Fig. 112. Thaumantia pilosella.
The Thaumantia pilosella, a member of the naked-eyed group, is like an inverted watch-glass ([fig. 112]), less than an inch in diameter. The roof of this umbrella is much thicker than the sides, and gradually thins off towards the rim. The proboscis, or stomach, descends from the centre of the sub-umbrella, but not so far as to the edge of the rim: it ends in a mouth with four sensitive fleshy lips. Four slender canals, which originate in the cavity of the stomach, radiate from the centre of the roof of the umbrella and extend to its margin, where they unite at the quadrants with a canal which encircles the rim, and are prolonged beyond it in the form of tentacles armed with numerous thread-cells containing poisonous darts. These tentacles must be formed of muscular fibre, for they are very irritable: each of them may be extended and contracted separately or along with the others; they guide the medusa through the water, and can anchor it by twisting round a fixed object.
The prey caught is digested in the stomach, the refuse is ejected by the mouth, and the nutritious fluid that has been extracted is carried up through the base of the stomach into the four radiating canals, to supply the waste and nourish the system. The digestive cavity and canals are lined with a soft membrane, covered with cilia, whose vibrations maintain the circulation of the juices and perform the duty of a heart; for the medusæ have none, nor have they any special respiratory system: their juices are aërated through the under-surface of the rim of the umbrella, while passing through the circular canal lying either within the water or on its surface.
A fringe of filamental tentacles hangs down into the water from the rim of the disc or umbrella, which is studded at equal distances by fleshy bulbs, each of which has a group of fifty dark eye-specks, being the rudiment of an eye; and if the animal be disturbed when in the dark, each eye-speck shines with a brilliant phosphoric light, and the umbrella looks as if it were begirt with a garland of stars.
Fig. 113. Otolites of Magnified Thaumantias.
Close to the edge of the canal which encircles the margin of the umbrella, there are eight hollow semi-oval enlargements of the flesh, two in each quadrant formed by the four radiating canals: they are the eight ears of the medusa, for in these hollow organs there are from thirty to fifty solid, transparent, and highly refractive spheres, arranged in a double row, so as to form a crescent, those near its centre being larger than the more remote. The solid spheres are analogous to the otolites in the ears of the more highly organized animals. Mr. M‘Cready has discovered nerve-centres behind each tentacle, and under each marginal coloured speck in several species of the open-eyed medusæ, which places this group of Acalephæ in a higher grade than any of the preceding orders. The medusæ swim by the muscular energy of their umbrellas: at each rhythmical contraction the water, which enters by the mouth and fills the great central cavity within the umbrella, is forced out again through an orifice at the other end, and by its reaction the medusa is impelled with considerable velocity in the contrary direction, so that the top of the umbrella goes first, and all its tentacles are dragged after it.
The medusæ are diœcious: in the males four reproductive cells full of reddish or purple granular matter surround the cavity of the stomach, and appear like a coloured cross through the top of the gelatinous umbrella. In the females, at a point just before the four radiating canals enter the marginal canal, the flesh on the exterior of the umbrella swells out into bulbs, containing vessels full of clear eggs with minute globular yolks. These eggs, when fertilized, are hatched, and the young are developed within these ovaries, so that they come into the water as a kind of infusorial ciliated animalcule destitute of a mouth. One end of the creature acquires a suctorial disc, fixes itself to an object, and uses its cilia. The other end opens into a mouth, round which tentacles like fishing lines spring forth; the central part is converted into the cavity of the stomach, and thus a perfect hydra is formed, capable of being propagated naturally by budding, or artificially by being cut in pieces, each piece becoming a perfect hydra, differing in no respect from a common simple fresh-water Hydra.
