The deuterostomes Flashcards
Evolutionary branch of deuterostomes
- Bilateria
- Chordata (all invert chordates are marine)
Body layers
- Mesoderm
- Two layers
- Coelomates
Examples of Echinoderms?
- Starfish
- Sea Urchins
- Sea Cucumber
Echinoderm features
- Pentamerous radial symmetry
- Body divided into 5 parts
- Not related to other radially symmetrical groups
- Variation in groups
Odd one out = sea cucumber - e.g. ‘burnt sausage’ - secondarily bilaterally symmetrical
Mouth location on echinoderms
- Mouth on top: oral surface
- Anus on top: aboral surface
Some variation:
- Starfish & Sea urchins - aboral surface -mouth underneath
- Sea lilies & Feather stars - oral surface - mouth on top
Irregular urchins: mouth has moved to underneath at one end - moving towards bilateral symmetry
Echinoderm larval stages symmetry
Bilateral symmetry
Larvae stages show convergent evolution
Echinoderms
Although not closely related:
- Sea cucumber and starfish have similar larval stages
- Sea urchins and brittle stars have similar larval stage
- Some groups have lost the planktonic larval stage (also shows convergent evolution)
How do Echinoderms go from bilateral symmetrical larvae to radially symmetrical adults?
- Adult arises from cells set aside in the larval stage (maximal indirect development)
- Anterior-posterior body axis shifts, body twists around new axis (aboral-oral axis)
- Coelomic cavities undergoes reorganisation
Ossicle features
- Each calcareous ossicle separated by living tissue
- Increase size without moutling
- Unlike arthropods, does not need to shed
How can ossicles be modified?
- Ossicles can fuse together, are hinged (e.g. urchins)
- Aristotle’s lantern - used for feeding(grinds away at algae). Are modified ossicles
- Sea cucumber ossicles are either very minute or absent
Echinoderm internal skeleton
Ossicles
Echinoderms tube feed
Podia
Podia modifications
- Can be large - used for locomotion and feeding
- Starfish with long legs use podia for latching onto prey - can pry open bivalves and inverts stomach into shell to digest prey by releasing enzymes.
How Brittle Stars uses their Podia
- Don’t use podia for locomotion
- Use podia for feeding
- Secrete mucus which created sticky surface that food gets stuck to
- Moves around by thrashing legs
How Sea cucumber use their Podia
Deposit feeder or suspension feeder using modified podia
Echinoderm: Water vascular system that controls the podia
- Modified coelom
- Sea water is circulated around the animal
- Podia can contract and relax via water circulating and entering the body and ampulla
Echinoderm: Mutable connective tissue
- Able to change their connective tissue
- Able to change how stiff it is
- Change in viscosity of collagen fibres
- Under nervous control so doesn’t use much energy
- Very rapid
- Defence mechanism
Echinoderm: Mutable connective tissue (Examples)
- Sea cucumbers can release their guts out of their back end
- Sea urchins spines can change rigidity for movement
Echinoderm: Nervous System
Nerve net
- Central ring with branches going down legs (in starfish)
- Part can take on a temporary brain function
What are Hemichordates?
- “Half chordates”
- They lack a true notochord
Two groups of Hemichordates
- Enteropnuets: Acorn worms
- Pterobranchs: Sea angels
Enteropnuets: Acorn worms
Hemichordates
- Solitary
- Vary in length long
- Body made up of 3 regions (proboscis, collar and trunk)
- Proboscis collects food, gills slits used to respiration
- Stomochord - supports heart and excretory organ
- Larval stage (toraria) - similar to echinoderms
- Nerve net (concentrated in collar and can be hollow)
Pterobranchs: Sea angels
- Lophophores
- Still same structure as acorn worms
- Very small 1-5mm long
- Sessile
- Colonial
- Some have gill slits
- Simple nerve net
- Stomochord - supports oral shield
- Can be traced back to the graptolites (cambrian - devonian)
Three groups of Chordates
- Urochordates: Tunicates
- Cephalochordates: Lancelets
- Vertebrates
Chordate features
Chordates have the following at some point in their lives:
- Notochord - stiffening rod used as support
- Dorsal, hollow nerve cord
- Pharyngeal (gill slits)
- Post-anal tail
Invertebrate chordates:
Urochordates (tunicates)
- In larval stage they have all the chordate features (‘Tadpole’ larva)
- In adults all that is left is pharyngeal slits
- Covered in ‘tunic’ which contains cellulose
Invertebrate chordates:
Urochordates (tunicates) - Larvae to adult metamorphosis
- Larval stages swims, settles on a surface (head down) and undergoes metamorphosis
- Notochord loses turgor
- Tail absorbed
- Rapid growth between buccal syphon & papillae
Unusual example of a tunicate: Larvacea
- Retains larval features in adult form
- Larvacea or Appendicularia (called both names)
Ceplalaochordates (amphioxus/lancelet)
- All features of chordates
- Sirri - tentacles - sensitive to chemicals and also passes water into the mouth over gills - filter feeders.
- Endostyle - transforms into thyroid gland in vertebrates
Phylogentic tree of echinoderms, hemichordates and Chordates
Two sister groups within the Deuterostomes
- Ambulacaria: Echinoderms & Hemichordates
- Chordates: Urochordates & Vertebrates
- Gill slits lost in Echinoderms
- Nerve net lost in Chordates (dorsal nerve chord)
