METAZOANS: CHARACTERISTICS, TYPES, HABITATS AND DISEASES - BIOLOGY - 2025

The metazoan, or animals, are multicellular eukaryotic organisms that develop from embryonic leaves and are unable to synthesize their own food. In classical taxonomy, the animal kingdom was divided into two large groups, the Protozoa and the Metazoa.

The Protozoa comprised the unicellular "animals", while the Metazoa group contained the multicellular animals. By excluding protozoa from the animal kingdom, the term metazoa became synonymous with all animals.

Diversity of metazoan (animalia). Taken and edited from: User: Stemonitis

Apparently, metazoans developed from colonial choanoflagellate organisms. This theory is supported by similarities observed in ribosomal RNA, mitochondria, and the constitution of flagella in both groups.

There are other theories that try to explain the origin of metazoans from symbiotic associations between different protists, or from only multinucleated ciliated protists. However, they are less accepted by the scientific community.

characteristics

Organization

Metazoans are multicellular eukaryotic organisms. Its cells are generally functionally organized into tissues and organs, and even into organ systems. Sponges and placozoans, however, do not possess true tissues.

Reproduction

Although some groups, or some species, can present asexual reproduction, basically all metazoans present oogamous sexual reproduction. Oogamy is reproduction by means of a pair of gametes that are different in size and shape.

The smaller gamete is generally flagellated (sperm), while the larger gamete generally lacks a flagellum, which is why it lacks movement (ovum). This type of reproduction generally implies the existence of a pair of parents.

Development

Product of sexual reproduction, a zygote is obtained, which after several mitotic divisions will transform into a blastula. All metazoans are diblastic or triblastic, that is, they develop from two or three embryonic leaves.

The diblastic organisms present ectoderm and endoderm (for example the cnidarians), while the triblastic organisms additionally present a mesoderm between these two embryonic leaves (for example the chordates).

Nutrition

The nutrition of metazoans is heterotrophic; that is, they must feed on organic matter already produced. They are unable to synthesize their own food from inorganic matter by photosynthesis or by chemosynthesis.

Metabolism

Metazoans are almost exclusively metabolically aerobic. That is, they require oxygen to carry out their vital processes.

Cell structure and composition

All metazoans lack a cell wall and use collagen as a structural protein. They also lack chloroplasts, so they cannot photosynthesize.

Symmetry

Animals are, with the exception of sponges, organisms of radial or bilateral symmetry. This means that they present one (bilateral symmetry) or many (radial) imaginary cutting planes that divide the organism into two equal and opposite halves.

Movement

Even when there are species with limited or no movement capacity, one of the most outstanding characteristics of animals is their wide movement capacity. This characteristic, however, is not exclusive to animals.

Types and examples

There are several ways to divide the different types of metazoans. One of the most traditional ways is to divide them according to the presence, or absence, of the spinal column. In this way two groups are obtained: vertebrates and invertebrates. These two groups are widely used for their practicality; however, they lack taxonomic validity.

According to the current order, at least 35 animal phyla are taxonomically recognized, from the Porifera to the chordates. All these phyla have invertebrate representatives, since vertebrates are only a subphylum of the chordate phylum. Some of the best known phyla are the following:

Porifera

Porifers are the most primitive metazoans. Its level of organization, according to some authors, is tissue. Others however argue that sponges lack true tissues. Most sponges also lack symmetry, although a few may show radial symmetry.

Its name comes from the fact that they have numerous pores in their body (ostioli), through which water penetrates thanks to the action of cells called choanocytes. There are about 5500 described species, all aquatic and the vast majority marine. Examples: Ircinia and Cliona.

Porifera, Ircinia sp. Taken and edited from: Zoe Richards et al

Cnidaria

Cnidarians are metazoans that have radial symmetry and develop from two embryonic leaves (diblastic). They have two different body shapes, a polypoid shape and a medusoid shape.

Some groups present alternation of generations between both body forms, while in other groups only one of the forms is present.

These organisms do not have cephalization (they lack a head), nor do they have a respiratory, circulatory or excretory system. The digestive system, for its part, is represented by a sac-like structure, with a single opening through which food enters and through which undigested waste is expelled.

They are aquatic organisms, almost exclusively marine, with about 10,000 known species. Among the representatives of this phylum are corals, anemones, sea fans and jellyfish.

Annelida

Annelids are a group of segmented worms that are characterized, among other aspects, by having a coelomic cavity formed by a process called schizocelia (schizocoelomates), having bilateral symmetry, a closed circulatory system, and excretion by metanephridia.

