Echinodermata Flashcards
Echinoderms (Echinodermata) Unique characteristics
● Endoskeleton of calcareous plates (Ossicles)
● Catch connective tissue (aka Mutable collagenous
tissue)
● Water-Vascular System
● Pedicellariae (claws used for defence)
● Dermal Branchiae (aka Papulae)
● Secondary radial symmetry - often Pentaradial
Crinoidea Feather stars & sea lilies
● All are suspension feeders
● Crinoids are some of the most ancient echinoderms
and the living classes are some of the most primitive
● Adults typically have pentaradial symmetry, with
body composed of a basal attachment stalk and a
crown with 5 or more flexible arms
● Oral surface points up (unlike all other echinoderms)
● Lacks many of the more specialised features we’ll
see in other echinoderm classes
Asteroidea (sea stars & starfishes) basic body plan
● Central disk with arms – organs spread throughout disk and arms
● Often pentaradial adult forms (pentamerous repeating of 5 arms)
● Oral surface has mouth. Anus on opposite(aboral) surface, together with mandreporite (inlet for water vascular system we’ll explore later)
● Ambulacral grooves contain 2 or 4 rows of
tube feet, and can close up with spines protecting the feet
Two stomachs:
● Evertable stomach that can be inserted into
mollusc shells or spread across corals to eat
them – secretes enzymes and partially digests
prey – and is then pulled back
● Food is then passed to the Pyloric stomach,
which extends into arms and a series of glands secrete digestive enzyme
Ophiuroidea (brittle stars & basket stars)
● Generally scavengers & detritivores: ambulacral grooves are closed, and tube feet inside the arms pass small organic particles along to the mouth. Basket stars can be suspension feeders using mucus to catch particles
● Central disc contains all organs
● Tube feet are not used for locomotion, instead muscles control lateral movement of arms
● No anus (food goes in and out of the mouth)
● Bursal slits open at the base of arms for gas exchange and excretion, and gamete release (typically two per arm)
Holothuroidea (sea cucumbers)
● Deposit or suspension feeders: using 10 elaborate buccal tentacles (modified tube feet), which arearranged in a ring around the mouth
● Elongated body – cucumber shaped
● Endoskeleton made up of microscopic ossicles
● Unique respiratory system called respiratory tree, a branching structure inside anus for gas
exchange. The anal sphincter and cloaca work topump water through the system
● Undefended by spines, but have interesting
defensive mechanisms including: discharge of
Cuvierian Tubules (long, sticky, soapy, sometimes
toxic threads), or evisceration: discharge of
digestive tract, respiratory tree and small parts of
the gonad
Echinoidea (sea urchins & sand dollars)
● Generally herbivores feeding on algae, but also
omnivorous consumers and predators
● Adults are typically spherical or flattened (sand dollars), with pentaradial symmetry
● Endoskeletal test made of fused ossicles
● Movable spines and tube feed used for locomotion
● Unique feeding apparatus called Aristotle’s lantern for scraping algae from substrate
Aristotle’s lantern
● Composed of 5 calcareous plates– each is
shaped somewhat like an arrowhead with the point projected towards the mouth
● Special muscles enable the lantern to be
partially protruded and retracted through
the mouth
● Combined muscle action retracts and protrudes the lantern, for pulling and tearing in addition to scraping
● Self sharpening – constant addition of
Platelets
Ossicles
● The outer surface is covered by an epidermis
● Below the epidermis is a thick layer of body wall
connective tissue that forms the endoskeletal
system, providing protection & rigidity
● Endoskeleton composed by calcareous ossicles
● Spines are modified ossicles that rest on the
dermal ossicles or are an extension of the ossicles
● Urchins: ossicles are fused together to make a
shell called the test
● Sea cucumbers: ossicles are microscopic
Catch connective tissue (AKA Mutable collagenous tissue)
Allows the stiffness and extendibility of their collagenous connective tissues (outside of cells) to be altered rapidly (1 s to minutes) and reversibly
Catch connective tissue features:
● Under direct nervous control
● Doesn’t use muscles (but coordinates with them), and does not use ATP (energy) at the point of use
● Used in locomotion, posture, defence and
reproduction
↳e.g. starfish stiffen body posture when feeding on prey
↳Used for autotomy (dropping arm when
attacked by predator)
Water vascular system
● Unique to echinoderms
● System of canals & tube feet- main function locomotion / hydraulic skeleton
● System is filled with fluid; mostly sea water with some extra proteins, and high potassium ions (driving osmotic influx through tube feet)
● Madreporite connects the system to the outside world, using cillia to pump water in and maintain positive pressure (in addition to osmosis)
● Water then flows through the ring canal, into lateral canals, and radial canals
● Valves on lateral canals control pressure going to each ampulla and podium (tube foot)
Tube-foot stepping motion under hydraulic control:
● Muscle contractions cause the ampulla to contract, forcing water into each tube foot (podium), causing them to extend
● Each podium secretes an adhesive that causes them to stick to any surfaces they contact
● When muscles in the podium contract, water is forced back into the ampulla, shortening the tube foot and causing it to pull back
● Another secretion from the podium breaks the chemical adhesive bond, causing the podium to release
● Many podia working together can cause locomotion and allow the animals to stick to surfaces tightly
Echinoderm body walls have specialised structures including
Pedicellariae: articulated ossicles, role in defence
and predation
Papulae: on cross-section of arm (specialized gills, for respiration and waste removal)
Echinoderm Nervous system
● Dispersed central nervous system consisting of a number of radial nerves interconnected by a
ring nerve around the mouth opening
● Sufficient for basic behavioural responses
including hunting, tracking scent, opening prey,
and anti-predator responses
● The radial and ring nerve systems have both
sensory and motor components
↳Sensory system integrates mechanical, chemical and photoreceptor inputs
↳Motor system controls the tube feet and arms; if one arm detects an attractive odour it becomes dominant over other arms and
dictates direction of movement
Photoreception:
● Dispersed photoreception is found across echinoderms, with sensors in the tube
feet and pedicellariae, often used to seek out dark places to hide (negative
phototaxis)
● Ocelli (compound eyes) have only been found in sea stars and in a single species of sea cucumber
↳Sea star compound eyes at the tip of arms, at the end of the radial nerve
↳Very low acuity (50-300 ommatidia in each eye, up to 8 degrees visual angle)
Echinoderms are a major pest species
● Major pest of mussel & oyster beds
● Crown of thorns are native to great barrier reef, but population booms pose a major threat to
coral reef
