This series of facts, presented here in their connection, had been observed separately before their true relation was understood. Investigations had been made on the Hydroid stock, described as _Coryne_, and upon its Medusoid offspring, described as _Sarsia_, named after the naturalist Sars, whose beautiful papers upon this class of animals have associated his name with it; but the investigations by which all these facts have been associated in one connected series are very recent. These transformations do not correspond to our common idea of metamorphoses, as observed in the Insect, for instance. In the Butterfly's life we have always one and the same individual,--the Caterpillar passing into the Chrysalis state, and the Chrysalis passing into the condition of the Winged Insect. But in the case I have been describing, while the Hydroid gives birth to the Medusa, it still preserves its own distinct existence; and the different forms developed on one stock seem to be two parallel lives, and not the various phases of one and the same life. This group of Hydroids retains the name of Coryne; and the Medusa born from it, Sarsia, has received, as I have said, the name of the distinguished investigator to whose labors we owe much of our present knowledge of these animals.--Let us look now at another group of Hydroids, whose mode of development is equally curious and interesting. The little transparent embryos from which they arise, oval in form, with a slight, scarcely perceptible depression at one end, resemble the embryos of Coryne already described. They may be seen in great numbers in the spring, floating about in the water, or rather swimming,--for the motion of all Radiates in their earliest stage of existence is rapid and constant, in consequence of the vibratile cilia that cover the surface. At this stage of its existence such an embryo is perfectly free, but presently its wandering life comes to an end; it shows a disposition to become fixed, and proceeds to choose a suitable resting-place. I use the word "choose" advisedly; for though at this time the little embryo seems to have no developed organs, it yet exercises a certain discrimination in its selection of a home. Slightly pear-shaped in form, it settles down upon its narrower end; it wavers and sways to and fro, as if trying to get a firm foothold and force itself down upon the surface to which it adheres; but presently, as if dissatisfied with the spot it has chosen, it suddenly breaks loose and swims away to another locality, where the same examination is repeated, not more to its own satisfaction apparently, for the creature will renew the experiment half a dozen times, perhaps, before making a final selection and becoming permanently attached to the soil. In the course of this process the lower end becomes flattened, and moulds itself to the shape of the body on which it rests. Once settled, this animal, thus far hardly more than a