1. Deuterostomia Phylum Chordata (Ascidians) Ambulacraria Hemichordates
Cephalochordates
Urochordates
(Ascidians)
Vertebrates
Phylum
Chordata
Ambulacraria
Hemichordates
Echinoderms

2. SubPhylum Cephalochordata SubPhylum Vertebrata
Phylum Hemichordata
Enteropneusta (acorn worms)
Pterobranchia (colonial)
Phylum Chordata
SubPhylum Urochordata (the Tunicates)
Ascidiacea (Ascidians)
Thaliacea (Salps)
Appendicularia (Larvaceans)
SubPhylum Cephalochordata
SubPhylum Vertebrata

3. Phylum Hemichordata Class Enteropneusta (acorn worms, 75 spp.)
Class Pterobranchia (colonial; 25 spp.)
wormlike body with proboscis (prosome), collar (mesosome),
and trunk (metasome), each with a (coelomic compartment)
- pharyngeal gill slits and dorsal hollow nerve chord like
in chordates; no notochord
- separate sexes; asexual reproduction
common (in pterobranchs)
- mouth + anus form secondarily
after blastopore closes
- unique excretory structure, the
glomerulus
Acorn
worm

4. Enteropneusts (acorn worms)
proboscis used for movement
in burrows, and for deposit-
feeding on organic matter on
the surface outside the burrow

5. Enteropneusts (acorn worms)
water out,
thru slits
water in
- cilia draw water through mouth (where food particles may be
captured), then water exits pharyngeal gill slits

6. Class Pterobranchia Colonial filter-feeders with 0-1 gill slits
Bryozoan-like features:
1) tentacles move water in same
way as a lophophore
2) mesocoel extends into ciliated
tentacles used in filter-feeding
3) live in secreted hard tubes
4) U-shaped gut, anus opening
outside the tentacles
water flow
food

7. Class Pterobranchia Colonial filter-feeders with 0-1 gill slits
Bryozoan-like features:
1) tentacles move water in same
way as a lophophore
2) mesocoel extends into ciliated
tentacles used in filter-feeding
3) live in secreted hard tubes
4) U-shaped gut, anus opening
outside the tentacles
5) colony of zooids produced by
asexual growth
immature
asexual
bud

8. Deuterostomia Phylum Chordata (Ascidians) Ambulacraria Hemichordates
Cephalochordates
Urochordates
(Ascidians)
Vertebrates
Phylum
Chordata
Ambulacraria
Hemichordates
Echinoderms

9. SubPhylum Cephalochordata SubPhylum Vertebrata
Phylum Hemichordata
Enteropneusta (acorn worms)
Pterobranchia (colonial)
Phylum Chordata
SubPhylum Urochordata (tunicates)
Ascidiacea (ascidians)
Thaliacea (salps)
Appendicularia (larvaceans)
SubPhylum Cephalochordata
SubPhylum Vertebrata

10. Phylum Chordata At some stage of development, all chordates possess:
- Pharyngeal gill slits
- Dorsal, hollow nerve chord
- Dorsal notochord (hollow flexible tube)
- Post-anal tail, tadpole stage at some point in lifecycle
- Gland in pharynx that processes iodine:
- endostyle
(Urochordata,
Cephalochordata)
- thyroid gland
(Vertebrata)

11. pharyngeal gill slits:
Out-pockets in the pharynx
Become gill chambers and gills in aquatic chordates
Become jaws, inner ear, and tonsils in terrestrial chordates
notochord: a firm, flexible rod derived from mesoderm cells;
becomes the endoskeleton in vertebrates
dorsal nerve cord: a hollow tube above the notochord;
becomes the spinal cord and brain in vertebrates
The combination of these characters is a synapomorphy for Chordata, but all 3 traits rarely show up in the adult stage

12. SubPhylum Urochordata
3,000 spp.
- numerous gill slits on pharynx used in suspension feeding
- oral + atrial siphons
- notochord and nerve chord present in tadpole larvae
- no coelom or bony tissue
- adult body covered in secreted tunic, a polysaccharide coating
Class Ascidiacea (Ascidians)
- benthic; solitary or colonial
Class Thaliacea (Salps)
- pelagic, colonial
Class Appendicularia (Larvaceans)
- pelagic, solitary
- adults retain larval characteristics

