Puzzling Spiny-Skinned Creatures

The echinoderms show several derived traits that are peculiar among deuterostomes

"I salute the echinoderms as a noble group especially designed to puzzle the zoologist"

Libbie H. Hyman (1955), The Invertebrates, Vol. 4. Echinodermata

Overview of Echinoderms

• "Spiny-skinned"
• 6,500 species
• By far, the largest group of invertebrate deuterostomes
• All are marine, mostly benthic
• 20 Classes described, mostly fossils
• We will consider 4 of 5 living Classes

Characteristics of Echinoderms

• Pentamerous secondary radial symmetry
• External fertilization
• Larvae are pelagic, adults usually benthic
• Movable and immovable calcareous spines
• Calcareous ossicles in dermal connective tissue
• A water vascular system

Class Stelleroidea

• Sea stars and brittle stars
• Arms in multiples of 5 (pentamerous)
• Two distinct groups:
  • Subclass Asteroidea
  • Subclass Ophiuroidea

Subclass Asteroidea

• 1500 spp, especially in cold waters such as Pacific NW
• Epidermis with numerous spines and pedicellariae
• Large perivisceral coelom (coelomic dominance)
• 2nd and 3rd coelomic pouches form water vascular system
  • aboral madreporite --> stone canal --> ring canal --> 5 radial canals --> numerous ampullas bearing tube feet
• Digestive system
  • cardiac stomach (eversible)-->pyloric stomach-->paired digestive glands (pyloric cecae) in each arm
• Nervous and circulatory systems also arranged radially

Class Ophiuroidea

• Brittle stars, basket stars
• 2000 species, largest group of Echinoderms
• Central disc well-defined, usually 5 arms
• Plates and spines on surfaces (no ambulacral groove)
• Articulating ossicles (vertebrae) fill perivisceral spaces in arms - muscles surrounding ossicles move legs
• Water vascular system for feeding rather than locomotion, lack ampullae
• Suspension feeders (mucus strands on arms), or predators (catch prey by arm looping)

Class Echinoidea

• Sea urchins, "hedgehog-like"
• 1,000 species, mostly benthic herbivores
• Imagine a starfish with arms turned upward and plates filling the spaces between arms
• 5 ambulacral and 5 interambulacral plates bearing spines
• Water vascular system similar to asteroids
• In irregular echinoids (sand dollars), aboral openings have migrated to margin
• Aristotle's Lantern - mouthparts consist of 5 calcareous plates with teeth for scraping algae off surfaces

Class Holothuroidea

• Sea cucumbers, "polyp-like"
• 1,200 species, benthic or suspension feeders
• An echinoderm without arms lying down: ventral surface with ampulla and tube feet in ambulacral grooves
• No spines or calcareous plates; ossicles are microscopic
• Mouth is surround by 10-30 feeding tentacles that are modified tube feet of water vascular system
• Paired respiratory tree that branches from the rectum
• Some species capable of "evisceration"

Class Crinoidea

• Sea lillies and feather stars ("lilly-like")
• 600 living species, many more fossil species
• Only living echinoderms that are sessile
• Oral surface is upright on same side as anus
• Body may be stalked, with pentamerous crown of feeding arms (cirri) attached
• Each feeding arm has an ambulacrum and pinnule branches that bear tube feet
• Food particles are captured by tube feet and funneled along ambulacral grooves to mouth