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How do tunicates differ from cephalochordates?
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tunicate
Blue spot sea squirt (Clavelina moluccensis), a colonial tunicate.
Tunicates and cephalochordates are both invertebrate subphyla within the phylum Chordata, but they exhibit significant anatomical and developmental differences:
- Notochord Distribution: In tunicates, the notochord is restricted to the tail and is typically present only during the larval stage, often regressing during metamorphosis. In cephalochordates, the notochord extends the entire length of the body and remains into adulthood.123
- Nervous System: Cephalochordates possess a nervous system with segmentally repeated nerves arising from the dorsal hollow nerve cord, a feature they share with vertebrates. Tunicates lack these segmentally repeated nerves.4
- Body Structure and Lifestyle: Most adult tunicates are sessile, benthic organisms encased in a tough tunic containing cellulose. Cephalochordates are small, fishlike, free-swimming marine invertebrates that lack a tunic and possess well-developed body segments.513
- Internal Anatomy: Cephalochordates have a single atrium with a ventral opening and lack a heart. Tunicates generally possess a heart, and their atrium typically forms from pouches that fuse into a single cavity.12