What is porifera?

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Hint: Porifera are the lowest multicellular animals which belong to kingdom Animalia and refers to pore bearing species. It includes nearly 5000 species and is the first pore bearing multicellular animals.

Complete answer:
Porifera contains pores called Ostia and have a spongy appearance. That is why they are also called sponges. Poriferans are found attached to the substratum and thus they do not move. They can absorb and withhold fluids. Poriferans were considered as plants initially due to the green colour and their symbiotic relationship with algae. However, they were later included in the animal kingdom when their life cycle and feeding system were discovered.
Porifera are usually found in marine water with only few found in freshwater. Their cells are loosely organized with their body cylindrical in shape. They have either radial symmetry or asymmetrical. No organs are found in their body and they show cellular grade of organization. The body of Porifera consists of pores called ostia and osculum and have a central cavity called spongocoel or atrium which opens to the outside through the osculum. The scleroblast secretes spicules while spongin fibres are secreted by spongioblast.
Furthermore, poriferans reproduce by budding and fragmental and are holozoic in nature. They do not have a nervous system but have neurosensory cells. The lost parts have the ability to regenerate again. Using these pores they exchange respiratory gases and nitrogenous wastes by diffusion.

Note: Common examples of Porifera include Spongilla, Euplectella, Cliona, Hylonema, and Sycon and Phylum Porifera is classified into three classes: Calcarea, Hexactinellids and Desmospongiae.