Group of Animal-Trees (Zoophytes, or Cœlenterata) in the Mediterranean. On the upper half of the plate is a swarm of swimming medusæ and ctenophora; on the lower half a few bunches of corals and hydroid polyps adhering to the bottom of the sea. (Compare the system of Zoophytes, vol. ii. p. [132], and on the opposite page their pedigree.) Among the adhering Zoophytes at the bottom of the ocean there is, below on the right hand, a large coral-colony (1), which is closely akin to the red precious coral (Eucorallium), and like the latter belongs to the group of corals with eight rays (Octocoralla Gorgonida); the single individuals (or persons) of the branching stock have the form of a star with eight rays, consisting of eight tentacles, which surround the mouth. (Octocoralla, vol. ii. p. [143].) Directly below and in front of it (quite below on the right), is a small bush of hydroid polyps (2), belonging to the group of bell-polyps, or Campanulariæ (vol. ii. p. [146]). A larger stock of hydroid polyps (3), belonging to the group of tube-polyps, or Tubullariæ, rises, to the left, on the opposite side, with its long thin branches. At its base is spread a stock of silicious sponges (Halichondria) (4), with short, finger-shaped branches (vol. ii. p. [139]). Behind it, below on the left (5), is a very large marine rose (Actinia), a single individual from the class of six-rayed corals (Hexacoralla, vol. ii. p. [143]). Its low, cylindrical body has a crown of very numerous and large leaf-shaped tentacles. Below, in the centre of the ground (6), is a sea-anemone (Cereanthus) from the group of fourfold corals (Tetracoralla). Lastly, on a small hill on the bottom of the sea, there rises, on the right above the corals (1) a cup-polyp (Lucernaria), as the representative of the stalked-jellies. (Podactinaria, or Calycozoa, vol. ii. p. [144].) Its cup-shaped, stalked body (7) has eight globular clusters of small, knotted tentacles on its rim.
Among the swimming Zoophytes which occupy the upper half of Plate [VII]., the hydromedusæ are especially remarkable, on account of their alteration of generation. (Compare vol. i. p. [206].) Directly above the Lucernaria (7) floats a small tiara jelly (Oceania), whose bell-shaped body has a process like a dome, the form of a papal tiara (8). From the opening of the bell there hangs a wreath of very fine and long tentacles. This Oceania is the offspring of a tube-polyp, resembling the adhering Tubularia below on the left (3). Beside this latter, on the left, swims a large but very delicate hair-jelly (Æquorea). Its disc-shaped, slightly arched body is just drawing itself together, and pressing water out of the cavity of the cup lying below (9). The numerous, long, and fine hair-like tentacles which hang down from the rim of the cup are drawn by the ejected water into a conical bunch, which towards the centre turns upwards like a collar, and is thrown into folds. Above, in the middle of the cavity of the cup, hangs the stomach, the mouth of which is surrounded by four lobes. This Æquorea is derived from a small bell-polyp, resembling the Campanularia (2). The small, slightly arched cap-jelly (Eucope), swimming above in the centre (10), is likewise derived from a similar bell-polyp. In these three last cases (8, 9, 10), as in the majority of the hydromedusæ, the alternation of generation consists in the freely swimming medusa (8, 9, 10), arising by the formation of buds (therefore by non sexual generation, vol. i. p. [192]), from adhering hydroid polyps (2, 3). These latter, however, originate out of the fructified eggs of the medusæ (therefore by sexual generation, vol. i. p. [195]). Hence the non-sexual, adhering generation of polyps (I., III., V., etc.) regularly alternates with the sexual, freely swimming generation of medusæ (II., IV., VI., etc.). This alteration of generation can only be explained by the Theory of Descent.
The same remark applies to a kindred form of propagation, which is still more remarkable, and which I discovered in 1864, near Nice, in the Elephant-jellies (Geryonida), and called allœogony, or allœogenesis. In this case two completely distinct forms of medusa are descended from one another; the larger and more highly developed generation (11), Geryonia, or Carmarina, is six-rayed, with six foliated sexual organs, and six very movable marginal filaments. From the centre of its bell-shaped cup, like the tongue of a bell, hangs a long proboscis, at the end of which is the opening of the mouth and stomach. In the cavity of the stomach is a long, tongue-shaped bunch of buds (which on Plate [VII]. (n) is extended from the mouth on the left like a tongue). On this tongue, when the Geryonia is sexually ripe, there bud a number of small medusæ. They are, however, not Geryoniæ, but belong to an entirely distinct but very different form of medusa, namely, to the genus Cunina, of the family of the Æginida. This Cunina (12) is very differently constructed; it has a flat, semi-globular cup without proboscis, consists in early life of six divisions, later of sixteen, and has sixteen bag-shaped sexual organs, and sixteen short, stiff, and strongly curved tentacles. A further explanation of this wonderful allœogenesis may be found in my “Contributions to the Natural History of the Hydromedusæ.” (Leipzig, Englemann, 1865), the first part of which contains a monograph of the Elephant-jellies, or Geryonida, illustrated by six copper-plates.
