A vast and varied group of animals, including some of the most well-known species like dogs, cats, birds, and fish, are called chordates, and they are fascinating animals. They are distinguished from other animals by a distinct set of traits that they all have at some point in their lives.
The existence of a notochord, a flexible rod that supports the body, is one of the most distinctive characteristics of chordates. Even though it might not be apparent in mature animals like dogs and cats, it is still a crucial stage in their growth. Chordates have a dorsal nerve cord in addition to the notochord; in more advanced species, this nerve cord develops into the brain and spinal cord.
Chordates are diverse animals that range in size and shape from small fish to large mammals, and they are all endowed with special adaptations. Studying these creatures can provide unexpected insights into the world we live in, particularly regarding the daily companions we keep.
- Chordates originated over 590 million years ago
- Embryological data classify chordates as deuterostomes
- There are three subphyla of chordates
- All chordates share a common feature: the presence of a notochord
- The life cycle of chordates begins with fertilization of the egg
- Chordates are found in all major habitats
- The nervous system of chordates ranks highest among all living organisms
- Vertebrates have the most complex digestive system
- The circulatory system of chordates has a characteristic pattern
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Chordates originated over 590 million years ago
Since their ancestors had soft bodies, they left almost no fossil legacy. Pikaia gracilens, a primitive cephalochordate that dates to approximately 505 million years ago, is the earliest known fossil chordate. Scientists disagree as to whether older organisms, like the 530 million-year-old Yunnanozoon lividum and the ancient Haikouella, which share several chordate characteristics, qualify as chordates. In the vast fossil record, vertebrates are present. It all started roughly 400 million years ago.
Embryological data classify chordates as deuterostomes
Animals classified as deuterostomes have bilateral symmetry and do not have a mouth that emerges from the blastopore. Echinoderms and hemichordata are also members of this group. Chaetognatha is also included by some taxonomists.
Given that hemichordates have gill slits and other characteristics that distinguish them from other animal species, they are most likely the closest relatives of chordates. The similarity of the larvae in some groups of hemichordates and echinoderms suggests a somewhat more distant relationship with echinoderms. Because chordates share characteristics with some fossil echinoderms, including organs resembling gill slits, scientists disagree as to whether or not chordates are descended from these animals. There are theories that trace their origins to other groups of organisms (such as annelids, nemerteans, and arthropods), but these theories are not very popular today.
Smithsonian Institution image
There are three subphyla of chordates
Among the subphyla of chordates are the following:
- Tunicata (subphylum Tunicata). Tunicatas are small animals, typically between one and five centimeters long, with a minimum body length of about one millimeter and a maximum length of just over 20 centimeters. They can form colonies up to 18 meters long. Over 1,000 species of tunicates have been described.
- Cephalochordata (subphylum Cephalochordata). Cephalochordates are tiny marine fish-like organisms. Cephalochordates range in length from one to three centimeters. They have well-developed muscles and can swim quickly by swinging their bodies. There are about 30 species of cephalochordates.
- Vertebrata (subphylum Vertebrata). Vertebrates range in size from tiny fish to giant whales, the largest animals in the world. This is the most highly developed subphylum of chordates.
Photo: Bio Master
Hemicordates were formerly categorized as members of the phylum Chordates, but they are currently apart.
All chordates share a common feature: the presence of a notochord
Chordates, as their name implies, have a rigid dorsal supporting rod (the notochord) at some point in their life cycle. When fully grown, this covers the majority of the body’s length. The notochord, which is ventral to the central nervous system and dorsal to the gut, stiffens the body and provides support for movement.
The gills and other body parts of tunicates and cephalochordates are supported by cartilaginous material. When a vertebrate reaches adulthood, its skeleton usually consists mostly of bone rather than cartilage.
Other typical traits of chordates consist of:
- bilateral symmetry
- segmented body, including segmented muscles
- single, dorsal, hollow nerve cord, usually with an enlarged anterior end (brain);
- tail protruding beyond the anus at some stage of development;
- ventral heart with dorsal and ventral blood vessels and a closed circulatory system;
- pharyngeal sacs present at some stage of development;
- complete digestive system;
- endostyle;
- bony or cartilaginous endoskeleton present at at least one stage of development.
The life cycle of chordates begins with fertilization of the egg
In its most basic form, external fertilization takes place in water. Many vertebrates, including tunicates, reproduce asexually (females of certain fish and lizards can reproduce without fertilization). Fish and tunicates that possess both male and female reproductive organs are known as hermaphrodites; otherwise, the sexes are distinct.
