Sponges

Sponges are the simplest of the multicellular animals. They have no organ systems and are characterized by numerous canals and chambers that open to the outside by way of pores which giving this phylum its name. Many sponges are asymmetrical, but some exhibit radial symmetry. They are almost plant-like in their simplicity and are often confused as being plants.

The body plan of a typical sponge is shown in figure1. Water (carrying suspended plankton and other potential food) enters numerous small pores called ostia (singular, ostium). The ostia are surrounded by donut-shaped cells called porocytes that open and close to control water flow. For the sponge depicted in figure 1, water flows directly into an open chamber called the spongocoel (the term "coel" refers to an open space or body cavity in an animal). Water leaves the spongocoel by a larger opening (the osculum).

Figure 1. General body plan of a simple sponge. (A) part of a colony, (B) longitudinal section including details

of choanocytes and an amoebocyte. The inside of the sponge (spongocoel) is to the left in B.

The interior of the spongocoel is lined with flagellated cells called choanocytes (or collar cells; Fig1). The choanocytes have a tubular collar facing the spongocoel. A flagellum extends from the center of this collar, the movement of which creates currents that force water through the sponge's "plumbing system". Suspended food particles (plankton, larvae, etc) in the water are drawn through the collar from below, trapped on the outside of the collar, and then phagocytized.

The collar cells do not digest the captured food by themselves; instead it is passed to a second cell (an amoebocyte) waiting in the mesohyl (the acellular portion of the sponge). Amoebocytes carry the food to other cells that require nutrition and thus partly compensate for the sponge's lack of a circulatory system.

Amoebocytes carry on other jobs. They can, for example, undergo developmental changes to turn into any other cell type that may be required. This allows for growth, repair, and reproduction of the sponge. In addition, they are responsible for producing the sponge's skeleton (a network of fibers flexible protein (spongin) and needle-like spicules). Spicules are usually made of calcium carbonate or oxides of silicon and the shape of the spicules is important in classification.

Most sponges are marine and reproduce through both asexual (budding and fragmentation) and sexual reproduction. Sexual reproduction is timed to environmental cues such as water temperature, tides, and the phase of the moon. During the mating season sperm are released into the water and eventually enter the porocytes of the female sponge. The sperm are later taken up by choanocytes, and then passed on to amoebocytes which carry them to egg cells located in the mesohyl. Sponges may be either dioecious or monoecious; depending on the species.

Class Demospongiae- General Structure. This is the largest of the sponge classes. They are bath sponges, which lack calcareous or siliceous spicules and is therefore soft to the touch. These sponges have spicules composed of a protein called spongin. Most of the large pores on the outer surface of this sponge are oscula (the excurrent canals). The tiny pin holes on the surface represent, for the most part, the ostia (incurrent canals).

Class Hexactinellida (Hyalospongiae). These sponges have their skeletons formed from silicon oxides which gives them a glassy appearance (their common name is "glass sponges"). Nearly all are deep-water forms, living at depths between 200 and 1000 m. Venus'-flower-basket (Euplectella) is an interesting specimen. Inside, two shrimp can be found (one is male, the other female). Numerous larval shrimp entered the sponge through the sieve plate at the top. The first pair to reach sexual maturity killed off other immature shrimp that had entered with them. In Japan Venus'-flower-baskets are given as wedding presents to symbolize lifelong devotion and fidelity (this tradition ignores, of course, the carnage as they reached maturity and the fact that the shrimp are trapped and can't escape).

Class Calcarea (Calcispongiae). These sponges have their spicules made from rocky substances such as calcium carbonate or sulfur compounds.Because of this, they are hard (at least in appearance) and are commonly known as rock sponges.