Chordate

Chordata

Kryptopterus 2

Branchiostoma lanceolatum

BU Bio

Pacific hagfish Myxine

Chordates (Chordata) are a large and diverse phylum of animals that includes the vertebrates, along with several closely related invertebrates. Chordates are defined by having at some point in their life cycle a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail. These features may not all be present in the adult form, but they appear at some stage of development.

Characteristics

Chordates are distinguished by several key anatomical features:

• Notochord: A flexible, rod-shaped body found in the embryonic stage of all chordates and in the adult stage of some chordate groups. It provides skeletal support.
• Dorsal Nerve Cord: Unlike other animal phyla, chordates possess a nerve cord running along their back in a dorsal position. In many chordates, this develops into the spinal cord and brain.
• Pharyngeal Slits: Openings in the pharynx that may develop into gills in aquatic species or into other structures in terrestrial species.
• Endostyle: A glandular groove in the floor of the pharynx, which in higher chordates develops into the thyroid gland.
• Post-anal Tail: A tail extending beyond the anus, present during some stage of development, which is used for propulsion in aquatic species.

Classification

Chordates are divided into three subphyla:

• Urochordata (tunicates), which are mostly sessile marine animals.
• Cephalochordata (lancelets), small, fish-like marine animals.
• Vertebrata (vertebrates), which include fish, amphibians, reptiles, birds, and mammals.

Vertebrates

Vertebrates are the most well-known group of chordates, characterized by their vertebral column, or spine, which replaces the notochord during early development. This group encompasses a vast diversity of life forms, from the smallest fish to the largest mammals.

Evolution

The origin of chordates is a subject of ongoing research and debate. Fossil evidence suggests that chordates originated in the Cambrian period, over 500 million years ago. The earliest known chordate is believed to be Pikaia gracilens, a small, eel-like creature found in the Burgess Shale.

Habitat and Distribution

Chordates are found in a wide range of habitats across the globe, from deep ocean trenches to high mountain ranges. While the majority of chordates are vertebrates and thus animals that are more familiar to us, the invertebrate chordates play crucial roles in marine ecosystems.

Importance

Chordates include many species of significant ecological, economic, and medical importance. Fish serve as a vital source of food for billions of people, while mammals, including humans, have profound impacts on ecosystems worldwide. Research on chordate development and genetics provides valuable insights into human biology and disease.

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