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.