Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 34.1 Chordates Have a Notochord and a Dorsal, Hollow Nerve Cord
Chordates include all animals with a notochord and dorsal, hollow nerve cord—features that appear at some stage of development. These traits define the phylum Chordata, which includes both invertebrate chordates (like lancelets and tunicates) and vertebrates, such as fishes, amphibians, reptiles, and mammals.
Shared Derived Characters of Chordates
All chordates exhibit four key features at some point in life:
- Notochord: Flexible support rod; replaced by vertebral column in vertebrates (remains as discs in humans)
- Dorsal, Hollow Nerve Cord: Forms brain and spinal cord; unique to chordates
- Pharyngeal Slits or Clefts: Functions include filter feeding, gas exchange, or structural roles (e.g., ears in humans)
- Muscular, Post-Anal Tail: Aids in movement; often reduced in adults
Lancelets (Cephalochordata)
- Blade-like marine animals that retain all four traits as adults
- Filter feeders using cilia and mucus nets in the pharynx
- Have segmental muscles from mesodermal somites, like vertebrates
- Use muscle contraction along the notochord for movement
Tunicates (Urochordata)
- More closely related to vertebrates than lancelets
- Larvae resemble tadpoles with all four chordate traits
- Adults are sessile filter feeders; only retain pharyngeal slits
- Use incurrent and excurrent siphons to feed
- Have only 9 Hox genes and embryonic cells similar to neural crest cells
Evolutionary Insights
- Ancestral chordate likely resembled a lancelet with all 4 traits
- Hox gene patterns in lancelets align with brain region organization in vertebrates
- Suggests vertebrate traits evolved from a lancelet-like ancestor
In a Nutshell
Chordates share four critical traits: notochord, dorsal nerve cord, pharyngeal slits, and post-anal tail. While lancelets retain all these traits into adulthood, tunicates display them only as larvae. Despite their simplicity, these invertebrates provide vital clues about the origin of vertebrates—including the genetic and structural foundations of the brain and nervous system.