224 MANUAL OF ZOOLOGY. 



body by distinct apertures, called " stigmata," and usually branch- 

 ing freely as they penetrate the tissues internally. The walls 

 of the tube are generally 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 sup- 

 plied with blood ; the vascular surface thus formed being in- 

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

 membranous lamellae, or vascular plates, which project into 

 the interior of the cavity. Like the tracheae, the pulmonary 

 sacs communicate with the exterior by minute apertures, or 

 "stigmata" (fig. 74, 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 some of the lower 

 forms the articulate type of nervous system is lost, and there is 

 merely a ganglionic mass which is traversed by the gullet. In 

 none of the Arachnida are compound eyes present, and in none 

 are the eyes supported 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 great majority of the Arachnida 

 are oviparous, and in most cases the larvae are like the adult in 

 all except in size. In some cases, however (Acarina) the 

 larvas have only six legs, and do not attain the proper four 

 pairs of legs until after some moults. 



The Arachnida may be divided into two great sections or 

 sub-classesviz., 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 trachea. 

 Eyes never more than four in number. 



The Trachearia comprise three orders viz., the Podosomata, 

 the Acarina, or Monomerosomata, and the Adelarthrosomata. 



