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?

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 and cellulose like

pharynx, tunic;

composed of proteins, salts, • Tunic encloses a basket 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

particles that are present in the water.

that strain larger particles • Feed by secreting a mucous net across the gill slits to filter out food