CHAPTER XV.

SPONGES.

Their Animal Nature-Their remarkable Structure-Their Skeleton-Spicula -Sensibility and Spontaneous Movements-Their Mode of Propagation-Their Importance in the Household of the Seas.

THE Sponges, which were formerly supposed to belong to the world of plants, have been proved by modern researches to form a peculiar group of Protozoa. Attached to a solid base, they revel like the polyps in every variety of shape and tint, imitate like them every form of vegetation, and adorn like them the submarine grounds with their fantastic shrubberies. More than sixty different species have been discovered in the British waters alone, and as they go on, increasing in number and beauty until they attain their highest development along the shores of the Tropical Ocean, they no doubt hold a conspicuous rank among the living wonders of the sea.

Portion of a Sponge (Halichondria) with and in some species entirely spicules projecting from the fibrous

network.

replace the horny fibre, though

they are still so arranged as to preserve the reticulated

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character. These spicules are of a wonderful variety and elegance of form, for their shapes are not only strictly determinate for each species of sponge, but each part of the sponge, it is believed, has spicula of a character peculiar to itself. Sometimes they are pointed at both ends, sometimes at one only, or one or both ends may be furnished with a head like that of a pin, or may carry three or more diverging points, which sometimes curve back so as to form hooks. Sometimes they are triradiate, sometimes stellar, in some cases smooth, in others beset with smaller spinous projections like the lance of the saw-fish. As they are generally composed of flint, it may well be imagined that our household sponge entirely owes its value to their absence in its highly flexible structure. The sponge-skeleton is covered externally and along the internal surfaces of the canals with a gelatinous or slimy substance, similar to that which constitutes the body of the Rhizopod, and which, seemingly inert and unorganised, is yet the seat of whatever life the sponge contains. It is by this slime, which may be pressed out with the finger, that the net-work is deposited, and from it the whole growth of the mass proceeds.

On examining a sponge, the holes with which the substance is everywhere pierced may be seen to be of two kinds: one of larger size than the rest, few in number, and opening into wide channels and tunnels which pierce the sponge through

Currents passing inwards through the pores (a.a), traversing the internal canals (6), and escaping by the larger vents (c.d).

its centre; the other minute, extremely numerous, covering the wide surface, and communicating with the innumerable branching passages which make up the body of the skeleton.

Through the smaller openings or pores the circumambient water freely enters the body of the sponge, passes through the smaller canals, and ultimately reaching the larger set of vessels, is evolved through the larger apertures or oscula. Thus by a still mysterious agency (for the presence of cilia has as yet been detected but in one genus of full-grown marine sponges), a constant circulation is kept up, providing the sponge with nourishing particles and oxygen, and enabling its system of channels to perform the functions both of an alimentary tube and a respiratory apparatus.

Dr. Grant describes in glowing terms his first discovery of this highly interesting phenomenon. Having put a small branch of sponge with some sea-water into a watch-glass, in order to examine it with the microscope, and bringing one of the apertures on the side of the sponge fully into view, I beheld for the first time the spectacle of this living fountain vomiting forth from a circular cavity an impetuous torrent of liquid matter, and hurling along, in rapid succession, opaque masses, which it strewed everywhere around. The beauty and novelty of such a scene in the animal kingdom long arrested my attention, but after twenty-five minutes of constant observation, I was obliged to withdraw my eye from fatigue, without having seen the torrent for one instant change its direction, or diminish in the slightest degree the rapidity of its course.'

Subsequent observations have proved that the living sponge has the power of opening and closing at pleasure its oscula, which are capable of acting independently of each other, thus fully establishing the animal nature of these simple organisations, in whom latterly even traces of sensibility have been detected, such as one would hardly expect to meet with in a sponge. For these creatures, as we are entitled to call them, are able to protrude from their oscula the gelatinous membrane which clothes their channels, and on touching these protruded parts with a needle, they were seen by Mr. Gosse to shrink immediately, a proof that the sponge, however low it may rank in the animal world, is yet far from being so totally inert or lifeless as was formerly imagined.

The propagation of the sponges is provided for in a no less wonderful manner than their respiration and nourishment. Their young eggs or sporules germinate on the sides of the

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canals, forming innumerable minute bud-like points. These, as they increase in size, are gradually clothed with vibratile cilia, and finally detaching themselves are cast out through the oscula into the world of waters. Here their wanderings continue for a short time, until, if they be not devoured on the way, they reach some rock or submarine body, on which, tired of their brief erratic existence, they fix themselves for ever, and bidding adieu to all further rambles, lead henceforth the quiet sedentary life of their parents.

In this manner the sponges, which otherwise would have been confined to narrow limits, spread like a living carpet over the bottom of the seas, and in spite of their being utterly defenceless maintain their existence from age to age. At the same time they serve to feed a vast number of other marine animals, for the waters frequently swarm with their eggs, and these afford many a welcome repast to myriads of sessile shells, worms, polyps, and other creatures small or abstemious enough to be satisfied with feasting on atoms.

CHAPTER XVI.

SEA-ANEMONES AND LITHOPHYTES.

Submarine Gardens.-Internal Structure of the Sea-anemones. - Tentacles. Urticating Organs.-Their Remarkable Tenacity of Life.-Their Modes of Locomotion.-Lithophytes.-Social Republicans.-Coral-islands.

WHO has ever sojourned on a rocky coast, worn and hollowed by the breakers of a thousand years, without admiring the crystal tide-pools, those charming relics of the receding flood, so full of all that can fascinate the naturalist, or enchant the poet. For the calm and transparent waters of these miniature lakes harbour a little world of animals and plants of such wonderful variety and elegance of form that the eye never tires of gazing on their loveliness, and the memory reckons them ever after among the chief beauties of the beautiful ocean. There, bathed in liquid crystal, delicate sea-weeds spread their graceful fronds, or clothe the naked rock with a velvet carpet; there annelides, and crustaceans, and molluscs of all forms and colours, reposing, wandering, darting, creeping, or swimming, enliven the ever-changing scene, and there, not the least ornament of these fairy gardens, the radiate Sea-anemones, emulating the daisies of the fields, expand their lustrous disks.

Desirous of plucking one of these elegant flowers of the ocean, you extend your hand, but at the slightest touch its beautiful coronet begins to curl and pucker its margin, and to incurve it in the form of a cup. If further annoyed, the rim of this cup contracts more and more, until the animated blossom, now transformed into a shrivelled shapeless mass, and receding all the time from the rude assault, retires under the cover of its rocky fortress, or clings with such tenacity to the stone to which it is attached that you will sooner tear it to pieces than make it forego its grasp.