Among the nomadic Polyps is a small floating Actinia, called Arachnactis, (Fig. 11,) from its resemblance to a spider. They are found in great plenty floating about during the night, feeling their way in every direction by means of their tentacles, which are large in proportion to the size of the animal, few in number, and turned downward when in their natural attitude. The partitions and the digestive cavity enclosed between them are short, as will be seen in Fig. 11, when compared to the general cavity of the body floating balloon-like above them. Around the mouth is a second row of shorter tentacles, better seen in a younger specimen (Fig. 12). This Actinia differs from those described above, in having two of the sides flattened, instead of being perfectly circular. Looked at from above (as in Fig. 13) this difference in the diameters is very perceptible; there is an evident tendency towards establishing a longitudinal axis. In the sea-anemone, this disposition is only hinted at in the slightly pointed folds or projections on opposite sides of the circle formed by the mouth, which in the Arachnactis are so elongated as to produce a somewhat narrow slit (see Fig. 13), instead of a circular opening. The mouth is also a little out of centre, rather nearer one end of the disk than the other. These facts are interesting, as showing that the tendency towards establishing a balance of parts, as between an anterior and posterior extremity, a right and left side, is not forgotten in these lower animals, though their organization as a whole is based upon an equality of parts, admitting neither of posterior and anterior extremities, nor of right and left, nor of above and below, in a structural sense. This animal also presents a seeming anomaly in the mode of formation of the young tentacles, which always make their appearance at the posterior extremity of the longitudinal axis, the new ones being placed behind the older ones, instead of alternating with them as in other Actiniæ. _Bicidium_. (_Bicidium parasiticum_ AG.) [Illustration: Fig. 14. Bicidium parasiticum; natural size.]