On the margin of the atolls, close within the line where the coral is washed by the tide, three species of Nullipores flourish; they are beautiful little plants, very common in the coral islands. One species grows in thin spreading sheets, like a lichen; the second in stony knobs as thick as a man’s finger, radiating from a common centre; and the third species, which has the colour of peach blossom, is a reticulated mass of stiff branches about the thickness of a crow’s quill. The three species either grow mixed or separately, and, although they can exist above the line of the corals, they require to be bathed the greater part of each tide: hence a layer two or three feet thick, and about twenty yards broad, formed by the growth of the Nullipores, fringes the circlet of the atolls and protects the coral below.
The lagoon in the centre of these islands is supplied with water from the exterior by openings in the lee side of the ring, but as the water has been deprived of the greater part of its nutritious particles and inorganic matter by the corals on the outside, the hardier kinds are no longer produced, and species of more delicate forms take their place. The depth of the lagoon varies in different atolls from fifty to twenty fathoms or less, the bottom being partly detritus, partly live coral. In these calm and limpid waters the corals are of the most varied and delicate structure, of the most charming and dazzling hues. When the shades of evening come on, the lagoon shines like the milky way with myriads of brilliant sparks. The microscopic medusæ and crustaceans invisible by day form the beauty of the night, and the sea-feather, vermilion in daylight, now waves with green phosphorescent light. This gorgeous character of the sea bed is not peculiar to the lagoons of the atolls; it prevails in shallow water throughout the whole coral-bearing regions of the Pacific and Indian Oceans.
Encircling reefs differ in no respect from the atoll ring, except in having islands in their lagoons, surrounded also by coral reefs. Barrier reefs are of the same structure as the atoll rings, from which they only differ in their position with regard to the land. They form extensive lines along the coasts, from which they are separated by a channel of the sea of variable depth and breadth, sometimes large enough for ships to pass. A very long one runs parallel to the west coast of New Caledonia, and stretches for 120 miles beyond the extremities of the island. But a barrier reef off the northeastern coast of the Australian continent is the grandest coral formation existing. Rising at once from an unfathomable depth of the ocean, it extends for a thousand miles along the coast with a breadth varying from 200 yards to a mile, and at an average distance of from 20 to 60 or 70 miles from the coast, the depth of the channel being from 10 to 60 fathoms. The pulse of the ocean, transcendently sublime, beats perpetually in peals of thunder along that stupendous reef, the fabric of almost microscopic beings.
SECTION V.
ANNULOSA, OR WORMS.
The Annulosa, which are the lowest grade of articulated animals, consist of four distinct orders: the Entozoa, which are muscle and intestine parasites; the Turbellariæ, fresh and salt-water animals covered with cilia; the Annelida, or Worms; and the Rotifers, or Wheel animalcules.
Entozoa.
There are three genera and numerous species of Entozoa. Every animal has one or more species peculiar to itself; fourteen infest the human race. They have a soft, absorbent body of a white or whitish colour, in consequence of being excluded from light, and living as they do by absorbing the vitalized juices of the animals they infest. Their nutritive system is in the lowest state of development; yet there are some of a higher grade. All are remarkable for their vast productiveness.
The Tænioïdæ, which belong to the inferior group, are intestinal, many-jointed worms, which have neither mouth nor digestive organs; and what is called the head has only hooks and suckers to fasten it to the internal membrane of the animal at whose expense it lives. The common Tænia, or Tape-worm, sometimes ten feet long, which is the type of the order, has four suckers and a circle of hooklets round a terminal proboscis to attach it to its victim. Though destitute of the organs of nutrition it is extremely prolific, for each segment of its long flat body is a reproductive monœcious zooid, which forms and lays its own eggs exactly as if it were a single independent animal, thus furnishing a very remarkable instance of the law of irrelative repetition, which is a series of organs performing the same functions independently of one another. Two pairs of canals containing a clear colourless liquid extend throughout the body of the worm.
Bags, or vesicles called cysts, had been found in the glands and muscles of various animals, afterwards discovered to contain young worms, which attain their perfect development within such creatures as eat the flesh containing the cysts. Under circumstances so unprecedented, it required no small skill and patience to determine the life-history of these singular creatures. The cysts differ in size and form according to the genera, and are embedded singly or in groups in the flesh of their victim, on whose ready prepared juices they live.
The greater number of the Tænia genus begin their lives as sexless cysted larvæ, and on entering their final abode, segments are successively added till the worm has arrived at its adult state. The tape-worm of the cat has its origin in the encysted larvæ found in the livers of the mouse and rat. One species of Entozoa, while in its primary state, inhabits the stomach of the stickleback, and only comes to perfection within the aquatic birds that feed on this fish. Another species infests the livers of the salmon tribe, and gets its perfect form in the pike and perch.