Sponges lack organs and tissue, and all the cells exhibit considerable independence. The sponge is made up of two single-cell-deep layers and an intermediate mesohyl (mobile cells plus extracellular matrix). The outer (sac) layer consists of flattened polygonal cells called pinacocytes. The middle (mesohyl) layer consists of gelatinous protein/carbohydrate material, a range of mobile cells, and a skeleton of calcareous or siliceous spicules, or of elastic proteinaceous fibers called spongin fibers. The inner layer consists of flagelled cells called collar cells, or choanocytes.
The body is permeated by numerous pores called ostia that open into inhalant canals that lead to the feeding chambers, which are made up of choanocytes; here also are large openings, termed oscules, fed by exhalant canals, that carry the water current from the choanocyte chambers to the exterior. The concerted whipping action of the choanocyte flagella creates a current of water from ostia through the sponge body oscules. The choanocytes filter plankton and small bits of organic detritus from the water and, like the pinacocytes, absorb oxygen. Food is digested in ameboid archaeocytes that pick up food vacuoles from the choanocytes, which ingest the mainly particulate food. Waste products are carried out through the osculum.
Different types of amoebocyte spongiocytes and sclerocytes are responsible for secreting the skeletal material. Achaeocytes give rise to egg cells and sperm derive from choanocytes. The body of most sponges is irregular in form, although an almost radial symmetry is displayed by some. Three types of sponge structure are recognized: the asconoid, the most primitive, is regular, tube-shaped, and radially symmetrical; the syconoid is a more irregular structure that displays some degree of folding of the body wall while still maintaining a basic radial symmetry; the leuconoid is highly irregular, displays the greatest degree of folding of the body wall, and has lost radial symmetry. In the leuconoid sponges choanocytes line the pockets formed by the convoluted body wall.
Sponges are limited in size by the rate at which water can flow in and out of the spongocoel, bringing in food-bearing water and oxygen and removing waste products. Because the asconoid type has the smallest surface area, sponges of this structure are among the smallest in the phylum; leuconoid sponges, with a large amount of surface area, represent some of the largest members of the phylum.
Pieces of sponge are able to regenerate into whole new sponges. Asexual reproduction occurs by budding or by fragmentation. The buds may remain attached to the parent or separate from it, and each bud develops into a new individual. Freshwater sponges, as well as several marine species, form resistant structures called gemmules that can withstand adverse conditions such as drying or cold and later develop into new individuals. Gemmules are aggregates of sponge tissue and food, covered by a hard coating containing spicules or spongin fibers. Sexual reproduction also occurs. Most sponges are hermaphroditic, the same individual producing eggs and sperm, but in some species the sexes are separate. The larvae are flagellated and swim about freely for a short time. After settling and attaching to a suitable substrate, the larvae develop into young sponges.
Sponges in this class are typified by skeletal spicules composed of calcium carbonate. The spicules often protrude through the epipinecodermal covering of the body wall, giving the organism a rough texture. Calcareous sponges are small, usually only a few inches high, and are generally dull in appearance, although several species are brightly colored. Members of this class are among the simplest sponges, and all three morphological types—asconoid, syconoid, and leuconoid—are represented. There are approximately 150 known species, exclusively marine and shallow-water dwellers.
These are deep-sea sponges. They lack an epidermal covering, and their skeletons are composed of spicules of silica. The spicules, which often form a latticework, have six points or some multiple thereof. Glass sponges are pale in color and are cup- or basket-shaped. The spongocoel is large, and the osculum is covered by a grillwork of fused spicules. When the living tissue is removed, the cylindrical skeletons often have the appearance of spun glass. The glass sponge known as Venus's-flower-basket (Euplectella) supplies a home for certain shrimps that become trapped by the lattice of spicules. The body plan of Hexactinellida is between syconoid and leuconoid.
Most sponges belong in this class. It includes sponges with a skeleton made up of silicon-containing spicules or spongin fibers or both. In the latter case, the spongin provides a matrix in which the spicules are embedded. The Demospongiae vary in size from small, encrusting forms to very large, irregular masses. All are leuconoid; many are brightly colored. The freshwater sponges (family Spongillidae) belong to this class; they are frequently green because of symbiotic algae that live in the amoebocytes. The fibrous sponges also belong to this class; they include the common bath sponges, Hippospongia communis and Spongia officinalis, and most of the other sponges used commercially. The boring sponges (family Clionidae) are extremely interesting because of their ability to bore into calcareous rocks and mollusk shells. They begin their boring as larvae and spend their lives in the tunnels they form. Sulfur sponges (Cliona species) are bright yellow boring forms inhabiting shallow waters on the east and west coasts of the United States.