In the Class of Polyps there are but two Orders,--the Actinoids and the Halcyonoids; and I have already said so much of the structure of Polyps that I think I need not repeat my remarks here in order to show the relation between these groups. The body of all Polyps consists of a sac divided into chambers by vertical partitions, and having a wreath of hollow tentacles around the summit, each one of which opens into one of the chambers. The greater complication of these parts and their limitation in definite numbers constitute the characters upon which their superiority or inferiority of structure is based. Here the comparison is easily made; it is simply the complication and number of identical parts that make the difference between the Orders. The Actinoids stand lowest from the simple character and indefinite increase of these parts; while the Halcyonoids, with their eight lobed tentacles, corresponding to the same number of internal divisions, are placed above them. We have the key-note to the common structure of the three Classes whose Orders we have been comparing in the name of the division to which they all belong: they are _Radiates._ The idea of radiation lies at the foundation of all these animals, whatever be their form or substance. Whether stony, like the Corals, or soft, like the Sea-Anemone, or gelatinous and transparent, like the Jelly-Fish, or hard and brittle, like the Sea-Urchins,--whether round or oblong or cylindrical or stellate, in all, the internal structure obeys this law of radiation. Not only is this true in a general way, but the comparison may be traced in all the details. One may ask how the narrow radiating tubes of the Acalephs, traversing the gelatinous mass of the body, can be compared to the wide radiating chambers of the Polyp; and yet nothing is more simple than to thicken the partitions in the Polyps so much as to narrow the chambers between them, till they form narrow alleys instead of wide spaces, and then we have the tubes of the Jelly-Fish. In the Jelly-Fish there is a circular tube around the margin into which all the radiating tubes open. What have we to compare with this in the Polyps? The outer edge of each partition in the Polyp is pierced by a hole near the margin. Of course when the partition is thickened, this hole, remaining open, becomes a tube; for what is a tube but an elongated hole? The comparison of the Acalephs with the Echinoderms is still easier, for they both have tubes; but in the latter the tubes are inclosed in walls of their own, instead of traversing the mass of the body, as in Acalephs, etc. * * * * *