- Sponges are sessile aquatic animals, mostly marine and largely inhabitants of hard substrates.
- They are primitive in their lack of organs, in¬cluding mouth and gut. There are different kinds of cells, but cellular differentiation has not fol¬lowed the common designs of other animals. 90 The Sponges
- The bodies of sponges are organized around a system of water canals, a specialization correlated with sessility.
- The small, vase-shaped asconoid body form, in which flagellated choanocytes line an interior atrial chamber, is the primitive sponge form. The evolution of the common leuconoid form, in which the flagellated cells are distributed within a vast number of minute chambers, has permitted the at-tainment of much larger size and great diversity of shape, since each addition to the sponge body brings with it all of the units necessary to provide the required additional water flow.
- The growth form of sponges is, in part, an adaptive response to the availability of space, the inclination of the substrate, and the current velocity.
- Support is provided by a skeleton of organic spongin fibers or siliceous or calcareous spicules, or a combination of spongin fibers and siliceous spicules.
- Feeding, gas exchange, and waste removalde pend on the flow of water through the body. Thi ability of the choanocyte collar to remove d tremely small particles from the water stream has probably been an important factor in the long, successful history of sponges.
- Probably because of their sessility, mosi sponges are hermaphroditic. Sperm leave Offi sponge and enter another in the currents flowing through the water canals. Eggs in the mesophylan fertilized in situ. They may then be released by wq of the water canals or brooded up to the larva stage. In most sponges the flagellated larva is a blastula, and reorganization equivalent to gastrulatioin occurs following settling.
- Sponges are probably an early evolution! side branch that gave rise to no other groups of animals.