The Parazoa animals are multicellular organisms.

They represent invertebrates lacking a vertebral column.

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This consists of the simplest group of organisms in the animal kingdom that is the least advanced in evolution.

These animals live in both freshwater and marine water.

The phylum Porifera and Placozoa belong to this subkingdom.

The phylum Porifera has around 15,000 species.

They lack a proper mouth, and their digestion is intracellular.

They are mostly filter-feeding animals.

They lack circulatory, respiratory, muscular, and nervous systems. Excretion occurs by simple diffusion.

The body wall of parazoa animals contains several pores called Ostia. It has an outer epidermal layer called pinacoderm, mesohyl is the middle layer having a jelly-like matrix and choanoderm is the inner layer having choanocytes series of water canals.

Several animals in this group are asymmetrical while some exhibit radial symmetry.

Some organisms in this subkingdom have regeneration property. An example is ground sponges.

Animals do not produce any specialized reproductive organs but sperm and ovum are produced. They reproduce sexually exhibiting both larval and adult stages. The larvae are planktonic that resembles a blastula while adults are sessile.

Classification of Parazoa animals:
The subkingdom of this animal kingdom is divided into two phyla which include,

Phylum Porifera: The Phylum Porifera refers to sponge animals. This term is derived from Latin porus – pore and fero – bear. These represent animals that lack true tissues and tissue-level organization. An example is sponges. The cells are loosely organized in sponges. Sponges are chiefly recognized in freshwater and marine water. Sponges are commonly found in deep-sea as well as shallow waters. They also hold on to rock surfaces. These animals internalize and process the solids and liquid food particles by the process of holozoic nutrition. Sponges are animals that firmly attach to the surface, therefore are sedentary. But larvae move using ciliated epithelium. There are altogether 5,000 species of which 150 species live in freshwater while the remaining thrives in seawater.

Sponges can be structurally classified into three types: Asconoid type – A simple sponge, E.g. Leucosolenia; Syconoid type – Complex than ascon type, example: Sycon; Leuconoid type – most complex large sponge, example: Euspongia.

Sponges are attached to the hard surface of rock or sea bed at one end and osculum at the opposite end. The pinacocyte covers the sponge that phagocytizes large food particles. The porocyte controls the flow of water through Ostia. The choanocyte generates a water current that filters food particles from water.