SPIDEKS : THEIR STRUCTURE AND HABITS. 65 



of the integument, or skin, of the abdomen ; the vascular surface 

 thus formed being increased by the development of 50 or 60 thin 

 triangular white leaflets, like the leaves of a book, all opening 

 into a common cavity, and communicating with the external air 

 by a pair of stigmata visible on the under-surface of the abdomen, 

 near its base. In the envelope of these gills is a tough ligament 

 which is attached to the pericardium ; consequently, the dilata- 

 tion and contraction of the heart alternately close and open the 

 gills, and by this simple arrangement respiration is effected. 



The stomach is situated in the cephalothorax, receiving food 

 from the mouth through an oesophagus, and discharging its 

 contents through a tube, or alimentary canal, running down the 

 abdomen into the rectum at its extremity. 



Passing now to the outward structure of the spiders, we 

 commence with the cephalothorax, the upper side of which is 

 called the shield, and has attached to it the eyes and the fakes ; 

 the lower side goes by the name of the breastplate, and is con- 

 nected with the mouth, the palpi, and the legs (Plate 7, Fig. 8). 



The eyes are simple, like the stemmata of insects, but in struc- 

 ture they bear some resemblance to the vertebrate type, although 

 apparently fixed and inexpressive. To compensate for their 

 immobihty, they are disposed in several pairs in various parts of 

 the forehead. Their mode of arrangement varies widely in the 

 different genera, and affords one of the best generic characters : 

 in the 8-eyed tribe they are often placed in two transverse rows 

 on the forehead, but such is by no means always the case, as may 

 be seen from the drawings in Plate 7, Figs. 2 — 4 ; in the 6-eyed 

 tribe there is even greater variety of position. Figs. 5 and 6. 



For seizing and disabling its prey, the spider is furnished with 

 a pair of falces, which are very formidable instruments in pro- 

 portion to their size : they are attached to the front edge of the 

 cephalothorax above the jaws, and consist each of two joints. 

 The lower joint, or base, is somewhat conical and fleshy ; the 

 upper, or fang, is horny and pointed, with an opening at the tip, 

 through which a poison is conveyed from a gland in the basal 

 joint. That this poison is an acid is proved by its reddening 

 litmus paper, but it has no taste perceptible by the tongue ; and a 

 series of experiments, carefully conducted by Mr. Blackwall, 

 appear to establish the conclusion that it produces no appreciable 

 pain or inflammation upon the human subject ; neither does it 

 exhibit any high degree of virulence in its effects upon other 

 spiders, or upon insects. This naturahst conjectures that upon 

 insects it may have a tendency to paralyse their organs of volun- 

 tary motion, and induce a determination of their fluids to the 

 injured part. The fang is attached to the base by a hinge-joint, 

 and in most famihes with a vertical or inclined articulation, 



