436 



INVEBTBBBATE MORFHOLOGT. 



may be united with the cephalothorax. In the Scorpions it 

 is divisible into an anterior portion, the prseabdomen, much 

 broader and stouter than the posterior postabdomen, an ar- 

 rangement also indicated in certain other forms. In the 

 adults the abdomen is usually destitute of appendages, though 

 they may be present in the embryos ; the Scorpions, however, 

 possess two highly-modified pairs, and it seems probable that 

 the four or six papillse upon which the ducts of the spinning- 

 glands open in the Spiders represent also modified append- 

 ages. 



A special respiratory system is entirely wanting in a few 

 forms. In the majority there occur on the sides of the body 

 from one to four pairs of pores termed stigmata (Fig. 201, st^'^). 

 In the Scorpions and some other forms each stigma opens 



into a cavity lined with chitin 

 continuous with that which covers 

 the general surface of the body, 

 and into this cavity there project 

 a number of lamellae arranged 

 like the leaves of a book (Fig. 

 199), whence the term lung-books 

 frequently applied to them. Each 

 lamella is hollow, trabeculse ex- 

 tending across the cavity from 

 one wall to the other, and the 

 cavities communicate with the 

 coelomic lacunae, so that blood can 

 readily flow into them and so 



Fis. 199.-TBANSVEKSE SECTION «^^°g® ^^^ g^«^« through the thin 



THROUGH THE LuNo-BooK OP A walls of the lamellsB. In other 



Spider (after moLeod). cases there occurs in connection 



ch = chitinogenous tissue. ^jth the lung-book apparatus, or 



g' = body- wall. , , ,. , i • -, . 



Ip = pulmonary lamella. ^Ise entirely replacing it, a tra- 



si = stigma. cheal system consisting of a num- 



t = last compartment of lung- ber of tubes ramifying through 



book, trachealike in char- ti^^ bo(jy_ j^ ^^^^ ^^^^^ ^^ strong 



tube or trachea arises at each 

 stigma and traverses the body, giving off branches to all parts 

 as it goes ; in others there is in connection with each stigma 



