Sponges inhabit every sea and shore, and differ very much in habit of growth. For whilst some can only be obtained by dredging at considerable depths, others live near the surface, and others, again, attach themselves to the surfaces of rocks and shells between the tide marks. Like the corals, they revel 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. The fine collection of West Indian sponges exhibited in the Crystal Palace, but to which fancy must add the additional ornament of colour, may serve to give some idea of their prodigal versatility of growth. More than sixty different species have been discovered in the British waters alone, and as they go on increasing in numbers, size, 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. The branched sponges, with a compact feltred tissue, are more common than others in the colder maritime domains, where the species of a loose texture, which grow in large massive forms, either do not exist or are very rare. Many sponges are of considerable size, such as the vase-like tropical species known under the name of Neptune's cup; others are almost microscopical; and while by far the greater number grow superficially from a solid base, some penetrate like destructive parasites into the texture of other animals. Thus the Halichondria celata establishes itself in the small holes which some of the smaller annelides drill in the shell of the oyster, eat further and further into the unfortunate mollusc's vitals, causing the softer parts of the shell to rot away, and spread through its whole substance, like the dry-rot fungus through a solid beam of timber, until, sinking under the weight of his misery, the poor victim perishes, and his loosened shell is cast to the mercy of the waves. On the other hand, some marine Acorn-shells nestle habitually in a sponge, the normal construction of the base of the shell being altered to suit the peculiarities of its habitation, so that in this instance, as in many others, there is a foreseen relation between two very dissimilar animals. Amongst the reticulated fibres of its spongy dwelling, the Acorn-shell finds a secure refuge in its infant state, and is soon enclosed by the growing fabric of the sponge-animal, except a small opening, which is kept clear by the vortex occasioned by the constant motion of its feelers or tentacula.
But very few of the manifold species of sponges are of any use to man. The common sponge of commerce (Spongia communis), so serviceable in our households, is most abundant in the Lycian seas, where it is found attached to rocks at various depths between three fathoms and thirty. When alive, it is of a dull bluish black above, and dirty white beneath. There are several qualities, possibly indicating as many distinct species.
"The most valued kinds," says Edward Forbes, "are sought for about the Gulf of Macri, along the Carian coast, and round the opposite islands. The species which live immediately along the shore near the water's edge, though often large, are worthless. These are of many colours; some of the brightest scarlet or clear yellow form a crust over the faces of submarine rocks; others are large and tubular, resembling holothuriæ in form and of a gamboge colour, which soon turns to dirty brown when taken out of the water; others are again lobed or palmate, studded with prickly points, and perforated at intervals with oscula. These grow to a considerable size, but, like the former, are useless, since their substance is full of needles of flint."
Large quantities of excellent sponge (Spongia usitatissima) are likewise imported from the West Indies.
[CHAP. XIX.]
MARINE PLANTS.
The Algæ.—Zostera marina.—The Ulvæ and Enteromorphæ.—The Fuci.—The Laminariæ.—Macrocystis pyrifera.—Description of the Submarine Thickets at Tierra del Fuego.—Nereocystis lutkeana.—The Sargasso Sea.—The Gathering of edible Birds'-nests in the marine Caves of Java.—Agar-Agar.—The Florideæ.—The Diatomaceæ.—Their importance in the economy of the Seas.
The dry land develops the most exuberant vegetation on the lowest grounds, the plains and deep valleys, and the size and multiplicity of plants gradually diminish as we ascend the higher mountain regions, until at last merely naked or snow-covered rocks raise their barren pinnacles to the skies: but the contrary takes place in the realms of ocean; for here the greater depths are completely denuded of vegetation, and it is only within 600 or 800 feet from the surface that the calcareous nullipores begin to cover the sea-bottom, as mosses and lichens clothe the lofty mountain-tops. Gradually corallines and a few algæ associate with them, until finally about 80 or 100 feet from the surface begins the rich vegetable zone which encircles the margin of the sea. The plants of which it is composed do not indeed attain the same high degree of development as those of the dry land, being deprived of the beauties of flower and fruit: but as the earth at different heights and latitudes constantly changes her verdant robe, and raises our highest admiration by the endless diversity of her ornaments, thus also the forms of the sea-plants change, whether we descend from the brink of ocean to a greater depth, or wander along the coast from one sea to another; and their delicate fronds are as remarkable for beauty of colour and elegance of outline, as the leaves of terrestrial vegetation.