13. Class Ascidiacea – the ascidians, or tunicates
- sessile, benthic adults; large, short-lived tadpole larvae
- solitary forms, or colonial (may be social or compound)
- colonies form by asexual reproduction
- hermaphrodites
- often colorful + chemically defended
- tropical species contain a photosynthetic symbiont, the
cyanobacterium Prochloron

14. Filter-feeding in ascidians
sheets of iodinated mucus
produced by ciliated tract
called the endostyle
- mucus sheets move
over the gills slits in
the pharynx, through
which water is pumped,
trapping food particles

15. Solitary Ascidians
Styela montereyensis

16. Colonial ascidians 1: In social species, individuals are clones
from asexual reproduction, but do not
share a common tunic (covering)

17. Colonial ascidians 2: In compound species, individuals are
clones that share a common tunic, and
may even share siphons
PHOTO B: colonial species in which each individual animal retains its
own inhalent and exhalent siphons
PHOTO C: colony in which individuals are packed in a ring and share
a centrally placed, common exhalent siphon (yellow arrow).

18. Tadpole larvae show the chordate features that are not
seen in the adult ascidian
- all ascidian larvae are lecithotrophic (non-feeding) and most
are very short lived, swimming for a few minutes to find a
suitable site for attachment and metamorphosis

19. Ascidian Metamorphosis

20. Class Thaliacea, the salps
- pelagic animals living in open water
- gelatinous + transparent
colonial salp
- atrial siphon is shifted to posterior end
- contraction of circular muscles compresses the tunic, forcing
water out of atrial siphon & causing jet-propulsion forwards

21. Salps can be solitary or colonial.
Colonial forms make long chains, which easily break apart.

22. Class Appendicularia, the larvaceans
- pelagic, live in a secreted mucous house
pharynx reduced, bears only 2 slits
complex mucus nets spun for filter feeding
- neoteny: larval form develops gonads for reproduction,
adult stage is gradually lost from the life cycle
- postanal tail beats to produce water currents
- notochord + reduced dorsal nerve chord present in tail

23. As webs and house become clogged with debris, they are cast
larvaceans live inside a 2-layer
gelatinous (jelly-like) house:
(1) pre-filter web, 1 meter across,
traps large particles
(2) inner mucus filter traps
small particles (2-20 mm)
which are eaten
As webs and house become clogged with debris, they are cast
off (15 times per day)
- animal spins a new house just
before leaving old one, quickly
inflates it
- old houses sink to deep sea
larvacean out of its house

24. larvacean out of its house
larvaceans live inside a 2-layer
gelatinous (jelly-like) house
larvacean out of its house

25. An outstanding mystery in marine science: what supplies
all necessary food to sustain animals of the deep sea?
- 30% of the carbon needed to keep the deep sea alive
could not be accounted for in sediment traps
Answer: sinking larvacean webs supply the missing nutrients
to feed the ocean floor (Robison et al., Science 2005, 308: )

26. Also remove up to 50% of particulate material that rivers dump
into the upper 5 meters of the sea
- important to keep water clear enough for light to penetrate
for photosynthetic organisms
Thus, larvaceans are critical living filters that sustain
communities living in near-surface waters and the deep-sea!

27. SubPhylum Cephalochordata
- Amphioxus, fishlike animals also called lancelets
- Ciliary mucus suspension feeders, much like ascidians
- water moved through pharyngeal gill slits by cilia
- iodinated mucous produced by endostyle traps food
- Closed circulatory system similar to that of fishes, but no heart
- Segmented muscle bands called myotomes
- notochord lasts through adult life, but never becomes vertebral
column and no brain develops (i.e., definitely not a vertebrate)
25 spp.

29. Expression patterns of
developmental genes are
also being used to study the
evolution of vertebrates
- whole nervous system of
lancelet-like ancestor was
essentially compressed into
the vertebrate brain

30. SubPhylum Vertebrata: the vertebrates
vertebral column houses dorsal nerve cord
there’s a lot of interest in studying the other chordates
to understand how our lineage evolved – who’s our
nearest relative among the invertebrates?

31. vertebrates
Analysis of the highly
conserved 18S gene
suggested that the
nearest relatives of
vertebrates were the
cephalochordates

32. New analyses of hundreds of protein-coding genes
consistently support Ascidiacea as the sister group
of the vertebrates, not the cephalochordates
Cephalochordata
Ascidiacea
Vertebrata