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Invertebrates are animals without a backbone or vertebral column. Vertebral column is made up of small bones called vertebrae.

The scientific name assigned to all animals without backbones is invertebrate, meaning without vertebrae

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Invertebrates are ectotherms (cold-blooded). Most invertebrates live in water or spend at least some part of their life in water.

Body symmetry is one of the important characteristics of the invertebrtaes; cnidarians have radial symmetrical bodies, while arthropods have bilateral symmetrical bodies.

Classification of Invertebrates

Invertebrates are categorized into eight major categories, called phyla (Figure 1).

Phylum Protozoa includes microscopic unicellular eukaryotes like amoeba, paramecium.

Phylum Porifera contain animals with pores bodies known as sponges.

Phylum Coelenterata, also called Cnidaria includes animals such as hydra, jellyfish, sea anemone, and corals. These animals are characterized by having special stinging organs called nematocysts.

Phylum Platyhelminthes includes flatworms like tapeworms.

Phylum Nematoda includes bilaterally symmetrical round worms like hookworm and pinworm.

Phylum Arthropoda includes animals with exoskeleton such as insects, crustacean, arachnid, myriapod.

Phylum Annelida includes segmented worm such as ringworm, earthworm, and leeches.

Phylum Mollusca include snails, slugs, scallop, octopus, and clams. They are called mollusks and are have soft body and shell.

Phylum Echinodermata consist of a group of animals with external spines, such as starfish, sea urchin, and sea cucumber.

Classifiaction of Invertebrates
Figure 1. Classification of Invertebrates

Table showing differences and similarities between Invertebrates and Vertebrates

Characteristics Invertebrates Vertebrates


Animalia Animalia


 Chordata  Chordata
Symmetry Bilateral and radial Bilateral
Physical Multicellular, no backbone, no cell wall Multicellular, backbone present, Advanced brain, neurons and integumentary system
 Nutrition  Parasitic and heterotrophic  Heterotrophic
 Size  Small and slow moving  Big
 Heart and circulatory system  Dorsal heart, open circulatory system  Ventral heart, closed blood vessel system
 Reproduction  Asexual and sexual  Sexual
 Species  Constitute 98% of the animal species  Constitute 2% of the animal species
 Examples  Sponges, insects, worms  Fish, snakes, birds, mammals

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