Beside the Jelly-Fishes arising from Hydroids, there are many others resembling these in all the essential features of their structure, but differing in their mode of development; for, although more or less Polyp-like when first born from the egg, they never become attached, nor do they ever bud or divide, but reach their mature condition without any such striking metamorphoses as those that characterize the development of the Hydroid Acalephs. All the Medusas, whether they arise from buds on the Hydroid stock, like the Sarsia, or from transverse division of the Hydroid form, like the Aurelia, or grow directly from the egg to maturity, without pausing in the Hydroid phase, like the Campanella, agree in the general division and relation of parts. All have a central cavity, from which arise radiating tubes extending to the margin of the umbrella-like disk, where they unite either in a net-work of meshes or in a single circular tube. But there is a great difference in the oral apparatus; the elaborate ruffled curtains, that hang from the corners of the mouth, occur only in the Species arising from the transverse division of the Polyp-like young. For this reason they are divided into two Orders,--the Hydroids and the Discophorae. The third Order, the Ctenophorae, are among the most beautiful of the Acalephs. I have spoken of the various hues they assume when in motion, and I will add one word of the peculiarity in their structure which causes this effect. The Ctenophorae differ from the Jelly-Fishes described above in sending off from the main cavity only two main tubes, instead of four like the others; but each of these tubes divides and subdivides in four branches as it approaches the periphery. From the eight branches produced in this way there arise vertical tubes extending in opposite directions up and down the sides of the body. Along these vertical tubes run the rows of little locomotive oars, or combs, as they have been called, from which these animals derive their name of Ctenophorae. The rapid motion of these flappers causes the decomposition of the rays of light along the surface of the body, producing the most striking prismatic effect; and it is no exaggeration to say that no jewel is brighter than these Ctenophorae as they move through the water. [Illustration: Idyia roseola; one of our Ctenophorae: a, anal aperture; b, radiating tube; c, circular tube; d, e, f, g, h, rows of locomotive fringes.] * * * * *