INTRODUCTION. Xlll. 



The cuticle of the abdomen is almost always continuous Ji.e.,. 

 without folds or articulated plates), and is either soft or, at most, 

 somewhat coriaceous or leathery in its texture. In some exotic 

 groups, however, it is hard and corneous. The surface is some- 

 times smooth ; at other times (and most frequently) it is clothed, 

 more or less, with hairs, bristles, and downy pubescence. In 

 the genus Gasteracantha and some other exotic spiders it is armed 

 with strong spines. It has generally a distinct pattern upon 

 it, formed either by colours and markings inherent in the 

 cuticle, or else dependent upon the hues of its hairy clothing. 

 In most cases the pattern on the upper side of the abdomen 

 assumes some modification or other of the form of a 

 central, longitudinal stripe on the fore part, followed by a 

 sories of transverse, curved, or angular lines, like circumflex 

 markings, whose apices are directed forwards. The central 

 stripe varies in its length, breadth, and form, but always 

 indicates the position of an important internal organ, which, 

 acting like the heart in the higher animals, receives the vital 

 fluid, and propels it throughout the whole system. Probably 

 the varied colour of this stripe depends on its immediate con- 

 nection with the internal organ above mentioned. 



In most spiders there may be observed some small impressed 

 spots, or depressions, symmetrically placed on the upper surface 

 of the abdomen. These are, apparently, different in number, and 

 oertainly different in size, in different groups of spiders ; probably 

 their number is, in reality, always the same, though not 

 externally visible in all cases. In the family Thomisides they 

 are often very distinct, and five or seven in number ; a single 

 one occupies the middle of the fore-extremity, and is followed by 

 two or three pairs of others ; thoso of each pair being widish apart 

 and forming a transverse line. These impressed spots indicate 

 points where the cuticle adheres by muscles, to the internal 

 organs. Although in most cases the epidermis of the abdomen 

 is continuous, yet in one (exotic) genus, Liphiatius, Schiodte, it is 

 broken up into distinct, transverse, corneous plates united at 

 their edges by a membranous fold. la some of even our own 



