The sponges or poriferans (from Latin porus "pore" and ferre "to bear") are animals of the phylum Porifera. Sponges

The Phylum Porifera contains approximate 5,000 species of sponges. Cells from fragmented sponges can reorganize and regenerate the sponge organism, something not possible with animals that have tissues. These asymmetrical animals have sac-like bodies that lack tissues, and are usually interpreted as representing the cellular level of evolution.

Sponges are often referred to as the “missing link” between single-celled and multi-celled organisms. They are so simple that many early scientists considered them to be plants. It was not until 1765 that sponges were classified as animals.

There are no true tissues in sponges: merely specialized cell layers. Epidermal cells in sponges line the outer surface. Collar cells line the inner cavity. Beating collar cells produce water currents that flow through pores in sponge wall into a central cavity and out through an osculum, the upper opening. A 10 cm tall sponge will filter as much as 100 liters of water a day. Amoeboid cells occupy the "inner" layer, along with hardened structures known as spicules.

Sponges do not have nervous, digestive or circulatory systems. Instead most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes, and the shapes of their bodies are adapted to maximize the efficiency of the water flow.

The body of the sponge would collapse if it did not have some type of supporting structure. Some sponges have a soft network of protein fibers called spongin. Most sponges have a combination of spicules and spongin, the ratio often determines how soft or hard the sponge is. Others have tiny, hard particles called spicules. Many of these spicules also stick out of the epidermis and provide the sponge with protection.

Sponges can reproduce asexually (by budding or from fragments) or sexually. The larval stage is able to move about while the adult is stationary. Sponges produce eggs and sperm that are released into a central cavity of the sponge, in which the zygote develops into a ciliated larva.

Environmental Problems Facing Sponges 1) Over harvesting of sponges Sponges are easy to harvest and sell Efforts at mariculture generally failed because of poaching and slow growth of sponges. Populations of sponges steeply declined wherever they were harvested. Remember the Tragedy of the Commons?

2) Sedimentation of the sponge beds Since sponges are filter feeders, they require fairly clean water in order to live. Areas that have been overharvested are also the areas where heavy sedimentation has occurred. This has led to an even sharper decline in sponge populations in areas such as the Gulf of Mexico and the Mediterranean.