Riley Galton
A. Echinoderm Classes
Asteroidea – Sea Stars:
l Multiple arms radiating from central disk
l Tube feet on bottom
Ophiuroidea – Brittle Stars:
l Distinct central disk with snake like arms
l Tube feet lack suckers
Echinoidea – Sea Urchins and Sand Dollars:
l No arms, but 5 rows of tube feet
l Have muscles that can move
l spines
Crinoidea – Feather Stars and Sea Lilies
l Flexible arms used in suspension feeding
l Haven’t changed in 500 million years
Holothuroidea – Sea Cucumbers
l Reduced endoskeleton
l Five rows of tube feet
Concentricycloidea – Sea Daisies
l Only two known species
l Disk shaped, less than 1cm in diameter
B. Echinoderm Sensory Structures
l Eyespots on the ends of ambulacra are sensitive to light
l Pedicellariae (jaw like skeletal structures used to keep organisms from settling on skin), tube feet, and spines are sensitive to touch
C. Echinoderm Reproduction
Sexual:
• Eggs and sperm are released into the water where they are fertilized
• Larvae are planktonic
• Internal fertilization has been observed in certain species of sea stars, brittle stars, and sea cucumbers
Asexual:
• Echinoderms can regenerate body parts and organs
• Asexual reproduction can occur (though not by will) if the echinoderm is torn with enough of the body on each part
Julia Heunis
D. Body Cavity (Coelom)
l PRESENT!
l Coelom = A fluid-filled cavity between body wall and gut; lined by mesoderm
l Two parts:
Perivisceral coelom – large, fluid-filled cavity where major organs, digestive tube and sex organs, are suspended
Water Vascular System - network of hydraulic canals unique to echinoderms that branches into extensions (tube feet) which function in locomotion, feeding and gas exchange
E. Body Symmetry
l evolved from animals with bilateral symmetry
l adult echinoderms possess radial symmetry
l All echinoderms exhibit fivefold radial symmetry in portions of their body at some stage of life (5 parts around central axis), even if they have secondary bilateral symmetry
l For the most part…
LARVAE ~ bilateral
ADULTS ~ radial
F. Excretory System
l NO true excretory system!
l Main opening of a sponge used only to EXPEL WASTE
l Anus leads directly from stomach/digestive tract
l Anus = top
mouth = bottom
Olivia Heunis
Julia Heunis
G. Skeletal Type Echinodermata: “Spiny Skinned”
l DO NOT possess an exoskeleton
l a thin outermost skin covers a mesodermal endoskeleton made of tiny calcified plates and spines - forms rigid support contained within tissues of the organism
l Skeleton composed of skeletal plates called ossicles
l Ossicles = small bones
l In some species, such as the sea urchin, plates of the skeleton are locked together to form a rigid structure.
l most sea stars and brittle stars can flex their ‘arms’ = skeleton has gaps, flexible plate junctures
H. Other Unique Features
l Echinoderms can REGENERATE
ex: sea cucumbers can eject a portion of gut in response to predators and regenerate when safe
l Water vascular system (described earlier) is unique to echinoderms; varies water pressure to control tube feet
l Crinoids and some brittle stars = passive filter-feeders, absorbing suspended particles from passing water
sea urchins = grazers
sea cucumbers = deposit feeders
starfish = active hunters
l Autonomy = The spontaneous self amputation of an appendage when the organism is injured or under attack. The autotomized part is usually regenerated.
l “arms” → multiples of 5; some have more because of regeneration
l mutable collagenous tissue ~ connect ossicles; can maintain different positions without much effort
I. Nervous System
l Decentralized
l central nerve ring surrounds gut, connect radial nerves.
l Radial nerves run under each arm, coordinate movement, etc.
l Do not have “brains,” but some have ganglia along radial nerves
J. Digestive System
l Complete digestive system (tubular gut), mouth to anus. (bottom to top)
l Crinoidia: filter-feed, use cilia
l Asteroidea: Cardiac (evert), pyloric stomach, digestive glands
l Urchins: “Aristotle’s lantern” (masticatory apparatus)
l Holothurians: suspension/deposit feeders, use tentacles.
K. Locomotion/Musculature
l Utilize water vascular system and tube feet
l Water in from madreporite, pumped into ampulla by radial canal
l Ampulla contracts, water to podia
l Podia contracts, water to ampulla
– Podia bend, shorten allows movement (vacuum and suction)
L. Gas Exchange
l Asteroids, crinoids: dermal gills (thin epidermis)
l Urchins: infolds of body wall (bursae) opening to outside
l Sand dollars: modified podia on top (petaloids) which are thin-walled and flaplike.
l Holothurians: highly branched hindgut called "respiratory tree"
M. Circulatory System
l Mostly in perivisceral coelom, enhanced by water vascular system and hemal system.
– Hemal system: series of canals and spaces mostly in coelomic channels. Fluid movement by cilia. Used to distribute nutrients. No respiratory system.
Quiz
- Echinodermata have ________ symmetry as larvae, but display ________ symmetry as adults.
- What allows Echinoderms to function in locomotion, feeding and gas exchange?
- Sea stars can grow new arms and sea cucumbers can replace their gut after ejecting them. What feature of echinoderms does this demonstrate?
- Name the stomachs of a sea star. Functions?
Answers
l Bilateral, radial
l Water vascular system
l Regeneration
l Cardiac, pyloric; evert and enzymes, further digestion
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