Module 15 Flashcards
Synapomorphies of chordata
Notochord
Hollow nerve cord
Pharyngeal slits
Post-anal tail
Endostyle/thyroid gland
Notochord
Rod-like, semi rigid tissue enclosed in a sheath
Acts as an internal skeleton to which muscles can attach to
Stiffens the body
Dynamics of notochord
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
Notochord in protochordates versus vertebrates
Protochordates:
- Notochord persists throughout life
Vertebrates:
- Replaced by vertebrae and only remains as intervertebral discs
Dorsal hollow nerve cord
Located dorsal to the digestive tract
Anterior end is an expanded region known as the brain
Evolutionary benefits of dorsal hollow nerve cord
The enlarged nerve network found on the anterior end enabled chordates to evolve highly complex and coordinated communication within the body
Pharyngeal slits
Openings or pouches between the pharyngeal cavity and the external environment
Pharyngeal slits in protochordates
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
Pharyngeal slits - Gills
Gills: network of capillaries that have thin walls permeable to gases
Pharyngeal slits provided chordates with a specialised and novel gas exchange system
Evolution of gills
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
Endostyle
Thyroid gland
- secretes iodinated compounds
- hormones that help regulate metabolism
- the presence of this organ has facilitated more complex and coordinated responses in vertebrates
Endostyle in protochordates and larval lampreys
Endostyle is locand at the base of the pharynx and secretes mucous to trap food
The post-anal tail
Associated with the musculature that surrounds the stiffening notochord
- structure provides motility
- used for locomotion, communication, and more
Discuss how the five synapomorphies enabled chordates to diversify in form and functions
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
Urochordata
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
Classes of Urochordata
Ascidiacea
Thaliaaceea
Appendicularia
Larval forms of Urochordata
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
Class Ascidiacea
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
Class Thaliacea
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
Class Appendicularia
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
Subphylum Cephalochordata
Lancelets
- slender, laterally flattened, translucent animals
- live in muddy and sandy bottoms of coastal waters
