The _sponges_ form a peculiar group in the animal world, which differs widely in organization from all the other metazoa. The innumerable kinds of sponges grow, as a rule, at the bottom of the sea. The simplest form of sponge, the _olynthus_, is in reality nothing more than a _gastraea_, the body-wall of which is perforated like a sieve, with fine pores, in order to permit the entrance of the nourishing stream of water. In the majority of sponges--even in the most familiar one, the bath-sponge--the bulbous organism constructs a kind of stem or tree, which is made up of thousands of these gastræads, and permeated by a nutritive system of canals. Sensation and movement are only developed in the faintest degree in the sponges; they have no nerves, muscles, or organs of sense. It was therefore quite natural that such stationary, shapeless, insensitive animals should have been commonly taken to be plants in earlier years. Their psychic life--for which no special organs have been differentiated--is far inferior to that of the mimosa and other sensitive plants. _The soul of the cnidaria_ is of the utmost importance in comparative and phylogenetic psychology; for in this numerous group of the coelenterates the historical evolution of the _nerve-soul_ out of the _tissue-soul_ is repeated before our eyes. To this group belong the innumerable classes of stationary polyps and corals, and of swimming medusæ and siphonophora. As the common ancestor of all the cnidaria we can safely assign a very simple polyp, which is substantially the same in structure as the common, still surviving, fresh-water polyp--the hydra. Yet the hydræ, and the stationary, closely related _hydropolyps_, have no nerves or higher sense-organs, although they are extremely sensitive. On the other hand, the free-swimming medusæ, which are developed from them--and are still connected with them by alternation of generations--have an independent nervous system and specific sense-organs. Here, also, we may directly observe the ontogenetic evolution of the nerve-soul (_neuropsyche_) out of the tissue-soul (_histopsyche_), and thus learn its phylogenetic origin. This is the more interesting as such phenomena are _polyphyletic_--that is, they have occurred several times--more than once, at least--quite independently. As I have shown elsewhere, the hydromedusæ have arisen from the hydropolyps in a different manner from that of the evolution of the scyphomedusæ from the scyphopolyps; the gemmation is terminal in the case of the latter, and lateral with the former. In addition, both groups have characteristic hereditary differences in the more minute structure of their psychic organs. The class of siphonophora is also very interesting to the psychologist. In these pretty, free-swimming organisms, which come from the hydromedusæ we can observe a double soul: the _personal soul_ of the numerous individualities which compose them, and the common, harmoniously acting psyche of the entire colony.