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.]
I trust I have succeeded in showing that the three Orders of the Acalephs are, like the five Orders of the Echinoderms, different degrees of complication of the same structure. In the Hydroids, the organization does not rise above the simple digestive cavity inclosed by the double body-wall; and we might not suspect their relation to the Acalephs, did we not see the Jelly-Fish born from the Hydroid stock. In the Hydroid-Medusae and Discophorae, instead of a simple digestive sac, as in the Hydroids, we have a cavity sending off tubes toward the periphery, which ramify more or less in their course. Now whether there are four tubes or eight, whether they ramify extensively or not, whether there are more or less complicated appendages around the margin or the mouth, makes no difference in the essential structure of these bodies. They are all disk-like in outline, they all have tentacles hanging from the margin, and a central cavity from which tubes diverge that divide the body into a certain number of portions, bearing in all the same relation to each other and to the central cavity. In the Ctenophorae, another complication of structure is introduced in the combination of vertical with horizontal tubes and the external appendages accompanying them.