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How do cephalochordates differ from tunicates?
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Chordate
X-ray fish (Pristella maxillaris), is an example of a chordate with a visible backbone.
Cephalochordates and tunicates are both subphyla within the phylum Chordata, but they differ significantly in their anatomy and life cycles.
Anatomical Differences
- Notochord: In cephalochordates, the notochord extends the entire length of the body and persists into adulthood. In tunicates, the notochord is typically restricted to the tail and is usually lost during metamorphosis into the adult form.123
- Body Structure: Cephalochordates are fishlike, free-swimming invertebrates with well-developed musculature and segments. Tunicates are often sessile (attached) as adults and are characterized by a tough outer covering called a tunic, which contains cellulose—a material rarely found in animals.1425
- Internal Systems: Cephalochordates lack a heart, whereas tunicates possess one. Additionally, cephalochordates have a single atrium with a ventral opening, while tunicates generally have an atrium that forms from fused pouches.23
Life Cycle and Habitat
Most adult tunicates are benthic and sessile, though some are pelagic. In contrast, cephalochordates are typically found partially buried in marine sand and gravel, retaining their chordate features throughout their lives, whereas most tunicates lose their primary chordate characteristics after metamorphosis.543