The first animals to appear on the face of the earth are believed to have been porifers.
There are several hypotheses about the origin of animals. One of the most widely accepted proposes that they were derived from colonial flagellated protists, giving rise first to the parazoan lineage (Parazoa sub-kingdom), represented by the porifers, and then to the eumetazoan lineage.
Sponges are animals. no symmetry or with radiated symmetry, diploblastics, accelerated and without digestive cavity.
All sponges are fixed in adulthood and colonial, living in the aquatic environment (freshwater or saltwater), usually from the low tide line to depths that reach 5500 meters. They always grow attached to immersed substrates such as wood, shells, rocks, etc. Many have an almost vegetal aspect (having been considered plants for many centuries), although they can be brilliantly colored.
Its animal nature was not recognized until 1765 but its systematic position remained uncertain until 1857.
The simplicity of the structure of the sponges is such that if they are crushed and sieved to separate their cells, they may regroup and form a sponge, all similar to the original. The cells of the body of the sponges even show a degree of independence without coordination by nerve cells.
They do not therefore present true tissues or organ systems. Another intriguing aspect of sponge biology is that they are the only animals whose main body opening is exhaling. However, most sponges react to touch, especially around their main opening, although stimuli are conducted slowly, probably cell to cell.
Sponges are still organisms, but capable of moving the water around them. Suspended food particles penetrate the sponge body through microscopic pores - inhalant pores - on its side wall and filtered water is drawn through a larger opening - kiss - in the area opposite the base. In certain species, the bone may be slowly closed. The oscule is almost always above the rest of the animal's body, an important adaptation because it avoids the recirculation of water to which food and oxygen have already been removed and residues added.
The wall of the body of the sponges delimits a central cavity, the atrium or spongiocelium. In certain more complex sponges there is not just a central cavity, but a maze of channels and chambers covered with flagellate cells - vibrating chambers. Breathing and excretion are done directly by diffusion with the aquatic environment, so the sponges cannot stand stagnant water.
The body wall of sponges is made up of several cell types, supported by skeletal elements of various types:
- Pinacocytes - flattened outer lining cells forming a kind of epidermis called pinacoderm (although not a true tissue);
- Choanocytes - Flagellate cells with a collar-shaped membrane expansion lining the spongiocelium and other internal vibrating chambers of the sponges. The movement of its flagella creates the water stream that brings nutrients and gases. The nutrients are filtered through the cell's “collar”, which is not a solid structure but rather a set of small erect rods separated by spaces. Any organic particle or plant microorganism trapped in the collar is directed downward towards the cell body and endocyted, with intracellular digestion occurring in digestive vacuoles. Subsequently the nutrients are diffused to the mesogleia or cell to cell.
- Amebocytes - Free cells of various types that move through amoeboid movements, present in the mesenchyme or mesogleia (gelatinous substance located between the pinacocyte and coanocyte layers) and which are responsible for growth and regeneration capacity, as they can originate all other cell types. (except for choanocytes) and produce skeletal spicules. These cells can also transfer the nutrients present in mesogleia to the remaining cells and remove the excretion products to the spongiocelium. They are also responsible for the formation of gametes;
- Porocytes - cells having a central portion, called the inhalant portion, which crosses them side by side. They are located at regular spaces in the body wall of the sponge, through which water enters the spongiocellium. These microscopic openings can be regulated by the animal.