bio 205 chapter 20: the echinoderms

phylum echinodermata

approximately 7,000 species
name translates to “spiny skinned”
radial symmetry as adults
triploblastic, coelomate, deuterostomes
bodies possess 2 surfaces
no segmentation

pentamorous radial symmetry

parts in multiples of 5

aboral surface

“top” dorsal surface typically where the anus is found, name means “opposite the mouth”

skin gills (papulae)

allows for gas exchange and waste removal from epidermal cells

pedicellariae

“forked-like” structures that allow for the removal of debris that may land on the aboral surface

anus

for the release of solid waste from the digestive tract

madreporite (sieve plate)

the opening of the water vascular system, sits on the central disk slightly off to one side

central disk

the center of the echinoderm body

arms (rays)

extend from the central disk, not possessed by all echinoderms

oral surface

“bottom” ventral surface where the mouth is found

mouth

centrally positioned on the central disk

oral spines

directed towards the mouth

ambulacral spines

directed towards the ambulacral groove

tube feet

sucker-like extensions from the internal water vascular system that are used for movement and food handling, extend from the ambulacral grooves

ossicles

pockets of calcium carbonate found within the skin of echinoderms that provide rigidity to the body and allow for some degree of protection

organ-systems that echinoderms lack

no blood circulatory system, no true respiratory organs, no true excretory organs

water vascular system

consists of numerous canals and sacs through which water moves and compensates for many of the missing or reduced systems and is encased in an endoskeleton of calcium carbonate
functions: movement, food handling, respiration of gases, excretion of ammonia, sensory perception

stone canal

runs vertically and carries the water from the madreporite (sieve plate) to the ring canal (circular canal) that can move water clockwise or counterclockwise

Polian vesicles

sacs that hang off the ring canal (circular canal)

Tiedemann’s bodies

muscular valves on each side of a Polian vesicle
if one constricts the ring canal, water will move into a Polian vesicle which then contracts to change the direction of the water movement through the ring canal

small lateral canals

extend off each side of a radial canal and carry water to the ampullae

ampullae

sacs that fill with water and activate the tube feet that extend below

ambulacral ridges

ridges of calcium carbonate that cover each radial canal and ampullae extend out of the sides
presence in each arm creates ambulacral grooves on the oral surface of each arm from which tube feet extend

evidence of radial symmetry

1) central disk from which arms (rays) extend
2) water vascular system is radially symmetrical with radial canals extending from ring (circular) canal
3) gonads in each arm
4) nervous system is radially symmetrical and consists of a nerve ring in the central disk with a radial nerve through each arm
5) digestive system is radially symmetrical with digestive glands extending from pyloric stomach out each arm

cardiac stomach

a spherical chamber for the storage of food

pyloric stomach

sits on top of the cardiac stomach and is responsible for the chemical/enzymatic digestion of food

digestive glands (pyloric cecae)

extend from the pyloric stomach out through each arm allowing for continued digestion and distrubution of nutrients out to the arms

intestine (rectal cecae)

sits on top of the pyloric stomach and will form solid waste that gets released out the anus

subphylum crinozoa

class crinoidea

subphylum asterozoa

class asteroidea, class ophiuroidea

subphylum echinozoa

class echinoidea, class holothuroidea

class crinoidea

sea lillies and feather stars
sessile as adults
spineless
tentacle-like arms with long tube feet adapted for food trapping
cirri and pinnules

cirri

branches from the bottom of the body that attach a crinoid to a substrate

pinnules

long tube feet that extend from the tentacle-like arms

class asteroidea

sea stars (starfish)
5 or more arms
arms are thick and blunt at the tip
spines
tube feet extend from below the arms

class ophiuroidea

brittle stars and basket stars
5 or more arms
arms are slender and serpentine, some branch
spines
tube feet extend from below the arms

class echinoidea

sea urchin, sand dollars and sea biscuits
armless
long, numerous spines
tube feet extend from the oral surface of the central disk

sea urchins

longest and most numerous spines of all echinoderms
movable spines
tube feet present along sides of central disk
Aristotle’s lantern

Aristotle’s lantern

a jaw-like structure in the mouths of sea urchins

sand dollars

flattened dorso-ventrally
spines only found on outer margin (edge)
some species have the anus on the oral surface along with the mouth

class holothuroidea

sea cucumbers
armless
spineless
tube feet appear in rows along sac-like, cucumber-shaped body
dendritic branching tentacles for food trapping

larval stage of class crinoidea

doliolaria larva, bilaterally symmetrical

larval stage for class asteroidea

bipinnaria larva, bilaterally symmetrical
brachiolaria larva, radially symmetrical

larval stage for class ophiuroidea

ophiopluteus larva, radially symmetrical

larval stage for class echinoidea

echinopluteus larva, radially symmetrical

larval stage for class holothuroidea

auricularia larva, bilaterally symmetrical