Phylum Echinodermata (Precambrian -Recent)

Subphylum Asterozoa (Ordovician-Recent)

Class Asteroidea (Ordovician - Recent)

Class Ophiuroidea (Ordovician - Recent)

Subphylum Echinozoa (Precambrian?, Camb-Rec.)

Class Echinoidea (Ordovician - Recent)

Class Edioasteroidea (Cambrian-Carboniferous)


Free-Living Echinoderm Morphological Terms

The terms below are pertinent to most all of free-living echinoderms and even to stalked pelmatozoans. See the attached figures to make sure you can identify each structure as well as know its function.

Ambulacra: one of the five rays of an echinoderm which support the tube feet; in echinoids it is usually covered with numerous ambulacral plates.

Interambulacra: region between ambulacral regions.

Apical system: the circlet of 10 plates surrounding the anal system (see periproct).

Madreporite: a modified opening which commonly has several openings which serve to regulate water pressure.

Periproct: anal opening, usually on the aboral (anti oral) side.

Aristotle's lantern: the mouth, jaw, and teeth parts of an echinoid made up of 40 different skeletal elements.

Spine: rod or club-like structure used in locomotion and/or defense.

Tubercle: attachment structure (usually a knob) to receive a spine.


This Subphylum includes the star shaped, mobile benthic echinoderms. The subphylum has two major groups: asteroids and ophiuroids based on the presence or absence of a central disc.


The Asteroids, or starfish, have thick, hollow arms with prominent ambulacral grooves extending to the tips of the arms on the ventral side. In life, the grooves were lined with the tube feet. The mouth is located on the ventral (under) side, and the anus, when present, is located on the ventral side next to the madreporite (see above for a definition). See the Recent example, making sure you can recognize the ambulacral grooves.


The Ophiuroids, or brittle stars, are characterized by whip-like arms made of articulated plates. The plates resemble tiny vertebrae. Ophiuroids have a mouth centrally located on the ventral surface but lack an anus. They do, however, have a madreporite on the dorsal surface. Additionally, ophiuroids have ambulacra on the ventral side leading towards the mouth. See the examples, making sure you can recognize the ambulacral regions of each appendage.


You will be responsible for three classes of echinozoans: echinoids, holothurians, and edioasteroids. Echinozoans occupy a variety of infaunal and epifaunal environments, yet like their pelmatozoan relatives, they are stenohyaline and require normal marine salinities.


Echinoids, including sea urchins and sand dollars, generally have skeletons which are sub-spherical, hear-shaped, or disk-shaped. Like the crinoid calyx, they are composed of a mosaic of plates; unlike the crinoids, they are covered by spines. These are tiny bristles in some, but can be quite large in others. The spines generally detach from the main skeleton after death, because they are held to it only by non-mineralized ligaments. There are no arms, and the ambulacral areas (where the perforated plates are found) stretch from pole to pole. Echinoids can be further divided into two groups which you will be responsible.

Regular Echinoids

Regular echinoids have five-fold radial symmetry; mouth and periproct at opposite poles (oral and apical sides). Plates in interambulacral areas have well defined tubercles. See the example and the plate arrangement, tubercles, spines, thickness of test, mouth and periproct, symmetry, and especially make sure you can identify the ambulacral vs. interambulacral regions. Regular echinoids are mostly epifaunal mobile grazers that sometimes occur in rocky subtidal and intertidal environments.

Figure 3 - Echinoid Morphology

From McRoberts (1998)

Irregular Echinoids

Irregular echinoids have bilateral symmetry; mouth toward anterior on ventral side, and periproct in posterior interambulacral area. Petals are conspicuous dorsal parts of ambulacral areas characterized by slit-like pores. Note that the anterior petal (ambulacral region) is cut by the plane of bilateral symmetry. Examine these examples - for things such as lack of symmetry, thickness of test, petals, mouth, and position of periproct.

Irregular echinoids occur primarily in infaunal environments. The depths at which individuals lived can sometimes be deduced by their external morphology.


Edioasteroids have a globular or discoid skeleton composed of many small interlocking plates. The upper surface contains five radiating ambulacra covered with even smaller plates. The five radial ambulacra may be straight or arranged in a spiral pattern. The mouth is centrally located next to a small hydropore. The anus usually occurs on the same side as the mouth, but within an interambulacral region. Commonly, the outer margin is comprised of a ring of more tightly interlocking plates. See the specimen encrusting the strophomenid brachiopod.

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