That these scuta are sometimes indicative of an obsolete segmentation would seem likely from the study of the remarkable species, _Tetrablemma medioculatum_ (Fig. 176), described by Pickard-Cambridge, from Ceylon. In addition to large dorsal and ventral scuta, the sides and posterior extremity are guarded by smaller scuta, the disposition of which is well seen in the figure. [Illustration: FIG. 176.—_Tetrablemma medioculatum_, much enlarged. =A=, Posterior view; =B=, profile, showing the scuta. (After Cambridge.) ] The normal smooth abdomen presents dorsally no very striking features. In species of variegated coloration there is very generally noticeable a median dentated band (Fig. 173), the “normal marking” of some writers, which would appear to have some correlation with the underlying dorsal vessel. Beneath the abdomen are to be seen the orifices of the breathing and genital organs, the spinnerets, and the anal aperture upon its tubercle. The breathing organs are, as will be explained later, of two kinds, lung-books and tracheae. The great majority of Spiders possess only two lung-books, and their transverse, slit-like openings (“stigmata” or “spiracles”) may be seen on either side of the anterior part of the abdomen. Where, as in the Theraphosae, there are four lung-books, the second pair open by similar slits a short distance behind the first. According to Bertkau, pulmonary sacs are entirely lacking in the genus _Nops_. The tracheae generally debouch by a single median stigma towards the posterior end of the abdomen, just in front of the spinnerets. This opening clearly results from the fusion of two stigmata, which in some species retain their paired arrangement. On a level with the openings of the anterior lung-books or pulmonary sacs there is usually observable a slight transverse ridge, the epigastric fold (Fig. 174), and in the centre of this is the genital opening. This is never visible until after the last moult, and in the male is always a simple inconspicuous aperture. This is also the case with the females of some groups (Theraphosae, Filistatidae, Dysderidae, etc.), but in most cases there is a more or less complicated armature, the “epigyne,” the special design of which is of great specific value. In its simplest form it is merely a plate, usually of dark colour, with one or two apertures (Fig. 174, _ep_), but in some families, notably the Epeiridae, it is more complicated, and is furnished with a hooked median projection, the “ovipositor” (“clavus” of Menge), which is often absurdly like a petrified elephant’s trunk in miniature.