Even more interesting and instructive than these remarkable relations are the vital phenomena of the Siphonophora, whose wonderful polymorphism I have frequently spoken of, and described in a popular manner in my lecture on “Differentiation in Nature and Human Life.”[(37)] (Compare vol. i. p. [270], and vol. ii. p. [140].) An example of this is given in Plate [VII]. in the drawing of the beautiful Physophora (13). This swimming stock or colony of hydromedusæ is kept floating on the surface of the sea by a small swimming bladder filled with air, which in the drawing is seen rising above the surface of the water. Below it is a column of four pairs of swimming bells, which eject water, and thereby set the whole colony in motion. At the lower end of the column of swimming bells is a crown-shaped wreath of curved spindle-shaped sensitive polyps, which also serve as a covering, under the protection of which the other individuals of the stock (the eating, catching, and reproductive persons) are hidden. The ontogenesis of the Siphonophora (and especially of this Physophora), I first observed in Lanzerote, one of the Canary Islands, in 1866, and described in my “History of the Development of the Siphonophora,” and added fourteen plates for its explanation. (Utrecht, 1869). It is rich in interesting facts, which can only be explained by the Theory of Descent.
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.
Plate [VIII]. represents the individual development of the first and non-sexual generation of Star-fishes, that is, of the nurses (usually, but erroneously, called larvæ). These nurses possess the form-value of a simple, unsegmented worm-individual. Fig. 1 represents the egg of the four Star-fishes; and it, in all essential points, agrees with that of man and of other animals. (Compare vol. i. p. [297], Fig. 5.) As in man, the protoplasm of the egg-cell (the yolk) is surrounded by a thick, structureless membrane (zona pellucida), and contains a globular, cell-kernel (nucleus), as clear as glass, which again encloses a nucleolus. Out of the fertilised egg of the Star-fish (Fig. A 1) there develops in the first place, by the repeated sub-division of cells, a globular mass of homogeneous cells (Fig. 6, vol. i. p. [299]), and this changes into a very simple nurse, which has almost the same shape as a wooden shoe (Fig. A 2-D 2). The edge of the opening of the shoe is bordered by a fringe of cilia, the ciliary movements of which keep the microscopically small and transparent nurse swimming about freely in the sea. This fringe of cilia is marked in Fig. A 2-A 4, on Plate [VII]., by the narrow alternately light and dark seam. The nurse then, in the first place, forms a perfectly simple intestinal canal for nutrition, mouth (o), stomach (m) and anus (a). Later, the windings of the fringe of cilia become more complicated, and there arise arm-like processes (Fig. A 3-D 3). In sea-stars (A 4) and sea-urchins (C 4) these arm-like processes, which are fringed with cilia, afterwards become very long. But in the case of sea-lilies (B 3) and sea-cucumbers (D 4), instead of this, the fringe of cilia, which at first, through winding in and out, forms one closed ring, changes subsequently into a succession of separate ciliated girdles, one lying behind the other.
In the interior of this curious nurse there then develops, by a non-sexual process of generation, namely, by the formation of internal buds or germ-buds (round about the stomach), the second generation of Star-fishes, which later on become sexually ripe. This second generation, which is represented on Plate IX. in a fully developed condition, exists originally as a stock or cormus of five worms, connected at one end in the form of a star, as is most clearly seen in the sea-stars, the most ancient and original form of the star-fishes. The second generation, which grows at the expense of the first, appropriates only the stomach and a small portion of the other organs of the latter, but forms for itself a new mouth and anus. The fringe of cilia, and the other parts of the body of the nurse, afterwards disappear. The second generation (A 5-D 5), is at first smaller or not much larger than the nurse, whereas, by growth, it afterwards becomes more than a hundred times, or even a thousand times, as large. If the ontogeny of the typical representatives of the four classes of Star-fishes be compared, it is easily seen that the original kind of development has been best preserved in sea-stars (A) and sea-urchins (C) by inheritance, whereas in sea-lilies (B) and sea-cucumbers it has been suppressed according to the laws of abbreviated inheritance (vol. i. p. [212]).