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What is notochord? What is dorsal nerve cord? What is coelom? What are vertebrates?
 Notochord
A notochord is a primitive beginning to the backbone. It appears in embryos as a small flexible rod made from cells from the mesoderm, which is one of the three layers of cells of embryos. Notochords are only found in the phylum chordata, a group of animals that includes humans. In fact, it is the defining feature of that particular phylum. Notochords, and furthermore backbones, are what separate us from the more primitive living creatures that we encounter.

The notochord is oriented head to tail and is found between the digestive tube and the nerve cord. Since it is composed of stiffer tissue, it allows for skeletal support of the organism. In certain chordates, like the lamprey and the sturgeon, the notochord remains there for life. In vertebrates, such as humans, a more complex backbone appears with only portions of the notochord remaining. As our backbone fully forms, the discs in our back is where the pieces of notochord are found as a sort of gelatinous material.

Dorsal nerve cord
The dorsal nerve cord is one of the embryonic features unique to chordates, along with a notochord, a post-anal tail, an endostyle, and pharyngeal slits. The dorsal nerve cord is a hollow cord dorsal to the notochord.

Coelom
A coelom is a fluid-filled body cavity that is completely lined by tissue created from the mesoderm, the middle layer of the primary cells found in an embryo. It is typically found in multicellular organisms, which are living things that have more than one cell. The most common types of multicellular organisms are found under the domain eukarya; more simplistic prokaryotic organisms are generally unicellular organisms. Most bilateral animals, those with a left and a right side, have a coelom, as well as vertebrates, or animals with a backbone.
Function
The coelom has numerous functions, the first of which involves acting as a cushion for the internal organs of the organism. Let's look at an example.

If you've ever been in a car accident, you may remember traveling forward and then coming to a sudden stop. However, during the stop, your organs continued to move forward due to inertia. This is because when something is traveling in a certain direction, it has a tendency to continue in that direction until acted upon. During the accident, the coelom, or your body cavity, helped to prevent further damage by cushioning your organs.

In organisms such as worms, the coelom acts to maintain pressure inside of the organism. This acts as the worm's skeletal system, which allows the worm's muscles to do work.

Vertebrates
Vertebrates are animals that have a backbone or spinal column, also called vertebrae. These animals include fish, birds, mammals, amphibians, and reptiles. Vertebrates are classified by the chordate subphylum vertebrata. Invertebrates are any other animal that is classified outside of that class.


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