METHODS OF STUDY IN NATURAL HISTORY. XIV. If I succeeded in explaining my subject clearly in the last article, my readers will have seen that the five Orders of the Echinoderms are but five expressions of the same idea; and I will now endeavor to show that the same identity of structural conception prevails also throughout the two other Classes of Radiates, and further, that not only the Orders within each Class, but the three Classes themselves, Echinoderms, Acalephs, and Polyps, bear the strictest comparison, founded upon close structural analysis, and are based upon one organic formula. We will first compare the three Orders of Acalephs,--Hydroids being the lowest, Discophorae; next, and the Ctenophorae highest. The fact that these animals have no popular names shows how little they are known. It is true that we hear some of them spoken of as Jelly-Fishes; but this name is usually applied to the larger Discophore, when it is thrown upon the beach and lies a shapeless mass of gelatinous substance on the sand, or is seen floating on the surface of the water. The name gives no idea of the animal as it exists in full life and activity. When we speak of a Bird or an Insect, the mere name calls up at once a characteristic image of the thing; but the name of Jelly-Fish, or Sun-Fish, or Sea-Blubber, as the larger Acalephs are also called, suggests to most persons a vague idea of a fish with a gelatinous body,--or, if they have lived near the sea-shore, they associate it only with the unsightly masses of jelly-like substance sometimes strewn in thousands along the beaches after a storm. To very few does this term recall either the large Discophore, with its purple disk and its long streamers floating perhaps twenty or thirty feet behind it as it swims,--or the Ctenophore, with its more delicate, transparent structure, and almost invisible fringes in parallel rows upon the body, which decompose the rays of light as the creature moves through the water, so that hues of ruby-red and emerald-green, blue, purple, yellow, all the colors of the rainbow, ripple constantly over its surface when it is in motion,--or the Hydroid, with its little shrub-like communities living in tide-pools, establishing themselves on rocks, shells, or sea-weeds, and giving birth not only to animals attached to submarine bodies, like themselves, but also to free Medusae or Jelly-Fishes that in their turn give birth again to eggs which return to the parent-form, and thus, by alternate generations, maintain two distinct patterns of animal life within one cycle of growth.