Parazoa facts for kids

Parazoa is the animal sub-kingdom that includes organisms of the phyla Porifera and Placozoa. Sponges are the most well-known parazoa. They are aquatic organisms classified under the phylum Porifera with about 15,000 species worldwide. Although multicellular, sponges only have a few different types of cells, some of which may migrate within the organism to perform different functions.

The three main classes of sponges include glass sponges (Hexactinellida), calcareous sponges (Calcarea), and demosponges (Demospongiae). Parazoa from the phylum Placozoa include the single species Trichoplax adhaerens. These tiny aquatic animals are flat, round, and transparent. They are composed of only four types of cells and have a simple body plan with just three cell layers.

Sponge Parazoa Barrel Sponge - Parazoa Gerard Soury/Stockbyte/Getty Images Sponge parazoans are unique invertebrate animals characterized by porous bodies. This interesting feature allows a sponge to filter food and nutrients from water as it passes through its pores. Sponges can be found at various depths in both marine and freshwater habitats and come in a variety of colors, sizes, and shapes. Some giant sponges can reach heights of seven feet, while the smallest sponges reach heights of only two-thousandths of an inch.

Their varied shapes (tube-like, barrel-like, fan-like, cup-like, branched, and irregular shapes) are structured to provide optimal water flow. This is vital as sponges do not have a circulatory system, respiratory system, digestive system, muscular system, or nervous system as do many other animals. Water circulating through pores allows for gas exchange as well as food filtration. Sponges typically feed on bacteria, algae, and other tiny organisms in water. To a lesser degree, some species have been known to feed on small crustaceans, like krill and shrimp. Since sponges are non-motile, they are typically found attached to rocks or other hard surfaces.

Sponge Body Structure Sponge Body Structure Adapted from work by Philcha/ Wikimedia Commons/CC BY Attribution 3.0 Body Symmetry

Unlike most animal organisms which exhibit some type of body symmetry, such as radial, bilateral, or spherical symmetry, most sponges are asymmetric, exhibiting no type of symmetry. There are a few species, however, that are radially symmetrical. Of all the animal phyla, Porifera are the simplest in form and most closely related to organisms from the kingdom Protista. While sponges are multicellular and their cells perform different functions, they do not form true tissues or organs.

Body Wall

Structurally, the sponge body is studded with numerous pores called ostia that lead to canals for channeling water to internal chambers. Sponges are attached at one end to a hard surface, while the opposite end, called the osculum, remains open to the aquatic surroundings. Sponge cells are arranged to form a three-layered body wall:

Pinacoderm - the outer surface layer of the body wall that is equivalent to the epidermis of higher animals. The pinacoderm consists of a single layer of flattened cells called pinacocytes. These cells are able to contract, thus reducing the size of a sponge when needed. Mesohyl - thin middle layer that is analogous to connective tissue in higher animals. It is characterized by a jelly-like matrix with collagen, spicules, and various cells embedded within. Cells called archaeocytes found in the mesohyl are amebocytes (cells capable of movement) that can transform into other sponge cell types. These cells aid in digestion, nutrient transport, and are even capable of developing into sex cells. Other cells called sclerocytes produce skeletal elements called spicules that provide structural support. Choanoderm - The inner layer of the body wall consisting of cells called choanocytes. These cells contain a flagellum, which is surrounded by a collar of cytoplasm at its base. Through the beating movement of the flagella, water flow is maintained and directed through the body. Body Plan

Sponges have a particular body plan with a pore/canal system that is arranged into one of three types: asconoid, syconoid or leuconoid. Asconoid sponges have the simplest organization consisting of a porous tube shape, an osculum, and an open internal area (spongocoel) that is lined with choanocytes. Syconoid sponges are larger and more complex than asconoid sponges. They have a thicker body wall and elongated pores that form a simple canal system. Leuconoid sponges are the most complex and largest of the three types. They have an intricate canal system with several chambers lined with flagellated choanocytes that direct water flows through the chambers and eventually out the osculum.

Sponge Reproduction Spawning Sponge Reinhard Dirscherl/WaterFrame/Getty Images Sexual Reproduction

Sponges are capable of both asexual and sexual reproduction. These parazoans reproduce most commonly by sexual reproduction and most are hermaphrodites, that is, the same sponge is capable of producing both male and female gametes. Typically only one type of gamete (sperm or egg) is produced per spawn. Fertilization occurs as sperm cells from one sponge are released through the osculum and carried by water current to another sponge.

As this water is propelled through the receiving sponge's body by choanocytes, the sperm is captured and directed to the mesohyl. Egg cells reside in the mesohyl and are fertilized upon union with a sperm cell. In time, the developing larvae leave the sponge body and swim until they find a suitable location and surface on which to attach, grow, and develop.

See also

In Spanish: Parazoa para niños