Chordate

A chordate is a special kind of animal that belongs to a group called Chordata. All chordates have five key features at some point in their lives. These features include a notochord, a hollow nerve cord on their back, a special structure called an endostyle or thyroid, pharyngeal slits, and a tail behind their anus.

Chordates are divided into three main groups. The first group is Vertebrata, which includes fish, amphibians, reptiles, birds, and mammals. These animals have a skull and a backbone that replaces their notochord. The second group is Tunicata, which includes sea squirts and salps. These animals only show the key chordate features when they are young. The third group is Cephalochordata, which includes lancelets. These animals look like jawless fish but do not have gills or a distinct head.

Chordates are one of the largest groups of animals, with over 81,000 living species. About half of these are ray-finned fishes, and most of the rest are tetrapods, which are animals that live on land and evolved to breathe air using lungs. Chordates have been around since the Cambrian explosion, more than 539 million years ago.

Etymology

The name "chordate" comes from a special part of their bodies called the notochord. This helps give chordates their shape and lets them move. Chordates also have symmetry on both sides, a space inside their bodies for organs, a system that moves blood around, and a body plan that repeats in segments. The name Chordata was first used by a scientist named William Bateson in 1885, but others were already using it before that. Another scientist, Ernst Haeckel, described these animals in 1866.

Anatomy

Chordates are animals that have special features at some point in their lives. These features include:

• A notochord, a stiff rod that runs along the middle of the body. In animals with backbones, this rod is replaced by parts of the spine.
• A hollow dorsal nerve cord, which becomes the spinal cord and helps control the body.
• Pharyngeal slits, which in fish become gills, but in other animals help with feeding.
• A post-anal tail, a muscular tail that sticks out behind the body.
• An endostyle, a groove in the throat that helps with feeding and storing certain materials.

These features help define what makes chordates special among animals.

Classification

This section shows how scientists group different kinds of animals called chordates. The groups are based on how these animals are related and what features they share.

The main group is called Phylum Chordata, and it has three big subgroups:

1. Subphylum Cephalochordata – These are small sea animals called lancelets.
2. Subphylum Tunicata – These include sea squirts and other ocean animals.
3. Subphylum Vertebrata – These are animals with backbones, like fish, amphibians, reptiles, birds, and mammals.

The chart shows many smaller groups within these big ones, helping scientists understand how all these animals are connected.

Subphyla

See also: List of chordate orders

Cephalochordata: Lancelets

Main article: Lancelet

Cephalochordates are one of the three main groups of chordates. They are small, fish-like animals without brains, heads, or special sense organs. These animals live buried in the sand and filter food from the water. They are the earliest group to branch off from other chordates.

Tunicata (Urochordata)

Main article: Tunicate (Urochordata)

Tunicates have three different adult shapes. All of their larvae have the basic chordate features, including long, tadpole-like tails. Their larvae also have simple brains and sensors to detect light and movement.

The smallest group of tunicates is the Appendicularia. They keep their tadpole-like shape and swim actively throughout their lives. For a long time, they were thought to be larvae of the other two groups.

The other two groups, sea squirts and salps, change shape as adults and lose some chordate features. Both groups are soft-bodied filter feeders that use mucus to catch food.

Sea squirts stay in one place on the sea floor their whole lives, feeding on tiny organisms in the water.

Salps float in the water, also feeding on tiny organisms. They have a special life cycle where one generation lives alone, and the next generation forms chains together.

Vertebrata (Craniata)

Main articles: Craniata and Vertebrata

All craniates have distinct skulls. This group includes hagfish, which do not have vertebrae. Most craniates have a backbone made of bone or cartilage that protects their spinal cord. Hagfish have incomplete skulls and no vertebrae, placing them in a special group. Recent studies suggest hagfish might actually be a type of vertebrate that lost their backbone.

Phylogeny

Overview

Scientists are still studying the simplest forms of chordates by comparing their DNA. This helps them understand how these animals are related, especially since some chordates may have lost certain features over time.

Researchers have found that chordates likely share a common ancestor. Evidence from proteins supports this idea. Early chordate fossils come from the Cambrian period, around 538.8 million years ago. These fossils include early fish and other chordates. Studying these fossils and comparing DNA helps scientists build evolutionary family trees, though this can be tricky.

Cladogram

Below is a family tree showing the probable evolutionary relationships between both extinct and living chordate groups.

Closest non-chordate relatives

The closest relatives of chordates are thought to be the hemichordates and Echinodermata, which together form the Ambulacraria. The Chordata and Ambulacraria together make up the superphylum Deuterostomia.

Hemichordates

Main article: Hemichordate

Hemichordates have some features like chordates, such as openings in the pharynx that look like gill slits, and a dorsal nerve cord. There are two living groups of hemichordates. The enteropneusts, known as "acorn worms", have long, worm-like bodies and live in seafloor sediments. Pterobranchs are tiny, colonial animals that live in connected tubes.

Echinoderms

Main article: Echinoderm

Echinoderms look very different from chordates. As adults they have a body shape like a wheel, and their bodies are supported by hard shells made of calcite. They also have special tube feet that help them move. Crinoids often look like flowers and stay in one place, while others like starfish and sea urchins can move around.