Phylum Chordata

Nonvertebratechordates

FishesAmphibians

ReptilesBirds

Mammals

Invertebrate ancestor

Chordate Cladogram

All chordates at sometime during their life history have:

• a notochord: a dorsal supporting rod located dorsally just below the nerve cord; it provides support and is replaced by the vertebral column in vertebrates

• a dorsal hollow nerve cord: a fluid-filled canal; spinal cord is protected by vertebrae

• pharyngeal gill pouches: openings that function in feeding, gas exchange, or both

Chordates that are not vertebrates

• Subphylum Urochordata

• contains 1,250 species

Tunicates (Sea Squirts), Larvae• The larvae of tunicates resemble the ancestral

chordate.• It has all three chordate characteristics and looks like

a tadpole.• The free-swimming larva develops into a sessile,

filter-feeding adult.Adult• The adult has a thick-walled body sac and an

incurrent siphon and an excurrent siphon.• Gill slits are the only chordate feature retained by the

adult form.• In some tunicates, the adult form may have been lost.

These animals retained the larval form as adults. Vertebrates may have evolved from forms like these.

Subphylum Cephalochordata

Subphylum Cephalochordata

23 species of lancelets are in the genus Branchiostoma

elongated, lance-shaped body resembles the lancelet, a two-edged surgical knife

inhabit shallow coastal waters; they lie partly buried in sandy substrates and filter feed

Lancelets retain the three chordate characteristics as an adult notochord extends from head to tail segmented muscles and their dorsal hollow nerve cord

Vertebrate Chordates

• Subphylum Vertebrata

• Animals with a vertebra column.