limbs. Trunk either homonomously 

 segmented with numerous pairs of jointed legs, or heteronomously segmented with 

 the limbs limited to the three segments of the anterior region of the trunk, the 

 thorax ; the posterior region, the hind-body, being limbless. The head everywhere 

 distinctly marked off from the trunk. Kespiration by means of tubular tracheae, whose 

 outer apertures (stigmata) are segmentally arranged. The heart in the homono- 

 mously segmented Myriapoda is a long dorsal vessel, supplied with many segmentally 

 arranged pairs of ostia ; it runs through the trunk longitudinally : in the Hexapoda 

 it is restricted to the abdomen. 



CLASS III. Chelicerota sive Arachnoidea. 



No preoral limbs comparable with the antennae of the Antennata or Protracheata. 

 Several anterior body segments (7, including the frontal lobes) are fused to form an 

 unsegmented region, called the cephalothorax. This carries 6 pairs of extremities, 

 the most anterior of which lies in front of the mouth. The two anterior pairs are 

 developed as oral limbs. The first are called the jaw-feelers (chelicerae), and the 

 second jaw-palps (pedipalps). The 4 remaining pairs of extremities are jointed 

 legs, generally long. Abdomen segmented or unsegmented, or fused with the 

 thorax, Avith no developed limbs. Respiration either exclusively through book-leaf 

 tracheae, or at the same time through book-leaf and tubular tracheae, or exclusively 

 through tubular tracheae. Number of stigmata limited, at the most 4 pairs. 

 The stigmata almost always lie in the abdomen. Heart, seldom wanting, restricted 

 to the abdomen. 



