ACELLOMATES: BIOLOGICAL CHARACTERISTICS, EXAMPLES OF SPECIES - BIOLOGY - 2025

The acellomates are animals that lack a cavity inside the body and their organs are supported by a set of cells called mesenchyme.

In the group corresponding to eumetazoan animals that present bilateral symmetry, there is a classification that groups them according to the characteristics of the internal body cavity into: acellomates, pseudocoelomates and coelomates.

By Eduard Solà, from Wikimedia Commons

These three groups described have no taxonomic value and are only used to describe a specific body plan model. This body cavity, called coelom, is completely surrounded by mesoderm, remembering that triblastic animals have three germ layers called ectoderm, mesoderm and endoderm.

Note that in the case of diblastic animals (such as cnidarians) they have only two germ layers and therefore there is no coelom. However, this group is not considered part of the acellomates since the term applies exclusively to animals that have a mesoderm.

The animals that conform to the body plan of an acellomate are flatworms (from the Greek platys meaning "flat" and helmis "worm"). To this phylum belong a series of vermiform animals - which means that they resemble a worm in shape - with a defined anterior end on a head and include free-living and parasitic species.

Biological characteristics

The acellomates comprise a simple animal group with bilateral symmetry characterized mainly by the absence of the coelom.

A typical cellulose organism does not have a digestive cavity, but instead has an intestinal cavity surrounded by a mass of tissue derived from endoderm and a mass of tissue derived from mesoderm. In addition, they may have cilia in the epithelium.

In contrast, a coelomed animal exhibits an intestinal cavity surrounded by a layer of mesodermal cells, occupied by fluid.

In the lineages after the flatworms, the coelom begins to develop, where the liquid present cushions the organs preventing any type of injury. Furthermore, said liquid present in the coelom is not compressed and for this reason it can fulfill the role of a hydrostatic skeleton.

According to phylogenetic analyzes, true coeloms and pseudocoeloms have been acquired and lost multiple times in the course of animal evolution.

Cellophane: flatworms

The acellomed organisms belong to the group of animals with the shapes of worms. Currently, there are two phylum of animals without a coelom: the phylum Acoelomorpha and the phylum Platyhelminthes.

Phylum Platyhelminthes

Organisms belonging to the Phylum Platyhelminthes are commonly known as flatworms. They measure approximately one millimeter, although some species can reach lengths greater than one meter. They can inhabit marine environments, fresh waters and humid terrestrial areas.

The Phylum is divided into four classes: Turbellaria, Trematoda, Monogenea, and Cestoda. The peat bogs are free-living and all members of the three remaining classes are parasites.

Among the free life forms we find the famous planaria and in parasitic individuals the flukes and tapeworms stand out. Free-living individuals present a ciliated epidermis, in contrast to a syncytial integument that covers the parasitic forms.

The muscular system is of mesodermal origin mainly and the circulatory, respiratory and skeletal systems are absent. Some forms have lymphatic channels and an excretory system with protonephridiums.

Phylum Acoelomorpha

Members of the phylum Acoelomorpha were in the class Turbellaria within the phylum Platyhelminthes. Now, two orders of the peat bogs, Acoela and Nemertodermatida, are present as two subgroups in the phylum Acoelomorpha.

This phylum includes approximately 350 species of small worm-shaped organisms, less than 5 mm in length. They live in marine environments, locating in sediments or in pelagic regions, although some species have been reported living in brackish water.

Most are free-living, although there are some species that live as parasites of other organisms. They present a cellular epidermis with cilium, which form an interconnected network. They do not have a respiratory or excretory system.

An important difference between Acoelomorpha and Platyhelminthes is the presence of only four or five Hox genes in the first group, while flatworms have seven or eight. These genes control the specific pattern of body structures.

Examples of species

Planaria

Organisms of the genus Planaria are the typical flatworms of the class Turbellaria. They are characterized by having cilia on the body surface that helps them create a "turbulence" in the water while they move and from there comes the name of the group.

References

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