Among the animal phyla, phylum Chordata is the most familiar one to us because it is the phylum to which humans and their relative vertebrates belong to. Phylum Chordata exhibits a distinguishable character from all other phyla viz. a notochord. All chordates are characterized by four features at different stages of their life:
- Notochord– It is a longitudinal, cartilaginous rod running between the nerve cord and the digestive tract. It acts as a support for the nerve cord. In vertebrates, it is replaced by the vertebral column after the embryonic stage.
- Dorsal nerve cord–It is a bundle of nerves which runs along the “back” and splits into the brain and the spinal cord
- Pharyngeal slits–They are the openings which allow the entry of water through the mouth without entering the digestive system viz. they connect mouth and throat.
- Postanal tail–It is an extension of the body to the anus. In chordates, the tail is composed of skeletal muscles which help in locomotion in fish-like species.
Other features of chordates include:
- Bilaterally symmetrical, triploblastic, coelomic and segmented body.
- The body design is complex and well-differentiated.
- The body has organ system level of organization.
Phylum Chordata: Classification
Phylum Chordata is classified into three subphyla types, namely Urochordata (tunicates), Cephalochordata (lancelets) and Vertebrata (vertebrates). Subphyla Urochordata and Cephalochordata are collectively known as protochordates, which are marine animals. They are invertebrates but they share attributes of chordates.
Urochordata: Tunicates possess notochord only during their larval stage and loses it once they turn into adults like Ascidia, Salpa, Doliolum.
Cephalochordata: Lancelets possess the notochord and nerve cord throughout their life. However, they lack the brain and bony vertebral column like Branchiostoma.
Vertebrata: Vertebrata includes humans and all other animals which have vertebrates. The subphylum Vertebrata is characterized by the cartilaginous or bony backbone (vertebral column). The notochord is present only in the embryonic stage and on maturity, it is replaced by the vertebral column. This is why it is said that ‘all vertebrates are chordates but all chordates are not vertebrates’. In addition to this, vertebrates have a muscular heart located ventrally, kidneys, appendages (as fins/limbs), etc.
Subphylum Vertebrata is further classified into seven classes. They are:
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