GENERAL CLASSIFICATION AND STRUCTURE. 



27 



,0V 



Pulmona 

 ry Sacs. 



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 network of muscular fibres, which are irregularly interlaced, and 

 which must enable the spider forcibly to compress the abdomen. The 

 muscles forming this layer are very faintly, if at all, marked with trans- 

 verse strise. (Meade.) 



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

 two gills, breathing holes, or pulmonary sacs (bg). They are scales or plates 

 symmetrically apposed on either side, form- 

 ing the covering of cavities com- 

 municating with the trachese or 

 air tubes. Externally they present 

 the appearance of simple transverse slits in 

 the venter. Each cavity contains about fifty 

 extremely thin, triangular, white leaflets, fast- 

 ened together at the edge of the breathing 

 hole. Each is double, being in fact a flat 

 pouch with an opening on the lower side 

 communicating with the outer air. The s})!- 

 racular plates are usually conspicuous objects 

 upon the venter, differing in color therefrom, 

 sometimes slightly pubescent, but more com- 

 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 

 Epig-y- 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 i)robably has the function of 

 an ovipositor. The form and structure of the epigynum 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 rei>roduction in the 

 female consist of two long ovoid plat(>s, longitudinally placed within the 

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

 oviduct, whose external o]»cning betwoon the s])ir;icnlar orilice is the opigy- 

 iium (ep). Fig. Kl. 



H ^ tTS- 



Fig. 16. Diagram of location of spinning 

 organs and their relations to other organs 

 of the abdomen, spn, spinnerets; py.g, 

 pyriform glands; tr.g, treeform glands; 

 cy.g, cylindrical glands ; ep, epigynum ; 

 bgl, breathing gills; ac, alimentary canal; 

 a, anus; ov, ovaries, showing the eggs ; h, 

 heart ; ex, cephalothorax. (After Under- 

 bill.) 



The ovaries, wliieh sliuitly before the deposition of eggs occupy a large 



