Module 15

Question 1

Synapomorphies of chordata

Answer 1

Notochord
Hollow nerve cord
Pharyngeal slits
Post-anal tail
Endostyle/thyroid gland

Question 2

Notochord

Answer 2

Rod-like, semi rigid tissue enclosed in a sheath
Acts as an internal skeleton to which muscles can attach to
Stiffens the body

Question 3

Dynamics of notochord

Answer 3

Notochord is pulled on by opposing muscle blocks on either side of the body relaxing and contracting
Notochord is flexible but not compressible - does not shorten when pulled upon by myotomes but instead bends

Question 4

Notochord in protochordates versus vertebrates

Answer 4

Protochordates:
- Notochord persists throughout life

Vertebrates:
- Replaced by vertebrae and only remains as intervertebral discs

Question 5

Dorsal hollow nerve cord

Answer 5

Located dorsal to the digestive tract
Anterior end is an expanded region known as the brain

Question 6

Evolutionary benefits of dorsal hollow nerve cord

Answer 6

The enlarged nerve network found on the anterior end enabled chordates to evolve highly complex and coordinated communication within the body

Question 7

Pharyngeal slits

Answer 7

Openings or pouches between the pharyngeal cavity and the external environment

Question 8

Pharyngeal slits in protochordates

Answer 8

Function as filter-feeding apparatus
- mucous secreted from the endostyle is carried dorsally by cilia and traps food
- mucous food string is then passed onto the gut

Question 9

Pharyngeal slits - Gills

Answer 9

Gills: network of capillaries that have thin walls permeable to gases
Pharyngeal slits provided chordates with a specialised and novel gas exchange system

Question 10

Evolution of gills

Answer 10

Water currents passing over the high surface area of the pharynx provide the conditions for the evolution of respiratory gills in fishes
Pharyngeal slits allowed for the exchange of gas to no longer be limited through the body wall
Enabled fishes to grow much larger - the reliance of organisms on their skin surface area for gas exchange was limited by the speed at which those gases can diffuse in and out of the blood

Question 11

Endostyle

Answer 11

Thyroid gland
- secretes iodinated compounds
- hormones that help regulate metabolism
- the presence of this organ has facilitated more complex and coordinated responses in vertebrates

Question 12

Endostyle in protochordates and larval lampreys

Answer 12

Endostyle is locand at the base of the pharynx and secretes mucous to trap food

Question 13

The post-anal tail

Answer 13

Associated with the musculature that surrounds the stiffening notochord
- structure provides motility
- used for locomotion, communication, and more

Question 14

Discuss how the five synapomorphies enabled chordates to diversify in form and functions

Answer 14

The presence of a notochord enabled chordates to evolve a variety of different body forms and mode of locomotion
- The notochord acts as an internal skeleton to which muscles can attach and the stiffening of the body enabled chordate bodies to withstand stronger forces
- In combination, these attributes led to faster forms of locomotion, including in dense media such as water

The enlarged nerve network that forms the brain enabled chordates to evolve highly complex and coordinated communication with the body, and in some vertebrate groups has led to development of cognition

Pharyngeal slits provided chordates with a specialized and novel gas exchange system, which meant they were no longer limited to exchanging gas through the body wall and enabled chordates to grow MUCH bigger

The presence of the thyroid gland has facilitated more complex and coordinated responses in vertebrates to their surroundings, including behaviours related to predation and defence.

Most vertebrates have a notable post-anal tail used for locomotion, communication and more
- The earliest forms of the tail were obviously used for swimming, the efficiency of which was improved with the evolution of different tail shapes and fins

Question 15

Urochordata

Answer 15

Mostly marine animals
- Sessile in adults form
- Sedentary filter-feeders
Have an outer covering called the tunic
- Thick, non-living matrix that protects the internal organs and is rich in tunic cells
- Tunic cells secrete the matrix and protect it from pathogens and parasites

Have all synapomorphies + neural crest

Question 16

Classes of Urochordata

Answer 16

Ascidiacea
Thaliaaceea
Appendicularia

Question 17

Larval forms of Urochordata

Answer 17

Only the larval forms bear all of the chordate synapomorphies
Free-swimming - aids in their dispersal
During metamorphosis, they lose many of their synapomorphy characteristics

Question 18

Class Ascidiacea

Answer 18

Example - Sea Pineapple Halocynthia roretzi
- sessile in adult form
- many have strong anti-predator chemical defences
-post-anal tail and notochord are present in larvae – lost during metamorphosis into a sessile adult

Question 19

Class Thaliacea

Answer 19

Transparent, marine pelagic, filter-feeders
Also known as salps
Bodies are cylindrical with the exhaling and inhalant siphons arranged at either end
- makes them jet powered by sucking in water at the anterior end and pushing it out at the posterior end to swim around
- fed using mucous produced by the endostyle passing over the mesh-like pharynx

Question 20

Class Appendicularia

Answer 20

Rely on mucous to feed
Primary feeding organ - specialised epidermal cells
- muscular tail beats inside the primary feeding organ to drive water into its through incurrent filters
- these block larger particles from entering
- the tiny particles that pass through are captured in feeding filters
- captured particles are then moved by the pharyngeal cilia into the mouth where they are gathered in mucous produced by the endostyle and both passed to the gut

These filters clog up and the primary feeding organ must be discarded and remade up to 5 times a day

Question 21

Subphylum Cephalochordata

Answer 21

Lancelets
- slender, laterally flattened, translucent animals
- live in muddy and sandy bottoms of coastal waters