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8.1. General characteristics of invertebrates

Invertebrate Characteristics

Invertebrate animals are all those members of the Metazoan or Animalia kingdom that are not included within the vertebrate subphylum of the chordate phylum. They receive this name due to the absence of a vertebral column  or notochord and an articulated internal skeleton.

95% of animal species are invertebrates, so it is a very diverse group that includes animals with very different structures that are included in different phyla (Phylum), type or trunk.

Invertebrates are ectotherms, so they cannot keep their body temperature constant, but instead have that of the environment in which they live.

The most important groups of invertebrate animals are the following:

  • Poriferous: they have the body perforated by a system of pores and channels through which water circulates. For example, sponges.
  • Cnidarians or coelenterates: they have a gastric cavity that communicates with the outside through a single opening. Its symmetry is radial, such as polyps and jellyfish.
  • Annelids: they have an elongated and cylindrical body divided into rings. For example, earthworms , leeches.
  • Molluscs: they have a soft body that may or may not be covered (by one or two shells). For example, clams, squid, octopus, snails.
  • Arthropods: they present an exoskeleton, they have a segmented body and articulated legs. For example, arachnids, insects, myriapods, crustaceans.
  • Echinoderms: they have a calcareous exoskeleton, radial symmetry, and an ambulacral apparatus that allows them to move. Some species have spines. For example, sea ​​urchins  and starfish.

The invertebrates of the aquatic environment: radial symmetry and bilateral symmetry

The symmetry that an animal presents is the imaginary division of its body into halves, which must be equivalent to each other. Our hands, for example, are not the same, they are symmetrical. The body parts of an organism can be arranged with respect to their axis in three ways:

  • Asymmetry. The body of the living being does not show any type of symmetry, as for example, the porifera.
  • Radial symmetry. The body can be divided by different imaginary planes, resulting in similar halves. Animals with this type of symmetry are usually sedentary or not very mobile, such as cnidarians.
  • Two-sided symmetry. There is only one plane of symmetry that divides the body into two similar halves. Bilaterally symmetrical animals are adapted to move forward. This type of symmetry is linked to the cephalization (concentration of the nervous system and sense organs in the head).

Clasificación de los invertebrados