Another circumstance, which is also only explicable by the Theory of Descent, is the remarkable change of generation in the higher medusæ, the disc-jellies (Discomedusæ, vol. ii. p. 136), a representative of which is given at the top of Plate VII., in the centre (rather in the background), namely, a Pelagia (14). From the bottom of the bell-shaped cup, which is strongly arched and the rim of which is neatly indented, there hang four very long and strong arms. The non-sexual polyps, from which these disc-jellies are derived, are exceedingly simple primæval polyps, differing very little from the common fresh-water polyp (Hydra). The alternation of generation in these Discomedusæ has also been described in my lecture on Differentiation,(37) and there illustrated by the Aurelia by way of example. Finally, the last class of Zoophytes, the group of comb-jellies (Ctenophora, vol. ii. p. 142), has two representatives on Plate VII. To the left, in the centre, between the Æquorea (9), the Physophora (13), and the Cunina (12), is a long and thin band like a belt (15), winding like a snake; this is the large and splendid _Venus’ girdle_ of the Mediterranean (Cestum), the colours of which are as varied as those of the rainbow. The actual body of the animal, which lies in the centre of the long belt, is very small, and constructed exactly like that of the _melon-jelly_ (Cydippe), which floats above to the left (16). On the latter are visible the eight characteristic fringed bands, or ciliated combs, of the ctenophora, and also two long tentacles which extend right across the page, and are fringed with still finer threads. PLATES VIII. AND IX. (_Between pages 170 and 171, Vol. II._) _History of the Development of Star-fishes_ (_Echinoderma_, or _Estrella_). The two plates exhibit their alternation of generation (vol. ii. p. 168), with an example from each of the four classes of Star-fishes. The sea-stars (Asterida) are represented by Uraster (_A_), the sea-lilies (Crinoida) by Comatula (_B_), the sea-urchins (Echinida) by Echinus (_C_), and finally, the sea-cucumbers (Holothuriæ) by Synapta (_D_). (Compare vol. ii. pp. 166 and 176.) The successive stages of development are marked by the numbers 1-6.