GENERAL CLASSIFICATION AND STRUCTURE. 27 



-spn 



the translucent epiderm, rather than by pubescence. These colors are often 

 very bright, shades of yellow and red prevailing, and bright metallic white 

 or silver being frequent. 



The tough integument which covers the abdomen consists of three 

 layers; the external one is a thin, transparent, horny membrane, nearly 

 colorless, but more or less densely covered with colored hairs. Beneath 

 this lies the soft layer of pigmentary matter upon which the peculiar color 

 of the body largely depends. The third or inner layer consists of an 

 expanded .netwoi'k of muscular fibres, which are irregularly interlaced, and 

 which must enable the spider forciblj'- to compress the abdomen. The 

 muscles forming this laj'cr are very faintly, if at all, marked with trans- 

 verse stria3. (Meade.) 



On the ventral side or venter near the base (anterior part) are situated 



two gills, lu-eathing holes, or pulmonary sacs (bg). Thej' are scales or plates 



symmetrically apposed on either side, form- ^ 



ing the covering of cavities com- • ■' 



mona- jj^m^jpating with the tracheae or /^^Plf^/^^K 



ry Sacs. . , '=' -^ .. ,, , /// ■{niJ\JJiU' ' \ 



air tubes. Externally they present /'( j ^^^^k^ \ 



the appearance of simj^le transverse slits in \\( t^^^^^^^^O^^-^ 



the venter. Each cavity contains about fifty •.'^..^vi//W^^^K^^^^. 



extremely thin, triangular, white leaflets; fast- ' ="-r:-^-^^r'-'"c5^§\?yS' 



ened together at the edge of the breathing j,g "~«P 



hole. Each is double, being in fact a flat ^^ ^^ Diagram of location of spinning 



pouch with an opening on the lower side organs and their relations to other organs 



... -.1 ,1 i -mi • of the abdomen, spn, spinnerets; py.g, 



communicating with the outer air. ihe Spi- py^iform glands; tr.g, treeform glands; 



racular plates are usually conspicuous obiectS cy.g, cylindrical glands; ep, eplgynum; 



, ,.„ '. "^ . , ^1 . bgl, breathing gills ; ae, alimentary canal ; 



upon the venter, dlftering m color therefrom, a, anus; ov, ovaries, shomng the eggs; h, 



.sometimes slightly pubescent, but more com- ^^art; ex, cephalothorax. (After Under- 



monly smooth, and of harder substance than 



the surrounding surface. (Cambridge.) In the male spider a minute orifice 

 which leads to the seminal organs is located between the spiracular plates. 

 In the same situation in the female is an aperture usually rather 

 Epigy- conspicuous, which is surmounted or surrounded with a corneous 

 ^^^' process of greater or less development. This aperture is the vulva. 



The process is known as the epigynum, and probably has the function of 

 an ovipositor. The form and structure of the epigjmum are characteristics 

 highly valued by systematic arachnologists in the determination of species. 

 But the organ is not present in immature spiders, and until the female 

 reaches maturity no aperture is visible. The organs of reproduction in the 

 female consist of two long ovoid plates, longitudinally placed within the 

 ventral surface of the abdomen. These unite and form a short broad 

 oviduct, whose external opening between the spiracular orifice is the epigy- 

 num (ep), Fig. 16. 



The ovaries, which shortly before the deposition of eggs occupy a large 



