Invertebrate

Invertebrates are animals that neither develop nor retain a vertebral column, commonly known as a spine or backbone. This group includes nearly all animal species—estimates suggest that about 97% of all animal species are invertebrates. They belong to many different groups, or phyla, such as arthropods, molluscs, annelids, echinoderms, flatworms, cnidarians, and sponges.

Invertebrates vary greatly in size, from tiny creatures just 10 micrometers long to the enormous colossal squid, which can reach lengths of 9 to 10 meters. Despite their name, some groups traditionally called invertebrates, like Tunicata and Cephalochordata, are actually more closely related to vertebrates than to other invertebrates. This makes the category of "invertebrates" a bit special in scientific classification, as it includes many different kinds of animals that do not form a single evolutionary group.

Etymology

The word "invertebrate" comes from a Latin word meaning "joint," which refers to parts of the spine in animals that have backbones. The idea of a "joint" comes from a root word meaning "to turn," and the prefix "in-" means "not" or "without." So, invertebrates are animals without backbones.

Taxonomic significance

The term invertebrates refers to animals without a backbone. Unlike groups such as Arthropoda or Vertebrata, "invertebrates" is not a scientific category but a useful way to talk about many different animals. These animals have many different body plans; some, like jellyfish, have soft bodies, while others, like insects and crustaceans, have hard outer shells. Familiar invertebrates include sponges, worms, mollusks, and arthropods.

Number of extant species

Most of the world's animal species are invertebrates, with insects making up the largest group. According to the IUCN Red List of Threatened Species in 2014, there are about 66,178 extant vertebrate species that have been described. This means that more than 95% of all described animal species are invertebrates.

Characteristics

The main feature that sets invertebrates apart is that they do not have a backbone, unlike vertebrates. This group includes a wide variety of animals, from tiny insects to large squids. Invertebrates are animals that must eat other organisms to get their energy.

Most invertebrates have bodies made of different types of tissues and often have a digestive system with one or two openings. Their bodies come in many shapes and sizes, some with symmetry like butterflies, and others without, like snails. Some invertebrates, such as spiders and bees, live together in groups and communicate with each other. The study of invertebrates includes many groups such as sponges, jellyfish, starfish, insects, and worms, each with their own special traits and ways of living.

Classification

Invertebrates can be grouped into several main categories, many of which are still useful even though scientists sometimes debate them. Some of these groups include sponges, comb jellies, medusozoans and corals, flatworms, bristleworms, earthworms and leeches, insects, springtails, crustaceans, myriapods, chelicerates, chitons, snails, slugs, bivalves, tusk shells, cephalopods, roundworms or threadworms, rotifers, tardigrades, starfishes, sea urchins, sea cucumbers, sea lilies and brittle stars, and many more.

These animals can also be split into two informal groups: macroinvertebrates, which are big enough to see without a microscope, and microinvertebrates, which are too small to see without help. This division is made for convenience, and the cutoff is roughly at a length of about 1mm.

History

The earliest animal fossils are of invertebrates. Some fossils, about 665 million years old, have been found in South Australia and are thought to be early sponges. Scientists believe animals may have appeared even earlier, possibly around 1 billion years ago. Tracks and burrows from the late Neoproterozoic Era show that worms lived at that time.

Around 453 million years ago, animals began to diversify, and many important groups of invertebrates separated from one another. Fossils of invertebrates are found in many types of rock layers and are often used to study the history of Earth.

Carl Linnaeus divided animals into two groups: insects and worms. Later, Jean-Baptiste Lamarck created the term "invertebrate" and split these groups into ten. Today, invertebrates are classified into over 30 phyla, ranging from simple sea sponges and flatworms to more complex arthropods and molluscs.

In research

For many years, scientists paid less attention to invertebrates, focusing instead on larger animals. However, this changed in the 1700s thanks to important work by Linnaeus and Lamarck. By the 1900s, studying invertebrates became very important in science, helping us learn about medicine, genetics, ancient life, and how plants and animals interact with their environment.

Today, two tiny invertebrates are very popular in labs: the fruit fly Drosophila melanogaster and a small worm called Caenorhabditis elegans. These creatures were among the first living things to have their genes mapped, which helps scientists understand life better. Invertebrates are also important for checking water quality and studying how the world is changing due to climate change.