- Urochordata, also known as tunicates, are marine chordates characterized by having a notochord only in their larval stage. They belong to the subphylum Urochordata within the phylum Chordata. - Sea squirts have a free-swimming larval stage with a notochord and tail, but undergo metamorphosis where the notochord is resorbed and they attach themselves permanently to surfaces as sessile filter-feeding adults encased in a tunic. - The largest class of urochordates is the Ascidiacea, which have a larval stage that transforms into solitary or colonial sessile adults that feed by

1. CLASSIFICATION
Kingdom: Animalia
UROCHORDATA
Phylum: Chordata
Subphylum: Urochordata (Tunicates)
Class: Ascidiacea (Sea Squirts)
Anatomy of the Sea Squirt
Larvacea (Larvaceans)
Thaliacea (Thaliaceans)

3. GENERAL CHARACTERISTICS
- Protochordates
• Morphological features: notochord, dorsal
hollow central nervous system, pharynx with
paired pouches; muscular, post-anal tail
- Group of marine chordates in which the
notochord is confined to the locomotor tail of
the larval stage

4. GENERAL CHARACTERISTICS
- Also known as Tunicates
- Enclosed in a delicate, nonliving, and
sometimes beautifully colored transparent tunic
- Filter-feeders.

5. CLASS ASCIDIACEA
→ largest class of urochordates
→ free-swimming larval stage
→ sessile as adults, either solitary or colonial

6. LARVAL STAGE
- Body consists of trunk
containing the immature
viscera and a muscular
notochord-supported tail.
- The nervous system
consists of a dorsal hollow
nerve cord, several
ganglia and nerves
- Respiratory water enters
the atrium chamber which
is then ejected through
the atriopore.
- At metamorphosis, the
notochord is resorbed and
becomes a source of
nutrients and
rearrangement of the
viscera takes place.

7. METAMORPHOSIS

8. ADULT
- Incoming water current becomes
trapped in the mucus, then the cilia
and esophageal papillae sweep the
mucus and entrap food particles into
the stomach.
- Respiratory water passes over the
gills and into the atrium, a fluid-filled
collecting chamber surrounding the
pharynx. Gas exchange occurs as
water circulates with the help of the
atrial muscles.
- One blood vessel from the heart
runs anteriorly under the endostyle,
and another runs posteriorly to the
digestive organs and gonads.
- Gametes may be shed through the
excurrent siphon for external
fertilization or eggs are retained for
fertilization and early development.

9. GARSTRANG’S HYPOTHESIS

10. UROCHORDATA
Anatomy of the Sea Squirt