Something remains to be said regarding the specialisation of tissues in this group. We have already mentioned the stinging cells, and the beginnings of muscular tissue, in Hydra. The former are a constant feature of the Cœlenterates, while the latter reaches a very considerable development in the higher forms, as may be judged from the surprising rapidity with which the Medusa can swim, or from the strength with which the sea-anemone can retract its tentacles and draw itself together. Important, further, is the nerve tissue. This consists of cells whose business it is to receive and transmit stimuli. They have long fibrous projections connecting them with each other, so that there is a network of communication throughout the whole animal. In the Medusa, where co-ordinated movements of various portions is necessary, there is a concentration of nerve cells into a double ring near the edge. Here also there are special organs, probably of sight and of the sense of balance; but as these cannot be regarded as the forerunners of the analogous organs in higher animals, we need not pause to describe them. The anatomy of the Cœlenterates will be better understood if the reader will study the diagrams in Figs. 35 and 36, while some idea of the beauty and variety met with in the group may be obtained from Fig. 37. [Illustration: FIG. 38.--Diagram of Ctenophore. _f_, Tentacle; _fs_, tentacle sac; _t_, central cavity; _tg_, upper canal; _rud_, plate bearing cilia; _g_, radial canal; _r_, longitudinal canal; _si_, sense organ.]