Hemichordates

Question 1

Enteropneusts

Answer 1

Acorn worms - marine
•Solitary and burrow in muddy deposits and deep-sea floors
•3 body regions - proboscis; collar and trunk
•Proboscis used to collect food
•Gills slits for respiration (trunk)
•Larval stage - tornaria, similar to echinoderms
•NS like nerve-net, conc of nerves in collar region that can be hollow…
•Stomochord - adds support to collar, completely different origin to notochord

Question 2

Pretobranchs

Answer 2

Sea angels - marine
•small and sessile - attached to substrate
•colonial
•proboscis, collar and trunk
•has lophophore used as feeding device 
•suspension feeders
•can withdraw into tube-like structures
•has 1 gill slit but still present 
•eve history can be traced back to graptolites to Cambrian-Devonian - ancient

Question 3

Hemichordate resemblance to chordates (or not)

Answer 3

• no notochord - stomochord completely different
• don’t have post anal tail
• nervous tissue in acorn worm hollow in collar - not dorsal chord though
• collect food differently - lophophore or proboscis externally (not gill slits like invert chordates)
• but do see gill slits - Pax genes are responsible for development of pharynx, expression the same

Question 4

Hemichordate resemblance to echinoderms

Answer 4

•in same evolutionary line
•similar larval stage
•similar NS - the nerve net
-acorn worm, the NS expresses same genes in same order as chordates - link between two
•some fossil forms of echinoderms have gill slits in at least 1 group - Homalozoans

Question 5

Closest group to hemichordates?

Answer 5

The echinoderms

-forms a group called the Ambulacrana - separate to the chordates