There are more than 16,000 species of annelids that can be terrestrial, marine or freshwater. These include earthworms, leeches, and polychaetes.

Arthropoda

This is the most diverse and abundant group within the metazoans. More than three-quarters of the known animals belong to this phylum, with more than a million species described. Its characteristics include a segmented body and the presence of a chitinous exoskeleton with articulated appendages.

Among the arthropods are mosquitoes, flies (insects), centipedes (chilopods), millipedes (diplopods), pan crabs (xiphosuros), sea spiders (pycnogonids), crabs, shrimp, lobsters (crustaceans), among others.

Arthropod, Macrobrachium amazonicum. Taken and edited from: Jonathan Vera Caripe.

Mollusca

Metazoa not segmented, with bilateral symmetry, which in some groups may be lost secondarily. Cephalization may be present (cephalopods) or absent (bivalves). The body is generally covered by a calcareous exoskeleton that can be bivalve, conical, or spiral-shaped.

Among the mollusks are clams (bivalves), chitons (polyplacophores), elephant tusks (scaphopods), snails (gastropods) and squid and octopus (cephalopods), among others.

Echinodermata

Metazoans with an internal skeleton composed of calcareous spicules, they lack kafalization and generally present radial symmetry in their adult stage. They present an aquifer vascular system, which is exclusive to the members of this phylum.

These organisms have a diffuse, non-centralized nervous system, and lack an excretory system. Some 7000 current species are known, including, for example, sea cucumbers (holothuridae), sea urchins and sand dollars (echinoids), starfish (asteroids), sea lilies (crinoids) and sea spiders (ofiuros).

Chordata

A metazoan phylum that has both invertebrate and vertebrate animals. They are characterized, among other aspects, by presenting, at some stage of their development, a hollow dorsal nerve cord, a notochord and pharyngeal gill slits.

There are just under 50,000 known chordate species, including sea squirts (urochordates), amphyoxes (cephalochordates), and also humans (vertebrate).

Habitat

Except for a few extreme habitats, where only prokaryotic organisms are capable of thriving, metazoans can be found anywhere.

Aquatic environments

Virtually all animal phyla have some marine representative. In fact, some are exclusive or almost exclusive to these environments. In the sea, organisms can live associated with the substrate (benthic) or with the water column (pelagic).

Metazoans can be found from the surface zone, to the greatest ocean depths (hadal zone). Most marine species remain in this habitat throughout their lives, while others can be maintained during stages of their life cycle in the terrestrial environment or in fresh water.

In the rivers also inhabit diverse animal species, although not as many as in the seas.

Terrestrial environment

Metazoans can be found from the intertidal zone (coastal areas of seas, rivers and lakes) to the highest altitudes and from the tropics to the poles. The vast majority of species that live in terrestrial environments are exclusive to this type of habitat, as they required deep adaptations to be able to conquer it.

However, some species, such as amphibians, or some arthropods, can spend part of their life between the terrestrial environment and fresh water. Other species, such as turtles and seabirds and some species of crustaceans, spend part of their life on land (although it is very brief, as in sea turtles) and part in the sea.

Although some metazoans, mainly birds and arthropods, have managed to conquer air spaces, no animal can spend its entire life cycle in the air.

Very few species of metazoans can survive long at temperatures above 50 ° C or below 0 ° C.

Diseases

Even though some metazoans can cause diseases, mainly helminths, in most cases they are vectors of diseases and not the true causes of them.

Transmitted

Metazoans can be vectors for diseases caused by viruses, protists, fungi, bacteria, and other metazoans. This should include sexually transmitted diseases such as syphilis, human papillomavirus, or acquired immunodeficiency virus.

Arthropods are vectors of a wide variety of diseases that affect human beings, for example anthrax, cholera, chikungunya, malaria, sleeping sickness, Chagas disease, or yellow fever, among others.

Rabies disease is another disease transmitted by animals, transmitted in this case by the bite of a mammal that suffers from the disease.

Produced

Parasitic metazoans can also cause disease in humans. The main responsible for this type of disease are helminths and arthropods. These diseases include taeniasis (cestodes), schistosomiasis (trematodes), onchocerciasis, filariasis, elephantiasis (nematodes), scabies (arthropods).

Mites are responsible for respiratory diseases (asthma, allergic rhinitis) and skin (atopic dermatitis).

Filariasis cycle. Taken and edited from: Image credit: CDC / Alexander J. da Silva, PhD / Melanie Moser. (PHIL # 3425), 2003

References

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