Oligochaeta which live in water; but these latter worms have a pore upon the head, which appears to be wanting in the earthworms. Dr. Michaelsen has thought that the head-pore serves to relieve the brain from undue watery pressure—to act, in fact, as a kind of safety-valve for the liberation of superfluous fluid. In some foreign worms the pores of the reproductive ducts are conspicuous external features (Fig. 197); even in our British species the turgescent male apertures upon the fifteenth segment are sometimes quite obvious. STRUCTURE OF THE BODY-WALL.—The body-wall consists in all Oligochaeta of three recognisable sheets of tissue. Outside is the epidermis, which always consists of a single layer of cells, except in the clitellar region of earthworms. It is a point of difference between the aquatic genera and the terrestrial forms that in the former the clitellum is only one cell thick, while in the higher Oligochaeta it is made up of more than one layer of cells. The epidermis is ciliated only in the genus _Aeolosoma_, and there only on the prostomium. It secretes a thin layer of chitin, which is defective opposite to the glandular cells, and becomes therefore perforated by numerous pores. The structure of the epidermis of _Lumbricus_ has been studied by Cerfontaine, whose recent account[407] of the same is the fullest and most accurate that exists. Underneath the epidermis comes a layer of circular muscle-fibres, {350}and underneath this again a layer of longitudinal muscles. In both layers the fibres have a softer core, outside which lies the radially striated muscular substance. The fibres are embedded in a granular matrix. It used to be considered at one time that such medullated fibres were distinctive of leeches as opposed to Oligochaeta. Their existence has been really known in the Oligochaeta since the researches of Ratzel; but Cerfontaine has fully described them, and emphasised the fact that the fibres of both circular and longitudinal coats are alike in this respect. [Illustration: FIG. 187.—Chaetae of Oligochaeta. × 10. (After Michaelsen, Stolc, and Vejdovsky). 1, 2, Penial chaetae of _Acanthodrilus georgianus_; 3, _Spirosperma_; 4, _Ilyodrilus_; 5, _Lophochaeta_; 6, _Tubifex_; 7, 8, _Nais_; 9, _Bohemilla_. Figs. 3-9 are ordinary chaetae.]