The great size of their locomotive flappers in proportion to the rest of the body, is characteristic of the young Ctenophoræ. They seem like large paddles on the sides of these tiny transparent spheres, and, owing to their great power as compared with those of the adult, the young move with extraordinary rapidity. The Pleurobrachia alone retains its quickness of motion in after life, and although its long graceful streamers appear only as short stumpy tentacles in the young ([Fig. 34]), yet its active little body would be more easily recognized in the earlier stages of growth than that of the other Ctenophoræ. Figs. [34], [35], and [36] show the Pleurobrachia at various stages of growth; [Fig. 34], with its thick stunted tentacles and short rows of flappers, is the youngest; the flappers themselves are rather long at this age, looking more like stiff hairs than like the minute fringes of the adult. In [Fig. 35] the tentacles are already considerably longer and more delicate; in [Fig. 36] the vertical tubes are already completed, while Figs. [27]-[29] present it in its adult condition.
| Fig. 34. Young Pleurobrachia still in the egg; t tentacle, e eye-speck, c c rows of locomotive flappers, d digestive cavity; greatly magnified. | Fig. 35. Young Pleurobrachia swimming about in the egg just before hatching; magnified. | |
| Fig. 36. Young Pleurobrachia resembling somewhat the adult; f funnel leading to anal opening, l lateral tubes, o o o' o' rows of locomotive flappers; magnified. | Fig. 37. Young Idyia, greatly magnified; lettering as in Fig. 36; d digestive cavity. |
Fig. 38. Young Idyia seen from the anal extremity, magnified; a anal opening, other letters as in [Fig. 36].
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.