The group, and Huxley's conception of its structure, are now absolutely accepted by anatomists, and have made one of the corner-stones of our modern idea of the arrangement of the animal kingdom. With the exception of sponges, concerning the exact relations of which there is still dispute, and of a few sets of parasitic and possibly degenerate creatures, all animals, the bodies of which are multicellular, from the simple fresh-water hydra up to man, are divided into two great groups. The structure of the simpler of these groups is exactly what Huxley found to be of importance in the Medusæ. The body wall, from which all the organs protrude, consists merely of a web of cells arranged in two sheets or membranes, and the single cavity consists of a central stomach, surrounded by these membranes, the cavity remaining simple or giving rise to a number of branching canals. The members of this great division of the animal kingdom are the creatures which Huxley selected and placed together, with the addition of the sea-anemones and the medusa-like Ctenophora, which, indeed, he mentioned in his memoir as being related to the others, but reserved fuller consideration for a future occasion. This group is now called the Cœlenterata, the name implying that the creatures are simply hollow stomachs, and it is contrasted in the strongest way with the group Cœlomata, in which are placed all the higher animals, from the simplest worm up to man; animals in which, in addition to the two foundation-membranes of the Cœlenterata, there is a third foundation-membrane, and in which, in addition to the simple stomach cavity with its offshoots, there is a true body-cavity or cœlome, and usually a set of spaces and channels containing a blood-fluid. The older method of naming groups of animals after some obvious superficial character lingered on for some years in text-books and treatises, but in this memoir the young ship-surgeon had replaced it by the modern scientific method of grouping animals together only because of real identity of structure. There is yet left to be noticed perhaps the most wonderful of all the ideas in this first memoir by Huxley. In the course of describing the two foundation membranes of the Medusæ he remarks: "It is curious to remark, that throughout, the outer and inner membranes appear to bear the same physiological relation to one another as do the serous and mucous layers of the germ: the outer becoming developed into the muscular system, and giving rise to the organs of offence and defence: the inner on the other hand appearing to be more closely subservient to the purposes of nutrition and generation."