BIO 385 Invertebrate Zoology
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Invertebrate Diversity
Phylum Echinodermata Echinoderms

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Echinoderm Characteristics

  • Most species exhibit pentaradial symmetry as adults
  • Larvae are bilaterally symmetric
  • Unique water vascular system: internal fluid-filled canals
  • Tube feet form external extensions of water vascular system
  • Calcareous endoskeleton composed of individual plates (ossicles)
  • Marine environments; most are benthic
Bipinnaria Larva
Bipinnaria larva of a sea star
Brachiolaria larva
Brachiolaria larva of a sea star

Class Crinoidea Sea Lilies & Feather Stars


  • Body (=calyx) is cup-shaped with oral surface upwards
  • Arms have numerous tiny branches (pinnules)
  • Anus opens on oral surface
  • Supported by aboral stalk (sea lilies) or tentacle-like cirri (feather stars)
  • Suspension feeder in deep-sea environments
Feather Star
Feather Star, possibly Comantheria sp.?
Feather Star
Feather Star, dried specimen
Scyphocrinites Fossil
Fossil of Scyphocrinites sp. Sea Lily calyx; Silurian Period, Morocco
Dichocrinus Crinoid Fossil
Fossil of Dichocrinus multiplex crinoid; Mississipian Period, IA
Floating Crinoid Fossil
Fossil of Culicocrinus thomasi (left) & Rhodocrinites douglassi (right); Mississipian Period, IA
Floating Crinoid Fossil
Fossil of Saccocoma pectinata, a floating crinoid; Jurassic Period, Germany

Class Asteroidea Sea Stars


  • Body star-shaped with 5 or more arms
  • Arms relatively thick; not distinct from central body disk
  • Tube feet lie in open ambulacral grooves
  • Gonads and digestive tract extend into arms
  • Madreporite (opening to water vascular system) and anus aboral
  • Mostly slow-moving predators
Seastar Dissection
Dissected Sea Star, Asterias sp.
See also labeled photo (2 views).
Ochre Seastar
Ochre Sea Star, Pisaster ochraceus, eating mussel (Mytilus sp.) (turned oral surface up; note ambulacral grooves and tube feet); CA
Giant-spined Star
Giant Spined Star, Pisaster gigantea; San Diego, CA
Red Knob Seastar
African Red Knob Sea Star, Protoreaster linckii, Kenya
Sand Star
Spiny Sand Star, Astropecten armatus
Bat Star
Bat Star, Patiria miniata, CA
Sunflower Star
Sunflower Star, Pycnopodia helianthoides; a top predator that feeds on other sea stars; CA.
Linkia Seastar
Linkia Seastar, Linckia sp.; the large arm was shed by the parent; the four shorter arms regenerated from this arm; Kenya.
Leather Star
Leather Star, Dermasterias imbricata; CA.

Class Ophiuroidea Brittle & Basket Stars


  • Five arms articulated; distinct from body disk
  • Arms may be branched (basket stars) or unbranched (brittle stars)
  • Arms lack open ambulacral groove
  • Lack anus
  • Most are deposit or suspension feeders
Spiny Brittlestar
Western Spiny Brittlestar, Ophiothrix spiculata; CA
Little Brittlestar
Little Brittlestar, Amphipholis pugetana; note regenerating arms; CA
Banded Brittlestar
Banded Brittlestar, Ophionereis annulata; CA
Banded-arm Brittlestar, probably Ophioderma appressum?
Basket Star
Basket Star, Gorgonocephalus sp.
Basket Star
Basket Star, Gorgonocephalus sp.

Class Echinoidea Sea Urchins


  • Body globular or disk-shaped; lack arms
  • Skeletal plates fused into rigid test
  • Pores in test allow tube feet to exit
  • Spines moveable
  • Protrusible jaw apparatus (Aristotle's Lantern)
  • Mostly herbivores or detritivores, but some predatory
Sea Urchin Closeup
Closeup of mouth of live reef urchin, Echinometra viridis
See also labeled photo.
Sea Urchin Dissection
Dissected Sea Urchin, Strongylocentrotus sp.
See also labeled photo.
Sand Dollar
Giant Keyhole Sand Dollar, Encope grandis; aboral and oral view of test.
See also labeled photo.
Eccentric Sand Dollar
Eccentric Sand Dollar, Dendraster excentricus
Shingle Urchin
Shingle Urchin, Colobocentrotus atratus; HI
Pencil Urchin
Pencil Urchin, Eucidaris sp.
Purple Urchins
California Purple Urchins, Strongylocentrotus purpuratus; CA
Sea Urchin
Reef Urchin, Echinometra viridis
Longspined Urchin
Black Long-spined Urchin, Diadema setosum

Class Holothuroidea Sea Cucumbers


  • Body elongated in oral-aboral axis
  • Secondarily bilaterally symmetric (no dorsal tube feet)
  • Skeleton reduced to isolated ossicles; soft-bodied
  • Ring of tentacles around mouth
  • Most are deposit feeders in soft sediments
Sea Cuke Dissection
Dissected Sea Cucumber, Cucumaria frondosa
See also labeled photo.
Burrowing Cucumber
Burrowing Sea Cucumber, Leptosynapta albicans, San Diego, CA
Calif Sea Cuke
Warty Sea Cucumber, Parastichopus parvimensis, showing "ventral" tube feet and tentacles.
Speckled Sea Cuke
Speckled Sea Cucumber, Actinopyga mauritiana; HI
Yellow Sea Cuke
Yellow Sea Cucumber, Colochirus robustus, a suspension feeder
Scaly Sea Cuke
Armored Sea Cucumber, Psolus chitonoides, a sedentary suspension feeder

Extinct Echinoderm Classes

In addition to the 5 extant classes, there are also many extinct groups of Echinoderms

  • Some were stalked suspension feeders, similar to crinoids
  • Primitive forms were bilaterally symmetric or asymmetric
  • You do not have to know these classes
Eocrinoid fossil
Fossil Eocrinoid, Gogia palmeri; Cambrian Period, ID (Class Eocrinoidea)
Carpoid fossil
Fossil Carpoid, Dendrocystites sp.; Ordovician Period, Morocco (Class Homoiostelea)
Cystoid fossil
Fossil Cystoid, Oklahomacystis sp.; Ordovician Period, OK (Class Cystoidea) In life, the stalk would attach at the right side of the photo, with the mouth at the left.
Cystoid fossil
Fossil Ctenocystoid, Ctenocystis utahensi; Cambrian Period, UT (Class Ctenocystoidea)
Edrioasteroid fossil
Fossil Edrioasteroid, Spinadiscus lefebvrei; Ordovician Period, Morocco (Class Edrioasteroidea)
Blastoid fossil
Fossil Blastoid, Pentremites godoni; Mississippian Period, IL (Class Blastoidea). In life, the stalk would attach at the lower left of the photo, with the mouth at the upper right.
This page last updated 12 March 2017 by Udo M. Savalli ()
Images and text Udo M. Savalli. All rights reserved.