The Idyia differs greatly in appearance at different periods of its development, and, indeed, no one would suspect, without some previous knowledge of its transformations, that the young Idyia, with its rapid gyrations, its short ambulacral tubes, like immense pouches (Fig. 37), its large pigment spots scattered over the surface (Fig. 38), was an earlier stage of the rosy-hued Idyia, which glides through the water with a scarcely perceptible motion. Figs. 37-40 represent the various stages of its growth. It will be seen how very short are the locomotive fringes (Fig. 39) in comparison with those of the full-grown ones (Fig. 33). It is only in the adult Idyia that these rows attain their full height, and the tubes, ramifying throughout the body (Fig. 40), are completed. [Illustration: Fig. 38. Young Idyia seen from the anal extremity, magnified; _a_ anal opening, other letters as in Fig. 36.] [Illustration: Fig. 39. Idyia somewhat older than Fig. 37, lettering as before; magnified.] [Illustration: Fig. 40. Young Idyia in which the ambulacral tubes begin to ramify; magnified, letters as before.] The Bolina, in its early condition, recalls the young Pleurobrachia. At this period it has the same rapid motion, and when somewhat more advanced, long tentacles, resembling those of the Pleurobrachia, make their appearance (Fig. 41); it is only at a later period that the tentacles become contracted, while the large lobes (Fig. 42), so characteristic of Bolina, are formed by the elongation of the oral end of the body, the auricles becoming more conspicuous at the same time (Fig. 43). A little later the lobes enlarge, the movements become more lazy; it assumes both in form and habits the character of the adult Bolina. [Illustration: Fig. 41. Young Bolina in stage resembling Pleurobrachia; greatly magnified.] [Illustration: Fig. 42. Young Bolina seen from the broad side, with rudimentary auricles and lobes; magnified.] [Illustration: Fig. 43. The same as Fig. 42, seen from the narrow side.] The series of changes through which the Ctenophoræ pass are as remarkable as any we shall have occasion to describe, though not accompanied with such absolutely different conditions of existence. The comparison of the earlier stages of life in these animals with their adult condition is important, not only with reference to their mode of development, but also because it gives us some insight into the relative standing of the different groups, since it shows us that certain features, permanent in the lower groups, are transient in the higher ones. A striking instance of this occurs in the fact mentioned above, that though the long tentacles so characteristic of the adult Pleurobrachia exist in the young Bolina, they yield in importance at a later period to the lobes which eventually become the predominant and characteristic feature of the latter. * * * * * DISCOPHORÆ.