“It is with these two kinds of creatures that the present work is concerned, and, if it seems almost impious to lay the ‘forced fingers rude’ of science upon living things of such exquisite beauty, let it be remembered that our human nature is not so much out of joint that the rational desire to know is incompatible with the emotional impulse to admire. Speaking for myself, I can testify that my admiration of the extreme beauty of these animals has been greatly enhanced—or rather I should say that this extreme beauty has been, so to speak, revealed—by the continuous and close observation which many of my experiments required: both with the unassisted eye and with the microscope numberless points of detail, unnoticed before, became familiar to the mind; the forms as a whole were impressed upon the memory; and, by constantly watching their movements and changes of appearance, I have grown, like an artist studying a face or a landscape, to appreciate a fulness of beauty, the esse of which is only rendered possible by the _percipi_ of such attention as is demanded by scientific research. Moreover, association, if not the sole creator, is at least a most important factor of the beautiful; and therefore the sight of one of these animals is now much more to me, in the respects which we are considering, than it can be to anyone in whose memory it is not connected with many days of that purest form of enjoyment which can only be experienced in the pursuit of science.” No matter how interesting investigation into any set of natural phenomena may be, probably none is more attractive than a study of primitive nervous systems. Alike in the survey of the whole of the animal kingdom and in the study of the development of any individual form there are certain broad truths evident. First among these may be mentioned the significant fact that the nervous system of all animals originates from some of the cells of that layer of the body which was originally the outermost. This is the lesson taught by nature that the prime necessity of living organisms is a knowledge of the outer world, and that the most sensitive and important system of organs primarily stands in a direct relation to the outer world. The investigations of Leuckart, Haeckel, Oscar and Richard Hertwig, and Prof. Schafer fully established the fact as to the origin of nerve fibres and sense-cells from the outer layer of the body, and as to the primitively diffused disposition of the central nervous system. This was first observed of the jelly-fish, but subsequent investigation proved it also to be the case with star-fish, sea-urchins and all the forms of echinoderms. Haeckel, in 1860, showed that the eyes of the star-fishes are nothing more than elongated epithelial cells provided with pigments, and throughout life quite superficial in position.