On Molecular and Microscopic Science, Volume 2 (of 2)

by Mary Somerville

Fig. 114. A, B, C, D, development of Medusa-buds; a, polype-body; b, tentacles; c, a secondary circle of tentacles; d, proboscis; e, new polype-bud.

From one of these, numberless successive generations of simple hydræ may be produced by budding, all catching their prey with their tentacles and digesting it in their stomachs. The limits to this budding-system seems to be indefinite: years may pass in this stage, but at length it ceases, and either the original hydra, or one of its descendants, undergoes a series of remarkable changes. The body of the hydra lengthens into a cylinder; it is then marked transversely by a number of constrictions beginning at the free end; these become deeper and deeper, till at length they break up the body into a pile of shallow cups, each lying in the hollow of the other, and leaving a kind of fleshy wall at the point of suspension or fixture. The edges of the cups are divided into lobes with a slit in each, in which the coloured rudiment of the eye is sunk. The cups are permanent, and characteristic of the group of naked-eyed medusæ. After a time, the cups begin to show contractile motions, which increase till the fibre of their attachment is broken, and then the superimposed cups are detached from the pile one after another, and swim freely away by the contractions of their lobes as young medusæ, leaving what remains of the parent hydra to repair its loss and again repeat this singular process. However, the young medusæ are not yet perfect. As they increase in size the divisions on the edge of the cup fill up; a proboscis-shaped stomach, with its four coloured cells and its square mouth, is developed from the centre of the sub-umbrella; the radiating canals extend from the central cavity, the encircling canal and fringe form round the umbrella-shaped cups, and the result is a highly organized Thaumantia pilosella, in whose life-history a simple hydra forms a singular stage.

Thus hydræ produce medusæ whose offspring are hydræ, and perfect medusæ produce hydræ whose offspring are perfect medusæ. However, the law of the alternation of generation is by no means peculiar to the Thaumantiæ. Many species of medusæ are subject to it, as the Turris neglecta, a beautiful little medusa not larger than a hempseed, common on the British coasts. It has a white muscular pellucid umbrella, a large proboscis of a rich orange colour at its upper part: in the orange-coloured flesh of it there are ovaries containing rose-coloured eggs, which are hatched within them, and come into the water as ciliated gemmules, which, after swimming about for a time, become fixed and are developed into small hydræ of a rich purple colour with sixty-four tentacles. From these hydræ others bud off indefinitely till the time comes when one of them becomes lengthened, constricted, divided into cups which drop off, and finally become a brood of the Turris neglecta.

The naked-eyed medusæ are extremely numerous. There are six orders of them and many genera, chiefly distinguished by the position and nature of their ovaries and the number of canals which radiate through their swimming organs. Both of the medusæ that have been described have four radiating canals; yet they belong to different orders, for the ovaries of the Thaumantia are in the edge of the umbrella, while those of the Turris are in the substance of the proboscis. Neither of these kinds have more than four ovaries, but some other kinds have eight ovaries and eight radiating canals. Most of the canals are simple, but in one genus they are branching. All are furnished with tentacles, some of them having stings, others none.

The covered-eyed group consists only of two natural divisions—the Rhizostoma, or many-mouthed medusæ, and the Monostoma, or one-mouthed medusæ. In both the coloured eye-specks at the margin of the umbrella are larger and more numerous, than in the naked-eyed group, and they are covered with a hood. The proboscis of the one-mouthed order terminates in a square mouth, the four angles of which are prolonged into tentacles with a solid hyaline axis. They have a fringed membrane along their under-surface, containing numerous stinging thread-cells. Sixteen canals, connected with the stomach or cavity of the proboscis, radiate over the flattish, cup-shaped umbrella; eight of these are branched, and terminate in the circular canal which runs round its fringed edge, and they form the nutrient and respiratory system of the animal, while the eight simple and alternate canals terminate in eight openings at the rim of the umbrella, through which the refuse or indigestible part of the food is discharged, thus forming an exception to the other pulmograde medusæ, and indeed to the Hydrozoa in general, which eject it at the mouth. All the canals are lined with cilia, whose vibrations maintain the circulation of the fluids, and perform the duties both of a heart and respiratory apparatus. Dr. A. Krohn has observed that in three species of the genus Pelagia belonging to the covered-eyed medusæ, the young are at once developed as medusæ without the intervention of the hydra form.

Fig. 115. Rhizostoma.

The disk of the Rhizostoma, or root-mouthed medusæ, is rather flat, and the large proboscis is unlike any other of the tribe. In the naked-eyed medusæ digestion is performed in the cavity of the proboscis; but in this order the proboscis is divided into four very long branches ending in club-shaped knobs ([fig. 115]), and nutrient tubes extend to their extremities from the great central cavity in the umbrella. Their broadish frilled borders are divided and subdivided along their whole lengths, and the nutrient canals, which follow all their ramifications, end in numerous fringed pores upon their edges and upon the club-shaped ends of the quadrifid proboscis. These numerous pores are mouths; they absorb minute animalcules, which are digested while passing through the united canals to the great central cavity of the umbrella, which receives the products of digestion. Eight canals radiate from that great cavity and traverse the umbrella; and the nutrient fluid, mixed with the sea-water, passes from the great cavity through these canals into an elegant network of large capillary tubes spread on the under-surface of the margin of the umbrella, which is always in contact with the water; and in this beautiful respiratory organ the carbonic acid gas is exchanged for the oxygen in the water of the sea. The indigestible part of the food is discharged through the mouths or pores, whose edges are prolonged into solid tentacles containing thread-cells, with their usual weapons of offence and defence. Besides these armed tentacles, which are very numerous in the covered-eyed group, the gelatinous umbrella has a multitude of oval thread-cells on its external coat, in each of which a very long filament is spirally coiled, which darts out to a considerable distance on the smallest touch, and stings severely.

A few only of the British pulmonigrade medusæ sting: the Cyanea capillata, one of the single-mouthed covered-eyed family, is most formidable. It has very long tentacles, which it can throw off if they get entangled, but they continue to sting, even after they are detached from the medusa.

This is one of the most remarkable instances of the inherent irritability of muscular fibre still in full force after the tentacles have been separated from the living animal. In many of the lower animals, as in the Hydra itself, vitality is so far from being extinguished in the severed members that it repairs the injury. Since the covered-eyed medusæ have eyes, ears, and very sensitive tentacles, it may be inferred that they possess nerves of sight, hearing, and touch, though none have been discovered, probably on account of the softness and transparency of their tissues. The stinging power by which they kill their prey and defend themselves may be classed among the consensual powers prompted by the sympathetic sensations of hunger or danger.