The Hydroids are not all attached to the ground,--some like the Physalia (Portuguese man-of-war), or the Nanomia, that pretty floating Hydroid of our own waters, move about with as much freedom as if they enjoyed an individual independent existence. As all these orders have their representatives on our coast, to be described hereafter in detail, we need only allude here to their characteristic features. But we must not leave unnoticed one very remarkable Hydroid Acaleph (Fig. 24), not found in our waters, and resembling the Polyps so much, that it has long been associated with them. The Millepore is a coral, and was therefore the more easily confounded with the Polyps, so large a proportion of which build coral stocks; but a more minute investigation of its structure (Figs. 25, 26) has recently shown that it belongs with the Acalephs.[2] This discovery is the more important, not only as explaining the true position of this animal in the Animal Kingdom, but as proving also the presence of Acalephs in the earliest periods of creation, since it refers a large number of fossil corals, whose affinities with the millepores are well understood, to that class, instead of to the class of Polyps with which they had hitherto been associated. But for this we should have no positive evidence of the existence of Acalephs in early geological periods, the gelatinous texture of the ordinary Jelly-fishes making their preservation almost impossible. It is not strange that the true nature of this animal should have remained so long unexplained; for it is only by the soft parts of the body, not of course preserved in the fossil condition, that their relations to the Acalephs may be detected; and they are so shy of approach, drawing their tentacles and the upper part of the body into their limestone frame if disturbed, that it is not easy to examine the living animal. [Footnote 2: See "Methods of Study," by Prof. Agassiz.] [Illustration: Fig. 26. Transverse section of a branch, showing pits, _a_ _a_ _a_ _a_, of the large Hydroids with the horizontal floors. (_Agassiz_.)]