parazoa

Parazoa is a term used in animal biology to designate the most morphologically simple multicellular animals that lack true tissues. It traditionally includes the phyla Porifera (sponges) and Placozoa, and is used to contrast with Eumetazoa, the animals with differentiated tissues and organs. The exact placement of Placozoa within Parazoa is a matter of ongoing research, but together these groups are viewed as representing an early Branch of Metazoa.

Organizationally, parazoans exhibit cellular-level organization rather than true tissues or organs. They lack nerves, muscles, and a gut. Feeding and digestion are primarily intracellular; many species are sessile filter feeders that draw water through pores and canal systems. Sponges (Porifera) possess specialized choanocytes for water flow and food capture, and their bodies are supported by a skeleton of spicules and/or the protein spongin. The body plans of sponges vary, including asconoid, syconoid, and leuconoid forms, which differ in canal complexity.

Placozoans are extremely simple and small, with a flat, plate-like body containing only a few cell types and no distinct tissues, nerves, or mouth. They move slowly and feed by phagocytosing suspended particles from the environment.

Reproduction in parazoans occurs both sexually and asexually. Sponges commonly reproduce by budding or fragmentation and can be hermaphroditic, while placozoans reproduce through simple life cycles that involve spreading and formation of new individuals.

Parazoa is significant for studying early metazoan evolution, offering insights into the origin of multicellularity and tissue specialization.