Hemichordata
Invertebrate Chordates
Evolutionary Perspective
• Echinoderms, Hemichordates, and Chordates
are most likely derived from a common (yet
undiscovered) ancestor, since they all share
deuterostome characteristics.
What characterizes a chordate?
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A dorsal tubular/hollow nerve cord
A notochord
Pharyngeal slits or pouches
Postanal tail
Which are shared by both Chordates and
Hemichordates?
• Pharyngeal slits
• Most have a dorsal (sometimes hollow) nerve
cord.
This shows that evolutionary ties b/w
Hemichordates and Chordates are closer than
b/w echinoderms and either of these phyla.
Hemichordates
• Hemichordata means “half chord”
• Includes: Acorn worms and Pterobranchs
• Live in or on marine sediments
5 Characteristics of Hemichordates
• Marine, deuterostomes w/bodies in 3
segements-proboscis, collar, and trunk
• Ciliated pharyngeal slits
• Open circulatory system
• Complete digestive tract
• Dorsal (sometimes hollow) nerve cord
Class Enteropneusta
• Acorn worms-named so b/c of the proboscis
at the anterior end
• 75 species, 10-40 cm usually
• Most occupy U-shaped burrows along the
sandy shore-line
• Cilia and a mucous-covered proboscis assist in
feeding
Class Pterobranchia
• Name means “wing or feather gills”
• Around 20 species-Rhabdopleura
• Small-0.1-5 mm, often living in secreted tubes in
asexually produced colonies
• Proboscis is sheild-like and secretes the tube
• Found mostly in deep oceans of the Southern
hemisphere
• Use cilia on arms/tentacles to filter and transport
food to mouth
Phylum Chordata
• Bilateral symmetry, deuterostomes
• Notochord, pharyngeal slits or pouches, dorsal
nerve chord, and postanal tail present at some
point during development
• Presence of endostyle (produces mucous) or
thyroid gland
• Complete digestive tract
• Ventral, contractile blood vessel (heart)
• Notochord-a supportive rod extending most of
body length
• Pharyngeal slits-series of openings along
pharyngeal region. Invert chordates use for filter
feeding. Some chordates for gas exchange.
• Dorsal hollow nerve cord-runs along length of
body, associated w/development of complex
sensory systems
• Postanal tail- extends posterior to the anus
SubPhylum Urochordata
• Marine animals; some species are solitary, others are colonial.
• Sessile adults, but motile during the larval stages
• Possess all 5 chordate characteristics as larvae
• Settle head first on
hard substrates and
undergo a dramatic
metamorphosis (e.g.,
tail, notochord, muscle
segments, and nerve
cord disappear)
SubPhylum Urochordata
cont.
• Adult body covered by an
outer envelope or tunic;
composed of proteins, salts,
and cellulose
• Tunic encloses a basketlike pharynx, that is
perforated by gill slits
• Tunicates are filter feeders; Oral siphon (mouth)-plankton is trapped
in a sheet of mucus and cilia later direct the food-laden mucus to the
stomach
• Water leaves the animal via an excurrent siphon
SubPhylum Cephalochordata- 45 species
• “Head chord”
•Exclusively marine, tadpole-like animals (up to 5cm)
• Although they are capable of swimming, they usually are buried in
the sand with only their anterior end being exposed
•World’s oceans in shallow waters clean sands
SubPhylum Cephalochordata cont.
•All chordate characteristics are present throughout their life history
• They are filter feeders: inside of the oral hood is lined with cilia wheel organ
• These cilia, plus cilia in the pharynx help generate a water current
• Water and suspended food particles pass through the oral hood,
equipped with projections called cirri that strain larger particles
• Feed by secreting a mucous net across the gill slits to filter out food
particles that are present in the water.