These infusoria are so closely related to bacteria that they may be confused with the latter, as indeed has been the case up to the last few years. A number of inferior beings were formerly called zoophytes, that is to say, animal plants; and it is sheer nonsense to suppose that they are conscious of pain. Sponges, corals, sea-anemones, star-fishes, sea-urchins, possess a nervous system which is so little developed, and reactions which are so indistinct, that we can scarcely suppose they possess an intelligent consciousness, and, consequently, sensibility to pain. Moreover, I do not see how their reactions would differ if they possessed the notion of pain. When we touch the tentacles of a star-fish, we notice, near the tentacles touched, a sort of agitation set up among the neighbouring tentacles, but this agitation does not extend to the tentacles of the others' arms; so that a general consciousness does not appear to exist, unless it be in a prodigiously rudimentary state, among inferior beings. In certain classes of the mollusca there is no head. Thus oysters and mussels, named on that account _acephala_, have in all probability no consciousness. I would have no scruple, therefore, either in eating living oysters, or in experimenting upon living oysters and mussels, since it seems to me evident that the notion of pain does not exist in them. It is not the same thing with insects; it is here that the first signs of pain begin to appear. Nevertheless, we must be careful to avoid confusing pain with signs of pain. When we take a worm and cut it into three segments, each of these segments will struggle and writhe in a perfect frenzy. It would, therefore, be necessary to admit that pain existed in each of these three segments--in other words, that each fragment possesses a central seat of pain, which is absurd; it is much more rational to suppose that the perturbed movements of the animal are the result of a strong nervous excitation, and that the injury is accompanied by defensive reflex movements but provokes no painful perception. Among the superior animals however, and especially among the vertebrata, pain exists. There can be no doubt about this, although it is impossible to know exactly in what consists the consciousness of pain in an animal; the most profound obscurity still reigns, and will perhaps always reign, over their consciousness and sensations. It would be ridiculous to deny that a dog suffers when his paw is crushed. Certainly, I fully believe that all pain is much less clearly perceived by the dog than by man. But, after all, it is a phenomenon of the same order and identical, save in intensity.