In the higher types of animals this space comes to be partially occupied by a structure that may be described as a cavity within a cavity--the cœlom. Most zoologists regard this as arising by a re-introversion of the _archenteron_ or primary alimentary sac. It is easily to be perceived that after the introversion which produces this digestive cavity, the wall of the cavity may be again introverted in such way as to intrude into the peri-visceral space. The cœlom thus formed is subsequently shut off. Becoming included among the more truly internal structures, and in part giving origin to certain lining membranes, it has for its chief function the formation of organs for the excretion and emission of nitrogenous waste and of the generative products: some portions of it retaining, as a consequence, indirect connexions with the environment and characters usually accompanying such connexions. Here we are not concerned with further details: the aim being simply to indicate the way in which out of the original layer, wholly external, there arise, by primary and secondary introversions, and the formation of intermediate membranes and spaces, the chief contrasts between outer and inner tissues, and how there simultaneously go on the differentiations accompanying different conditions. § 289_a_. Another all-important differentiation between outer tissues and inner tissues has now to be set forth--that by which the nervous system becomes established and distinguished. Strangely enough, like the one above described, it is sequent upon an introversion: the nervous system is primarily a skin-structure and develops by the infolding of this skin-structure. In creatures possessing the earliest rudiments of nerves these exist in certain superficial cells. Each has a small tubular orifice from which projects a minute hair, and each has on its under side processes running into the tissue below, and serving, as it seems, to conduct impressions from the projecting hair when it is disturbed by contacts with foreign bodies. A plexus of fibres bringing the inner processes of such cells into communication arises, and forms something like a nervous layer capable of propagating impulses in all directions. At a subsequent stage some of the superficial cells, ceasing to be themselves the recipients of external stimuli, sink inwards and become ganglion-cells connected with the nervous plexus--agents, as we must suppose, for the reception, multiplication, and diffusion of the impulses received from the outer cells.