I have said that the Radiates are the lowest type among animals, embodying, under an infinite variety of forms, that plan in which all parts bear definite relations to a vertical central axis. The three classes of Radiates are distinguished from each other by three distinct ways of executing that plan. I dwell upon this point; for we shall never arrive at a clear understanding of the different significance and value of the various divisions of the Animal Kingdom, till we appreciate the distinction between the structural conception and the material means by which it is expressed. A comparison will, perhaps, better explain my meaning. There are certain architectonic types, including edifices of different materials, with an infinite variety of architectural details and external ornaments; but the flat roof and the colonnade are typical of all Grecian temples, whether built of marble or granite or wood, whether Doric or Ionic or Corinthian, whether simple and massive or light and ornamented; and, in like manner, the steep roof and pointed arch are the typical characters of all Gothic cathedrals, whatever be the material or the details. The architectural conception remains the same in all its essential elements, however the more superficial features vary. Such relations as these edifices bear to the architectural idea that includes them all, do classes bear to the primary divisions or branches of the Animal Kingdom. The three classes of Radiates, beginning with the lowest, and naming them in their relative order, are Polyps, Acalephs or Jelly-Fishes, and Echinoderms or Star-Fishes and Sea-Urchins. In the Polyps the plan is executed in the simplest manner by a sac, the sides of which are folded inward, at regular intervals from top to bottom, so as to divide it by vertical radiating partitions, converging from the periphery toward the centre. These folds or partitions do not meet in the centre, but leave an open space, which is the main cavity of the body. This open space, however, occupies only the lower part of the body; for in the upper there is a second sac hanging to a certain distance within the first. This inner sac has an aperture in the bottom, through which whatever enters it passes into the main cavity of the body. A central opening in the top forms a kind of mouth, around which are radiating tentacles connecting with the open chambers formed by the partitions within. Cutting such an animal across in a transverse section, we shall see the radiation of the partitions from the centre to the circumference, showing still more distinctly the typical structure of the division to which it belongs. [Illustration: Vertical section of a Sea-Anemone of Actinia: _o_, mouth; _t_, tentacles; _s_, inner sac or stomach; _b_, main cavity; _ff_, reproductive organs; _g_, radiating partition; _eee_, radiating chambers; _cc_, circular openings in the partitions; _aa_, lower floor.] [Illustration: Transverse section of a Sea-Anemone or Actinia.]