GENERAL CLASSIFICATION AND STRUCTURE. 27 
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 strize. (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 trachee 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, beg in fact a flat ... 46, Dinguaiasof (penton eresoinniste 
pouch with an opening on the lower side organs and their relations to other organs 
communicating with the outer air. The spi- SV itorm glands; tre, trweform glonan, 
racular plates are usually conspicuous objects  cy-s, cylindrical glands; ep, epigynum; 
Fi : 3 bgl, breathing gills; ac, alimentary canal; 
upon the venter, differing in color therefrom, 4g anus: ov, ovaries, showing the eggs; h, 
sometimes slightly pubescent, but more com- heart; cx, cephslothorax. (After Under- 
hill.) 
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 
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 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 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 
Pulmona- 
ry Sacs. 
Epigy- 
num. 
