Lesson 11: Echinodermata (and Deuterostome)

Deuterostomia vs. Protosotmia

• bilateria → deuterostome rather than protostome

• main differences

  • radial cleavage

  • regulative embryo

  • blastopore becomes anus, mouth forms secondarily

  • coelom forms by outpocketing (enterocoelous)

Deuterostome Radial Cleavage

• radial four cell embryo cleavage (cut in half, and in half again) compared to the protostome spiral cleavage of the embryo

Deuterostome Regulative Embryogenesis

• regulative (indeterminate) 

• 4 cell radial cleaved embryo excises one blastomere, to create 2 normal larvae

Deuterostome Fate of Blastopore 

• blastopore envaginates to form the anus initially

• mouth forms secondarily 

Deuterostome Enterocoelous

• outpocketing 

• coelom forms by outpocketing 

Deuterostomia DIvision 

• Ambulacraria or Chordata

  • Ambulacraria divides into Echinodermata or Hemichordata 

Deuterostome Groups

• Ambulacraria

  • Echinodermata

  • Hemichordata

    • Enteropneusta

    • Pterobranchia

• Chordata

Echinodermata

• “spiny skin” 

• exclusively marine 

• lack cephalization 

• loss of gill slits 

• 5 unique features: 

  • pentaradial symmetry in adults

  • calcareous endoskeleton 

  • water vascular system 

  • pedicellariae 

  • dermal branchiae (papulae)

• class: 

  • asteroidea 

  • ophiuroidea 

  • echinoidea 

  • holothuroidea 

  • crinoidea 

Pentaradial Symmetry

• from a bilateral ancestor 

• mouth side= oral 

• opposite of mouth= aboral 

Madreporite

• a perforated plate by which the entry of seawater into the vascular system of an echinoderm is controlled.

Calcareous Endoskeleton

• mesodermal porous endoskeleton 

  • meshwork of ossicles form the stereom 

  • connected by mutable “catch collagen” 

    • therefore, can change from liquid to solid 

Water Vascular System

• hydraulic fluid system 

  • canals and tube feet 

  • only open on one end 

  • specialized coelomic system 

• functions: 

  • respiration 

  • excretion 

  • locomotion 

  • feeding 

• medreporite → stone canal → ring canal → radial canal → lateral canal → ampullae → podia (tube feet)

Ambulacra

• bands from which tube feet project within Echinodermata 

• singlular: ambulacrum 

Pedicellariae 

• usually on aboral surfaces (all around in sea urchins)

• functions:

  • cleaning

  • food capture

  • defense

Dermal Branchiae (Papulae)

• also called skin gills 

• projections of thin walled coelom 

Basic Internal Characteristics

• nervous system 

  • no brain or ganglia

  • nerve ring, radial nerves, and epidermal nerve net

  • sensory cells; eyespots at the tip of arms

• digestive system 

  • feeding mode varies (carnivores, scavengers, herbivores, deposit feeders, suspension feeders)

• reproduction 

  • often separate sexes with external fertilization 

Larval Development

• deuterostome embryology 

  • coloem forms via outpocketing

  • anus forms before mouth

  • radial cleavage

  • regulative

• bilateral free-swimming larvae

  • left side becomes the oral surface

  • right side becomes the aboral surface

  • tube feet

• metamorphosis to adult form

Regeneration

• only need 1/5th of the central disk and 1 arm 

• able to deliberately shed body parts of appendages 

  • autotomy 

  • used as an escape response to predators, or as a means of replacing infected or damaged appendages 

• in some cases, fragmentation not regeneration 

  • Linckia 

Asteroidea

• class of phylum Echinodermata

  • star form

• poster echinoderm 

  • central disc + 5, 7, or 50 arms 

  • two part stomach 

    • can evert cardiac stomach 

  • scavengers and carnivores 

    • can also suspension feed 

• keystone species 

  • purple intertidal seastar (controls BC mussel populations)

  • reef seastar 

• some can be deadly

  • crown of thorns

  • pacific sunflower seastar

Seastar Wasting Disease

• white lesions form, and water vascular system stops working 

  • leads to fragmentation, and death 

• associated with warmer temperatures and densovirus 

Ophinuroidea

• class of phylum echinodermata 

• “like a serpent” 

• abundant and can form huge carpets

• move with flexible, articulated arms 

  • closed ambulacral grooves

  • tube feet for feeding, no suckers 

• no pedicellariae, papulae, or anus

• madreporite on oral side

• organs inside central disc

• cryptic

  • move away from light

• fragile, with remarkable regeneration

Echinoidea

• class of phylum Echinodermata

• sea urchins, sand dollars, heart urchins

• compact body within a test 

  • ossicles as fused plates 

  • some secondarily bilateral 

• 5 ambulacra extends up and around towards anus (aboral)

  • no arms

• tube feet for locomotion with assist from spines in socket joints 

  • short spines on sand dollars

• many with venomous pedicellariae (3 jaws)

• mostly herbivorous 

  • aristotle’s lantern with 5 teeth 

  • sand dollars deposit feeders 

Ecological Impacts of Echinoidea

• kelp forest destruction leads to an urchin barren environment 

  • effects surrounding wildlife: otter, etc. 

Holothuroidea

• sea cucumbers 

• elongated oral-aboral axis 

• secondarily bilateral 

• reduced ossicles 

• ventral tube feet specialized for locomotion on sole 

  • no arms 

  • benthic crawlers (however, few pelagic species exist)

  • burrowers, therefore no tube feet. circular and longitudinal muscles instead 

• oral tentacles are modified tube feet for suspension or deposit feeding

  • important nutrient cyclers

• cloaca

• spacious coelomic cavity

  • hydrostatic skeleton

Cloaca

• all purpose orifice via anus of holothuroidea

• respiration and excretion

  • respiration via respiratory tree

• amazing self-defence

  • evisceration of cuvierian tubules

Crinoidea 

• class of phylum echinodermata 

• stalk with cirri attached to aboral side of calyx with many arms that branch into pinnules 

  • can move, crawl, sweep, or swim via cirri or feathery arms 

• sea lillies 

  • flower shaped on top of stalk attached to substrate 

• feather stars 

  • short stalk with many branched arms 

• no pedicellariae, spines or madreporite

• mouth and anus on oral side 

• many in deep water 

Hemichordata 

• gill slits 

• once included in chordates, but notochord is not homologous with the chordate notochord

• 3 part coelom

• 2 classes

  • Enteropneusta

  • Pterobranchia

Enteropneusta

• class of phylum hemichordata 

• acorn worms 

• solitary, exclusively marine

• deposit and suspension feeders via mucus on proboscis 

Pterobranchia

• class of phylum hemichordata 

• small, tube dwelling mostly colonial

• suspension feeders via crown of tentacles with coelomic extensions 

  • convergent with the lophophore 

Echinodermata Organization

Echinodermata Symmetry

Echinodermata Body Cavity

Echinodermata Development

Echinodermata Segmentation 

Hemichordata Organization

Hemichordata Symmetry

Hemichordata Body Cavity

Hemichordata Development 

Hemichordata Segmentation