Since water is vital to sponges for excretion, feeding, and gas exchange, their body structure facilitates the movement of water through the sponge. Structures such as canals, chambers, and cavities enable water to move through the sponge to nearly all body cells.
The morphology of the simplest sponges takes the shape of a cylinder with a large central cavity, the spongocoel , occupying the inside of the cylinder. Water can enter into the spongocoel from numerous pores in the body wall. Water entering the spongocoel is extruded via a large common opening called the osculum. However, sponges exhibit a range of diversity in body forms, including variations in the size of the spongocoel, the number of osculi, and where the cells that filter food from the water are located.
While sponges excluding the hexactinellids do not exhibit tissue-layer organization, they do have different cell types that perform distinct functions. Pinacocytes , which are epithelial-like cells, form the outermost layer of sponges and enclose a jelly-like substance called mesohyl. Mesohyl is an extracellular matrix consisting of a collagen-like gel with suspended cells that perform various functions. The gel-like consistency of mesohyl acts like an endoskeleton and maintains the tubular morphology of sponges.
In addition to the osculum, sponges have multiple pores called ostia on their bodies that allow water to enter the sponge. In some sponges, ostia are formed by porocytes, single tube-shaped cells that act as valves to regulate the flow of water into the spongocoel. In other sponges, ostia are formed by folds in the body wall of the sponge. Whereas pinacocytes line the outside of the sponge, choanocytes tend to line certain inner portions of the sponge body that surround the mesohyl.
The structure of a choanocyte is critical to its function, which is to generate a water current through the sponge and to trap and ingest food particles by phagocytosis.
Note the similarity in appearance between the sponge choanocyte and choanoflagellates Protista. This similarity suggests that sponges and choanoflagellates are closely related and likely share a recent common ancestry.
The cumulative effect of the flagella from all choanocytes aids the movement of water through the sponge: drawing water into the sponge through the numerous ostia, into the spaces lined by choanocytes, and eventually out through the osculum or osculi. In the meantime, food particles, including waterborne bacteria and algae, are trapped by the sieve-like collar of the choanocytes, slide down into the body of the cell, are ingested by phagocytosis, and become encased in a food vacuole.
Lastly, choanocytes will differentiate into sperm for sexual reproduction, where they will become dislodged from the mesohyl and leave the sponge with expelled water through the osculum. Watch this video to see the movement of water through the sponge body. The second crucial cells in sponges are called amoebocytes or archaeocytes , named for the fact that they move throughout the mesohyl in an amoeba-like fashion.
Amoebocytes have a variety of functions: delivering nutrients from choanocytes to other cells within the sponge, giving rise to eggs for sexual reproduction which remain in the mesohyl , delivering phagocytized sperm from choanocytes to eggs, and differentiating into more-specific cell types. Spicules can be made of silica or calcium carbonate. Skip to main content. Phylum Porifera Sponges Sponges are considered the oldest animal group. Features: Asymmetrical Organized as a collection of different kinds of specialized cells No tissues Skeleton lacking or made of spicules Porifera fact: Many sponges can filter their entire body volume in less than one minute.
The presence and composition of spicules form the basis for differentiating three of the four classes of sponges Figure 2. Sponges in class Calcarea produce calcium carbonate spicules and no spongin; those in class Hexactinellida produce six-rayed siliceous glassy spicules and no spongin; and those in class Demospongia contain spongin and may or may not have spicules; if present, those spicules are siliceous.
Sponges in this last class have been used as bath sponges. Spicules are most conspicuously present in the glass sponges, class Hexactinellida. Some of the spicules may attain gigantic proportions. For example, relative to typical glass sponge spicules, whose size generally ranges from 3 to 10 mm, some of the basal spicules of the hexactinellid Monorhaphis chuni are enormous and grow up to 3 meters long! The glass sponges are also unusual in that most of their body cells are fused together to form a multinucleate syncytium.
Because their cells are interconnected in this way, the hexactinellid sponges have no mesohyl. A fourth class of sponges, the Sclerospongiae, was described from species discovered in underwater tunnels. These are also called coralline sponges after their multilayered calcium carbonate skeletons. Dating based on the rate of deposition of the skeletal layers suggests that some of these sponges are hundreds of years old.
Figure 2. Several classes of sponges a Clathrina clathrus belongs to class Calcarea, b Staurocalyptus spp. Improve this page Learn More. Skip to main content. Many of the free-standing sponges are well known to most people. For example, nearly everyone has heard of the barrel sponge, a large tropical sponge which sometimes grows large enough to fit a whole person inside. Equally well known are the tube sponges of the tropics, coming in nearly every color of the rainbow.
While not all sponges are as colorful or as large as those found in the tropics, sponges are an ancient and efficient design which will probably continue to populate the world's oceans longer than people will populate the Earth.
Check out some more sponge pictures here. This barrel sponge is spawning. All images on these pages for non-profit educational use only.
Diagram of a simple sponge Although they may look plant-like, sponges are the simplest of multi-cellular animals. A non-toxic yellow dye has been squirted around the base of a purple tube sponge in the Caribbean.
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