200 MANUAL OF ZOOLOGY. 



vessel' of the Insecta. All the Araclmida breathe the air 

 directly, and the respiratory function is performed by the 

 general surface of the body (as in the lowest members of the 

 class), or by ramified air- tubes, termed ' trachea, ' or by dis- 

 tinct pulmonary chambers or sacs ; or, lastly, by a combination 

 of tracheae and pulmonary vesicles. The ' trachea ' consist of 

 ramified tubes, opening upon the surface of the body by dis- 

 tinct apertures, called ' stigmata,' and branching freely as 

 they penetrate the tissues internally. The walls of the tube 

 are prevented from collapsing by means of a chitinous fibre or 

 filament, which is coiled up into a spiral, and is situated 

 beneath their epithelial lining. The pulmonary sacs are 

 simple involutions of the integument, abundantly supplied 

 with blood ; the vascular surface thus formed being increased 

 in area by the development of a number of close-set mem- 

 branous lamellae, or vascular plates, which project into the 

 interior of the cavifcy. Like the tracheae, the pulmonary sacs 

 communicate with the exterior by minute apertures, or ' stig- 

 mata ' (fig. 57, 3). 



The nervous system is of the normal articulate type, but is 

 often much concentrated. In the Spiders there is a cephalic, 

 or ' cerebral,' ganglion, a large thoracic ganglion, and in some 

 instances a small abdominal ganglion. In none of the Aradi- 

 nida are compound eyes present, and in none are the eyes sup- 

 ported upon foot- stalks. The organs of vision, when present, 

 are in the form of from two to eight simple eyes, or 'ocelli.' 



In all the Arachnida, with the exception of the Tardigrada, 

 the sexes are distinct. 



The Arachnida may be divided into two great sections or 

 sub-classes, viz. the Trachearia, in which respiration is effected 

 by the general surface of the body, or by tracheae, and there 

 are never more than four ocelli; and the Pulmonaria, in which 

 respiration is effected by pulmonary sacs, either alone or com- 

 bined with tracheae, and there are six or more eyes. 



CHAPTER XXXVII. 



DIVISIONS OF THE ARACHNIDA. 



DIVISION A. TRACHEARIA. Respiration cutaneous, or by trachece. 

 Eyes never more than four in number. 



The Trachearia comprise three orders, viz. the Podosomata, 

 the Acarina, or Monomerosomata, and the Adelarthrosomata. 



