The phylum Porifera contains the simplest creatures of the animal kingdom. Sponges are aquatic sessile organisms (they are unable to move by themselves and they remain attached to substrates). They do not have tissue diversity and their bodies have pores (the feature form which their name is derived).
They are multicellular, like all organisms of the animal kingdom.
Sponges live exclusively in aquatic environments and are attached by their base to a substrate (fixation ground). Sponges are filtering animals, meaning that they nourish themselves from nutrients that enter their atrium brought in by water.
The shape of sponges is normally that of a tube or a globe with an opening at the top end. They have an internal central cavity and porous walls. The central cavity is called the spongocoel and the opening at the upper extremity is called the osculum.
Sponges are filtering organisms. They make water enter their bodies through their lateral pores. Water then circulates inside the central cavity and exits through the osculum.
Sponges can close their pores to stop water from entering their body in the presence of stimulus that may may indicate danger. However, this method is rudimentary and is actually a form of protection against harmful agents.
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Sponges contain an outer wall covered by flat cells called pinacocytes as well as pores surrounded by special cells called porocytes. Their internal wall is filled with choanocytes, flagellate cells specialized in the phagocytosis of food brought to the central cavity. The choanocyte flagella also maintain the water flow inside the sponge.
Between the outer and the inner sections of the body of a poriferan, there are cells with amoeboid movement (via pseudopods) called amoebocytes. Since they are embedded in connective tissue, amoebocytes move and distribute nutrients to other cells as well as produce spicules that fill the tissue and support the body structure like a primitive skeleton. (Some poriferans have an internal skeleton, which is an endoskeleton, made of spicules and organic fibers.)
Sponges are different from other animals in that they can only use intracellular digestion. They do not have a digestive system nor do they release digestive enzymes in the spongocoel to cause the extracellular break down of nutrients.
Apart from sponges, which do not have a digestive cavity where extracellular digestion takes place, all other animals have a digestive system with an internal cavity in which extracellular digestion occurs.
The gas exchange in sponges happens through diffusion from the exterior to the cells that absorb molecular oxygen and release carbon dioxide.
Sponges do not have a nervous, circulatory or excretory system.
Reproduction in sponges can be asexual by budding, gemmation or fragmentation (regeneration) or sexual with a larval stage (a ciliated amphiblastula larva).
Sexual reproduction in sponges, in addition to contributing to genetic diversity, also facilitates the colonization of additional environments by these organisms, since sperm cells and larvae are mobile and can swim in the exterior to compensate for the immobility of the adult.
Some chemical substances secreted by sponges have anti-inflammatory, antibiotic and anti-tumor effects and are used in the production of medicine. Since ancient times, the endoskeleton of some sponges has had a commercial value, being used as a personal hygiene product (bath sponges), as well as to wash animals, objects and so on.
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