The Actinozoa (anemonies, &c.) and the Hydrozoa (polyps) constitute the COELENTERA of Frey and Leuckart. "In all these animals," says Professor Huxley, "the substance of the body is differentiated into those histological elements which have been termed cells, and the latter are previously disposed in two layers, one external and one internal, constituting the ectoderm and endoderm. Among animals which possess this histological structure the Coelenterata stand alone in having an alimentary canal, which is open at its inner end and communicates freely by this aperture with the general cavity of the body," and "all (unless the Ctenophora should prove a partial exception to the rule) are provided with very remarkable organs of offence or defence, called thread-cells or nematocysts." In describing the Polyps we have given illustrations of these weapons. The remaining classes, which have been roughly associated as _Protozoa_, must evidently be rearranged. Sponges, Rhizopods (Amoebæ, &c.), and Gregarines, have strong resemblances, but recent researches may place the former higher. The Infusoria comprehend creatures too various to remain under one head, and very many of them too highly organized to be called "protozoons," or first life-forms. Those who wish to pursue this subject further may consult Professor Huxley's 'Elements of Comparative Anatomy,' from which the preceding quotations have been taken. A system of classification founded upon anatomical and developmental considerations frequently differs considerably from one we might arrive at if all the creatures were arranged according to the perfection of their faculties and the extent and accuracy of their relations to the external world. Such a classification would not in any way supersede the former, but it would prove very instructive and offer many valuable suggestions. Some years since, Professor Owen proposed to divide the Vertebrates according to the perfection of their brains, but other anatomists did not find his divisions sufficiently coincident with facts. Very little has been done towards an exact science of human phrenology. The difficulties remain pretty much as they were many years ago, and our comparative phrenology, if we may use such a term, is in a very imperfect state. When we come to the lower animals we do not know what peculiarities of the brain of an ant make it the recipient of a higher instinct, or give its possessor greater capacities for dealing with new and unexpected difficulties than are possessed by most other insects, and if any reader has a marine aquarium, and will make a few experiments in taming prawns, and watching their proceedings, he will discover symptoms of intelligence beyond what the structure of the creature would have led him to expect.