Chordates have a notochord, a supportive structure, which varies in shape and appearance across the three subphyla. The urochordates have it only in larval stages, cephalochordates have it in adulthood, and vertebrates have it in embryonic form, which is later replaced by spinal vertebrae. The notochord allowed animals to become larger and more complex.
A notochord is the defining feature of members of the phylum Chordata, a large and diverse biological group that includes all animals with spines or spines, along with more primitive chordates. The structure has very large cells which are densely arranged within a protective sheath. Chordates are divided into three subphyla, depending on the shape their notochords take and when they appear. At any stage of development, it acts as a form of support for the possessing animal, giving animals the ability to do things like walk upright.
The most primitive group of chordates, the urochordates, also called tunicates, have a notochord only in the larval stages of development. Animals in this group are pelagic, which means they are found in the world’s oceans, and there are a number of representative species, most of which are known only to biologists. However, these animals provide an insight into chordate development, showing the notochord in an early stage of development.
The next group of chordates, the cephalochordates, also called lancets, possess a notochord in adulthood and also live in the ocean. The structure runs throughout the body, even up to the head, and animals lack a protective layer of bone such as a vertebral column. It acts as an axial support, providing a strong core for the animal. The notochord is highly flexible, but non-compressible, allowing the animal to move freely without damaging the structure.
In the highest class of chordates, the vertebrates, the notochord exists only when the animal is in embryonic form. As the vertebrate develops, it is first enveloped and then replaced by spinal vertebrae, protective cases of bone that cover the delicate spinal cord. The vertebral column is capable of supporting a much larger and more complex organism, and is much stronger than the notochord. This allowed the first vertebrates to make the leap to land and develop into well-known species, such as humans.
In all chordates, the notochord exists in some form when the animal is in a larval or embryonic stage. His development from there helps shape the type of animal he’ll transform into, whether it’s a sack-like tunicate or a purebred Norwegian Fjord horse. The development of this structure represents a major advance in evolution, as it allowed animals to become much larger and more complex, a drastic departure from the simpler orders of animals that previously existed.
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