Sponge

Sponge, the skeleton of the animals forming the Porifera, a class of animals belonging to the phylum Coelenterata (q.v.). The members of the Porifera differ very markedly from the remaining Coelenterata by their very varied and variable shape; they form irregular masses, the individual members of which have no tentacles and thus appear far less highly organised than the compound Anthozoa (q.v.). A few, however, have a definite shape. This is typically shaped like a funnel, and is shown in its simplest form in such a genus as Leucosolenia; in others it becomes more complex, and consists of a tubular structure supported by a trellis-work of silica, as in the exquisite "Venus Flower Basket" (Euplectella); in others the sponge consists of a fleshy, creeping mass, as in Homoderma, the type-genus of the Homodermidae; in others (the family Clionidae) the animal lives in borings in shells. In attempting to form an idea as to the structure and affinities of a sponge it should be regarded as composed of a large number of cells belonging to two different layers, separated by the gelatinous, irregular material known as "mesogloea." The two layers are the ectoderm and endoderm; the former consists of flattened cells, and forms the external layer of the body. The endoderm is typically composed of larger cells, each with whip-like process or flagellum, which rises from the centre of a neck-like extension or collar; so these cells resemble a rounded bottle with a short, thick neck, with a cord rising from the centre of the neck. The endoderm lines a series of tubes, which ramify through the mass of the sponge; the typical "collared cells" may line the whole length of these (in the Homocoela) or be restricted to special parts of the tubes or bladder-like expansions of them known as ampullae (as in all but this order). These tubes may open to the exterior by a series of pores, which occur in great abundance over the whole surface of the sponge; they may be all of the same size, or some may be small (micropores) and others large (macropores). The pores are typically "inhalent" - i.e. the currents of water which bring the sponge its food and fresh water for respiratory purposes all enter the sponge by them; but in those sponges which have not also a series of larger exhalent apertures known as "oscula" some of the pores have to allow of the escape of the surplus water. The oscula begin as a single large aperture, which usually divides into many small ones. The tubes which ramify through the sponge form the "gastric cavity," which may consist of (1) a simple, central, vase-shaped cavity, as in such primitive forms as the Asconidae ; (2) this central cavity may be replaced by a series of radial exhalent canals as in the Syconidae; (3) it may consist of a series of canals with ampullae - i.e. pear-shaped or spherical expansions. The skeleton of the sponge consists of a series of "spicules" or small rods formed by cells in the mesogloea; they are composed either of a soft but tough material allied to silk and known as "spongin," as in the common sponge (Euspongia); of silica, as in the Venus' Flower Basket; or of carbonate of lime, as in the sub-class Calcarea. The spicules are of four main types: (1) monaxile - simple rods, curved or straight; (2) triaxile - composed of three rods crossing, and thus usually six-rayed, as in Hexactinellidae; (3) tetraxile - composed of four rods, but one of these is often suppressed - these occur in the Lithistidae; (4) polyaxile - of many axes, and forming stellate or globular spicules; these are free. The spicules are either united to one another by interlocking spines, or by spongin or the deposition of carbonate of lime or silica; or they may be free, and are then known as "flesh spicules." The sponges are classified in the main, according to the structure of the skeleton and the nature of the gastric cavity. By the first character they are divided into two sub-classes - the Calcarea, in which the skeleton is composed of carbonate of lime; and the Fibrospongiae or Non-calcarea, in which the skeleton is composed of silica or spongin, or is absent. The Calcarea are divided into two orders, according to the second character; thus in the Homocoela the gastric cavity is lined throughout with collared cells, and in the Heterocoela the collared cells occur only in special portions of the gastric cavity, either in radiating tubes or special bladder-like expansions known as ampullae. The first order includes the three families of the Asconidae, Homodermidae, and Leucopsidae; the latter consists of the families of Syconidae, Sylleibidae, Leuconidae, and Teichonidae. The Non-Porifera are divided into three orders:- (1) The Hyalospongiae, including all the Hexactinellida, which are very abundant as fossils, and range from the Cambrian period onward; the group is characterised by the possession of a siliceous skeleton composed of six-rayed spicules often united into a lattice-like tissue. The "Venus' Flower Basket," or Euplectella, is the best-known living species. (2) The Spiculispongiae, in which the skeleton is composed of siliceous spicules which are either four-rayed or consist of a single spine; these are often quite disconnected. It includes five sub-orders: the Lithistina, in which the skeleton is massive, and the spicules united to one another - this group is very important to the geologist; Tetractina, with four-rayed and one-ray spicules; Oligosilicina, with only small isolated flesh spicules; Pseudotetraxonia, with one-rayed and flesh spicules; and the Clavulina, including the boring Sponges, Cliona, (3) The Cornacuspongiae, with spicules if present only of a single spine, united by spongin, or formed only of spongin fibres. This includes two sub-orders: the Halichrondrina, of which the freshwater sponge (Spongilla) is the best-known form; and the Keratosa, including all the soft sponges of commerce. The systematic position of the sponges has been much debated, but embryological evidence is conclusive as to their being Coelenterata. The sponges are all marine, except the two freshwater genera, Spongilla and Meyenia. The others occur in all seas and at all depths, but the Hexactinellidae usually occur in deep water. The sponges vary in size from minute bodies, 1/20 inch in length, to enormous masses. Many species occur round the English coasts. The sponges used for washing purposes live in shallow water in warm seas; the best ones come from the Levant and Greek archipelago, but the largest part of the supply is yielded by the West Indian fisheries, especially around the Bahamas. Owing to the indestructibility of the spicules, the sponges are of great importance to the geologist, for they are abundant as fossils from the Cambrian period onward. Their spicules often occur in such abundance on "fossil sponge banks" as to form thick beds of siliceous rock, such as the cherts of the Lower Greensand.