Classes
Sponges were traditionally distributed in three classes: calcareous sponges (Calcarea), glass sponges (Hexactinellida) and demosponges (Demospongiae). However, studies have shown that the Homoscleromorpha, a group thought to belong to the Demospongiae, is actually phylogenetically well separated. Therefore, they have recently been recognized as the fourth class of sponges.
Sponges are divided into classes mainly according to the composition of their skeletons:
| Type of cells | Spicules | Spongin fibers | Massive exoskeleton | Body form | |
|---|---|---|---|---|---|
| Calcarea | Single nucleus, single external membrane | Calcite May be individual or large masses |
Never | Common. Made of calcite if present. |
Asconoid, syconoid, leuconoid or solenoid |
| Glass sponges | Mostly syncytia in all species | Silica May be individual or fused |
Never | Never | Leuconoid |
| Demosponges | Single nucleus, single external membrane | Silica | In many species | In some species. Made of aragonite if present. |
Leuconoid |
| Homoscleromorpha | Single nucleus, single external membrane | Silica | In many species | Never | Sylleibid or leuconoid |
Read more about this topic: Sponge
Famous quotes containing the word classes:
“Genocide begins, however improbably, in the conviction that classes of biological distinction indisputably sanction social and political discrimination.”
—Andrea Dworkin (b. 1946)
“My plan of instruction is extremely simple and limited. They learn, on week-days, such coarse works as may fit them for servants. I allow of no writing for the poor. My object is not to make fanatics, but to train up the lower classes in habits of industry and piety.”
—Hannah More (1745–1833)
“... too much attention is paid to dress by those who have neither the excuse of ample means nor of social claims.... The injury done by this state of things to the morals and the manners of our lower classes is incalculable.”
—Mrs. H. O. Ward (1824–1899)