Echinoderms Flashcards
(35 cards)
What kind of organisms does the echinoderms include?
Starfish, sea urchins, sand dollars, sea cucumbers and sea lillies.
What percentage of deep sea biomass is made up of echinoderms?
95%
Why do we have a good fossil record of echinoderms?
Their calcareous (calcium carbonate) skeletons are well preserved.
When do the earliest echinoderm fossils date back to?
The Cambrian.
What is a) stereo and b) stereom mesh?
a) A calcareous mineral with a magnesium oxide component
b) The porous stereom structure that makes up echinoderm skeletons
How is it that echinoderms can regrow their limbs?
The stereom mesh coalesces into ossicles that can grow in all directions.
Define an ossicle.
A bony plate.
Is the arrangement of ossicles the same in all the echinoderms?
No: in sea urchins they are very tightly packed, getting progressively looser from starfish to sea cucumbers whose ossicles are microscopic.
Echinoderms are highly regenerative. What advantage does this give them?
They evade predators by shedding limbs and regrowing them.
Echinoderm spines have birefringent optical properties. What does this mean?
They can refract light by 2 different indices.
Echinoderms occupy a wide variety of feeding niches, from predator to detritivores to filter feeding. True or false?
True.
Echinoderms are all marine yet they lack gills. How do they breathe?
Through the WVS
Echinoderms are all marine yet they lack gills. How do they breathe?
Through the WVS or water vascular system.
Explain how the WVS works.
A series of canals or ambulacra branch from a central ring canal around the mouth. Water is brought in through the madreporite. The madreporite is connected to the central ring by the stone canal. The ambulacra are connected to the podia (tube feet) via the ampulla.
The seawater and water in the WVS are in constant equilibrium. Why has this limited echinoderms?
They are unable to leave the sea.
What are the 3 functions of the WVS?
- Locomotion: hydraulic pressure extends/retracts the tube feet
- Respiration: gas exchange occurs across papillae (dermal gills)
- Feeding: hydraulic pressure causes ejection/retraction of the gut
The circulatory system runs parallel to the WVS. True or false?
True.
Define a coelomocyte.
Immune cells in the circulatory fluid
Is the nervous system central or decentralised?
Decentralised: There are radial nerves running down each limb that form networks. There is no head or ‘brain’, however each limb essentially has its own nervous system.
There are no obvious sensory organs in echinoderms. Some starfish have an optic cushion - what is this?
A group of photoreceptor cells. Some echinoderms also have sensory receptors at the end of their podia.
There are distinct sexes in echinoderms. Do they display dimorphism?
No
Is fertilisation internal or external?
External: one sex releases a chemical that induces gamete release in the other.
How have echinoderms been classified with the deuterostomes?
They have ~7000 deuterostome genes but only ~5000 protostome genes.
What are the 5 classes of echinoderm?
- Crinoidea: sea lillies
- Asteroidea: starfish
- Ophiuroidea: brittle stars
- Holothuroidea: sea cucumbers
- Echinoidea: sea urchins