Invertebrate animals are those that lack a dorsal spinal cord, vertebral column, and endoskeleton. Simply put, invertebrates do not have a backbone. They are generally small and have some type of protective structure or exoskeleton, such as shells.
According to taxonomic classifications, invertebrates are all those animals that do not fall within the subphylum Vertebrata. In turn, these animals belong to the chordata phylum of the animal kingdom.
Approximately 95% of the living species known to date belong to this group, which is why they represent the greatest biodiversity on the planet.
Due to their small size and in many cases, difficult location, the study of invertebrate animals was relegated for centuries. It was only in the 18th century that its potential in terms of scientific research began to be considered. Mainly due to the interest of the French naturalist Jean-Baptiste Lamarck, who dedicated himself to the study of invertebrate animals. He also was the first to classify them into 10 groups.
However, advances in zoology made it possible to make a new classification that is the one currently used:
- Arthropods
- Mollusks
- Porifera (sponges)
- Cnidarians
- Echinoderms
- Platyhelminthes (flatworms)
- Nematoda
- Annelida
Types of Invertebrates
1. Arthropods
They are invertebrates characterized by the presence of an exoskeleton. Their external skeleton molts in the growth phase. They also have articulated legs, a body segmented into two or three regions, with repetitive patterns.
Arachnids (mites, scorpions, and spiders), insects (ants, butterflies), myriapods (such as centipedes), and crustaceans (crabs, prawns) belong to this group.
2. Mollusks
They have a soft body, in some cases protected by a shell. Their bodies are symmetrical and without segmentation. Clams, octopuses, squid and oysters are some representatives of this group.
3. Porifera (sponges)
Marine sponges are simple invertebrate animals that live in aquatic habitats. Water leaves their body through a large opening at the top called the osculum. It enters through smaller pores located in the walls of its body.
4. Cnidarians
These are marine invertebrate animals with a sponge-like body and have a single opening that serves as a mouth and anus. These animals usually have one or more tentacles. Jellyfish, corals, and polyps are cnidarian invertebrates.
5. Echinoderms
These marine animals have pentaradial symmetry. This means they have an odd number of body parts arranged in circular fashion. They have an exoskeleton formed by limestone plates that, in some cases, have spikes.
The stars and sea urchins represent the group of echinoderms invertebrates.
6. Platyhelminthes (flatworms)
Also known as flatworms, they are the simplest animals with interneurons. Most are hermaphrodites and are in fact a parasitic life form, requiring one or more hosts during their life cycle.
Parasites like tapeworms are the best known species of flatworms.
7. Nematoda
Also known as cylindrical or round worms, these invertebrates are characterized by having a body without segmentation, with muscles that allow their movement. Some nematodes are transmitting agents of intestinal diseases.
The Necator americanus is an intestinal parasite, of the group of nematodes, causing a disease called hookworm infection.
8. Annelida
These invertebrate animals are characterized by metamerism. That is, they have linear series of body segments fundamentally similar in structure. Certain organs are repeated in each segment. Leeches and earthworms are two kinds of annelids.
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