Download 29.2 Invertebrate Chordates

 Phylum Chordata has three subphyla:
 Urochordata – the tunicates (sea squirts)
 Cephalochordata – the lancelets
 Vertebrata – animals with backbones
 Like all other chordates, they must have the 4 traits all
chordates have at some time in their development
 1. notochord
 2. a dorsal hollow nerve chord
 3. gill slits
 4. muscle blocks
 All chordates have bilateral symmetry
 Can be cut in half in only one plane
 They have a well developed coelom
 They have segmentation
Notochord
Found in all chordate embryos
 It is a semi-rigid rod-like structure found
between the digestive system and the
dorsal hollow nerve chord
Is retained into adulthood in invertebrate
chordates
Replace by a backbone in vertebrates
Develops from the mesoderm
Provides support for side to side
movement of the body
Move organisms through the water at
great speed
Develops from a plate of ectoderm that
rolls into a hollow tube
In most adult chordates, cells of
posterior nerve chord become the
spinal chord
Those in the anterior portion become
the brain
A pair of nerves connects chord to
muscle blocks
Paired openings located in the pharynx
Many invertebrate chordates have gill
slits only during embryonic
development
Those that keep them into adulthood
use them to strain food from the water
In some vertebrates, they develop into
internal gills for gas exchange
Modified body segments made of
stacked muscle layers
Ever pulled meat apart on a fish??
Anchored by the notochord
Gives them a firm structure to pull
against when they contract
Aid in movement of the tail
At some point in their development, all
chordates have a muscular tail
Subphylum Urochordata
Commonly called tunicates or sea
squirts
Adults don’t show chordate features
but larval stage does
Larva are free swimming and don’t eat
only for a few days after hatching
Only gill slits indicate the adults are
chordates
In the subphylum Cephalochordata
Spend most of their time buried in
sand
Like tunicates, they are filter feeders
They retain all their chordate features
into adulthood
Are fish-like
No distinct head but have light
sensitive cells on anterior end
Has a hood that covers their mouth
Tentacles around the mouth direct
water current and food particles toward
the mouth
Because they have no hard parts, fossil
record is incomplete