Vertebrates are animals that have a backbone or spinal column, such as mammals, birds, reptiles, amphibians, and fish. They belong to the subphylum Vertebrata, which is part of the phylum Chordata. Chordates are animals that have a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some point in their development. But did you know that not all chordates are vertebrates? In fact, there are two groups of invertebrates that are also chordates: cephalochordates and urochordates. These are the groups of invertebrates that are most closely related to vertebrates.
Cephalochordates: The Lancelets
Cephalochordates are also known as lancelets, because they have a lance-shaped body that is flattened from side to side. They are small, marine animals that live in shallow waters, where they burrow into the sand and filter feed on plankton. They have a notochord that extends throughout their body, and a nerve cord that runs above it. They also have pharyngeal slits that function as gills, and a post-anal tail that helps them swim. Cephalochordates lack a distinct head, brain, eyes, ears, and jaws. They have a simple circulatory system with a heart-like structure, but no blood cells or hemoglobin. They also have a simple excretory system with nephridia, but no kidneys or bladder.
Cephalochordates are considered to be the most primitive chordates, because they retain all the features of the chordate body plan throughout their life. They are also thought to be the closest living relatives of the earliest vertebrates, because they share many molecular and developmental similarities with them. According to Quizlet, cephalochordates are the group of invertebrates that is most closely related to vertebrates.
Urochordates: The Tunicates
Urochordates are also known as tunicates, because they have a tunic or covering made of cellulose that protects their body. They are also marine animals, but they have a more diverse range of habitats and lifestyles than cephalochordates. Some urochordates are sessile filter feeders that attach to rocks or other surfaces, such as sea squirts and salps. Others are free-swimming predators or parasites, such as larvaceans and thaliaceans. Urochordates have a notochord and a nerve cord only in their larval stage, which lasts for a few hours or days. When they metamorphose into adults, they lose these structures and become more specialized for their mode of feeding and living. They retain their pharyngeal slits as adults, which serve as both gills and filter-feeding organs. They also have a heart-like structure that pumps blood in both directions alternately, and nephridia for excretion.
Urochordates are considered to be more derived than cephalochordates, because they undergo significant changes during their development and lose some of the chordate features as adults. However, they are still closely related to vertebrates, because they share some molecular and genetic characteristics with them. According to Biology LibreTexts, urochordates are the second closest group of invertebrates to vertebrates.
Conclusion
Vertebrates are not the only chordates in the animal kingdom. There are two groups of invertebrates that also belong to the phylum Chordata: cephalochordates and urochordates. These groups are the most closely related to vertebrates, because they share some common features and ancestry with them. Cephalochordates are more similar to vertebrates than urochordates, because they retain all the chordate characteristics throughout their life. Urochordates are less similar to vertebrates than cephalochordates, because they lose some of the chordate characteristics during their metamorphosis into adults. However, both groups provide valuable insights into the evolution and diversity of chordates.
I hope you enjoyed reading this article and learned something new about these fascinating animals. If you have any questions or comments, please feel free to share them below. Thank you for your attention! š
