XVI. INTRODUCTION. 



The external aperture (in males) is very minute, but in the 

 female sex it is much larger, of very varied form, and frequently 

 has a process of diverse shape and structure, in different species, 

 connected with it ; this process is also often of considerable 

 length, and is probably of use in the deposition of the ova. 

 The form and structure of the genital aperture, and its processes 

 afford some of the most distinctive characters for the specific 

 determination of female spiders ; it is only visible as a distinct 

 orifice when the spider has attained maturity, of which it is the 

 unerring sign (see pi. i., fig. 2,/, and fig. 11, a; also plate ii., 

 fig. 2, JO- 

 INTERNAL STRUCTURE. 



In a monograph like the present, any lengthened details of 

 the internal structure, or anatomy of spiders, would be out of 

 place. Certain points have been already neoessarily touched 

 upon in explaining their external structure ; it will, therefore, 

 suffice here to refer to the internal organs in very general terms. 

 Organs of Digestion. 



These consist of a large sac or stomach, furnished with short 

 lateral branches, corresponding with the number of legs on each 

 side, and placed within the cavity of the cephalo-thorax. The 

 juices of insects, which form the spider's food, pass into this 

 stomach from the mouth parts, or throat, through an oesophagal 

 tube, and thence into the abdominal intestine, by means of a 

 canal which runs through the pedicle connecting the cephalo- 

 thorax and abdomen. At its hinder extremity this intestine 

 empties itself into a large, rounded pouch or rectum, and thence 

 all undigested matter is excreted through the anal orifice. There 

 are also appropriate glands in the cephalo-thorax for the 

 secretion of gastric juice, and others in tho abdomen for the 

 secretion of bile, appealing to subserve purposes similar to those 

 effected by the analogous products of the stomach and liver in 

 the higher animals. 



Organs ot Circulation. 



Circulation is effected by means of a large, elongate, four- 

 chambered vessel, running close beneath the cuticle, along the 



