INTRODUCTION. xvii. 



middle at the fore part of the upper side of the abdomen, and 

 called the "Dorsal vessel." In this vessel the vital fluid is collected, 

 and thence circulated by a regular system of arteries and veins. 

 The position of the dorsal vessel is almost always apparent on 

 the outside (as before noticed) by an elongated stripe, or band, 

 of varied form and colour. 



Organs of Respiration. 



These have already been referred to. The more localized nature 

 of the main breathing apparatus in spiders than among the Inseota 

 plainly marks a progress to a higher type of organization; 

 though the peculiarly pulmonary nature attributed to them, in 

 most of the general works on Arachnida, appears calculated to 

 mislead. 



The respiration of spiders is, in fact, effected not by pulmonary 

 organs, but by essentially tracheal ones, which may be described 

 as consisting of both Trachea proper, and Sac-trachea. The 

 latter form the principal breathing organs, their position being 

 shown outwardly by the spiracular plates detailed above ; they 

 consist of a collection of flattened membranous sacs, enclosed 

 within an outer membrane, and lying together somewhat like 

 the leaves of a book. The air is collected in these Sac-trachea, 

 and re-oxygenates the main body of the vital fluid in its 

 passage through the dorsal vessel ; while by the Trachea (pro- 

 perly so called) air is conveyed in small quantities to the vital 

 fluid in various parts of the system. To any one familiar with 

 the structure and functions of the lungs of mammals, the 

 essential difference between them and the Sac-tracheae of spiders 

 will be obvious. 



Nervous System. 

 Spiders also possess a very perfectly organized nervous system. 

 "Within the fore-part of the cephalo-thorax is one main bilobed 

 ganglion, or mass of nervous matter, emitting nervous threads 

 to each of the eyes, falces, and labium ; not far behind this, and 

 continuous with it, is a still larger mass, from which nerve- 

 threads issue to the legs and palpi ; a fascia of the same nature 



