多孔动物门门

海绵曾被认为是所有动物的最近共同祖先中最先分化的一支，在整个动物界的演化树上位于最基部，和其他所有动物（即真后生动物）形成姐妹群关系，但目前的研究指出栉水母比海绵更早出现。动物学中专门研究海绵的分支学科被称作海绵学（spongiology）。

下级分类

依据世界海洋物种目录（World Register of Marine Species，WoRMS），多孔动物门包含以下类群：

钙质海绵纲 Calcarea（= Calcispongia／Calcispongiae）

寻常海绵纲／普通海绵纲 Demospongiae（= 硬骨海绵纲 Sclerospongiae）

六放海绵纲 Hexactinellida（= 玻璃海绵纲 Hyalospongiae）

• 同骨海绵纲 Homoscleromorpha
• 地位未定 incertae sedis
• 肉质海绵目 Carnosa（非单系，已弃用）
• 属 Placodictyum ?（可能属于海参）

Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'). They are filter feeders that are typically bound to the seabed, being sessile. Sponges are one of the most ancient members of living seabed community (macrobenthos), with many species being important reef-builders. Some sponge species inhabit freshwater.

Sponges are multicellular organisms consisting of jelly-like mesohyl sandwiched between two thin layers of cells, and usually have tube-like bodies full of pores and channels that allow water to circulate through them. They have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. They do not have complex nervous, digestive or circulatory systems. Instead, most sponges rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove waste products, usually via flagellar movements of the so-called "collar cells".

Sponges may be the sister group to all other animals, although the evidence for this relationship remains inconclusive, and different studies may recover different placements between sponges and other animal clades. Fossil evidence of primitive sponges such as Otavia dates to as early as the Tonian period (around 800 Mya). The branch of zoology that studies sponges is spongiology. Historically, the skeletons of sea sponges were used as the cleaning tool of the same name, though this is no longer a common practice.

Etymology

The term sponge derives from the Ancient Greek word σπόγγος spóngos. The scientific name Porifera is a neuter plural of the Modern Latin term porifer, which comes from the roots porus meaning "pore, opening", and -fer meaning "bearing or carrying".