The larvae are structurally distinct from those of non-chordates. For tunicates and vertebrates, internal fertilization, viviparity (birth of cubs that have undergone embryological development), and parental care are typical.
Chordates are found in all major habitats
Alexander Semenov took this picture.
Benthic (bottom-dwelling) and pelagic (open-water) tunicates are marine animals that frequently form colonies through asexual reproduction. Their larvae swim in open water to develop into adults, or they search for a spot to attach and change into an adult. Adult cephalochordates live mostly or totally in sea sand and gravel, but they develop in open water.
The transition from gills to lungs occurred when vertebrates invaded freshwater environments and eventually reached land. Vertebrates were further divided into amphibians, reptiles, birds, and mammals through further evolution. Compared to other chordates, these vertebrates are all far more complex, and their habitats vary significantly based on how actively they seek food.
The nervous system of chordates ranks highest among all living organisms
In chordates, the anterior end of the main nerve cord expands to resemble a brain, but only in vertebrates is the brain fully developed. Both the sensory organs and the visual organs of tunicates are sensitive to light and gravity direction. Sharp variations in light intensity are detected by pigment spots and light receptors in the lancelets’ nerve cord. Vertebrates have more complex eyes and other sensory organs.
The nervous systems of cephalochordates and vertebrates share a common ancestor, as indicated by the presence of segmentally repeating nerves originating from a dorsal hollow nerve cord. There are no segmentally repeating nerves in the nervous system of tunicates. All vertebrates have a significantly larger brain that is separated into regions with specific functions.
A fascinating class of animals, chordates include animals like fish, dogs, cats, and birds. They are unique in that they have all had a notochord at some point in their lives, which aids in supporting their bodies. Vertebrates, which are more recognizable to us because they have a backbone, are also included in this group. Chordates are essential to the diversity of life on Earth because of their remarkable capacity for environmental adaptation as well as their distinctive physical makeup.
Vertebrates have the most complex digestive system
Cephalochordates and tunicates are both filter feeders of microscopic food particles suspended in water. A stream of water enters the mouth through the pharynx, where the endostyle (a gland situated beneath the two rows of gill slits) secretes a layer of mucus, due to the beating of cilia, which are hair-like projections of cells on the gill slits. Following suspension, the food particles are filtered out. The food-rich mucus layer is moved up through the gill slits by the ciliiaria lining the pharynx, where it is twisted and directed towards the back of the intestine. Through the atrial orifice, the water stream enters the atrium and leaves.
By Ricardo R. Fernandez in photo
Similar things occur in the larval stages of other vertebrates, especially in the case of the ancient jawless fish known as lampreys. They differ in that their food is directed by the pharyngeal muscles, which pump food particles and water through the gill slits, rather than by cilia, and it contains slightly larger particles. Vertebrates were able to hold larger food items once their jaws developed.
Primitive chordates have a simple tube that resembles a stomach with a pouch that is part of the digestive system. They only have the rudiments of specialized digestive organs found in vertebrates, such as the pancreas and liver.
The circulatory system of chordates has a characteristic pattern
Ugreen/Getty Images image
Blood is pumped by a separate heart in tunicates and vertebrates, and by blood vessel contractions in cephalochordates. Blood that lacks oxygen travels forward via the ventral aorta. The oxygenated blood flows to the body through a sequence of branchial arteries in the gills, where gas exchange takes place. The majority of the oxygenated blood then returns to its source via the dorsal aorta. Tiny blood vessels called capillaries help vertebrate blood flow through tissues. Blood travels through holes in the tissues of tunicates and cephalochordates because they do not have capillaries.
| Fact | Description |
| Backbone Presence | Chordates have a backbone or spinal cord that supports their body structure. |
| Three Main Groups | Chordates include vertebrates (like mammals), tunicates, and lancelets. |
| Wide Range of Sizes | Chordates range from tiny fish to the largest animals, like whales. |
| All Have a Notochord | In their early development, all chordates have a notochord, a flexible rod supporting the body. |
Chordates are truly fascinating creatures that show how diverse life on Earth can be. From tiny sea squirts to majestic mammals like dogs and cats, this group covers a wide range of forms and functions.
The fact that chordates share characteristics like a dorsal nerve cord and a notochord—features that give them their name—is among their most remarkable traits. All chordates have these characteristics at some point in their lives, demonstrating the close evolutionary ties between different species.
Chordates have evolved to live in nearly every type of habitat on Earth, whether it be on land, in the air, or in the ocean. Their astounding variety never ceases to pique our interest and foster reverence for the surrounding natural environment.
