Echinoid morphology

Echinoids, or sea urchins, have a robust, internal skeleton (the test) composed of numerous, fixed, calcite plates. Instead of arms the test has five narrow zones formed from perforated plates (the ambulacra) through which the tube feet emerge. These porous segments alternate with broader areas (the inter-ambulacra) that lack pores. The anus is on the upper (aboral) surface and is surrounded by a double ring of plates (Fig. 7.7). The mouth is on the underside (oral surface). The external surface of the test is covered with spines and pedicellariae, tiny spines with pincers that remove settling organisms (Fig. 7.8).

Echinoids can be divided into two main groups: (i) the regulars, rounded forms (e.g., sea urchins); and (ii) the irregulars, flattened and heart-shaped echinoids (e.g., sand dollars, heart urchins) (Fig. 7.7). Regular echinoids are always surface dwellers and usually feed by scraping seaweed from rocks using a complex jaw apparatus known as Aristotle's lantern. Articulated spines enable the animal to move slowly across the substrate aided by the tube feet. Irregular echinoids are often burrowers, and their spines are generally shorter and more densely spaced. The tube feet are highly modified. Some are used for digging and others are adapted for respiration, forming tubes that connect the animal with the sediment surface.

Fig. 7.7 Regular and irregular echinoid morphology.

Occular plate: connects with water vascular system

Apical system: double ring of occular and genital plates surrounding the periproct

Periproct: central hole that contains the anus

Peristome: area surrounding the mouth (Aristotle's lantern not preserved)

Regular Echinoid Morphology

Interambulacra

Regular echinoid

Ambulacra: plates with pore pairs through which tube feet emerge

Genital plate: plate with hole for release of gametes

Interambulacra area formed from large imperforate plates

Ambulacra

Interambulacra

Ambulacra

Mouth

Irregular echinoid

Mouth

Plastron: flattened area covered with paddle-shaped spines used in locomotion

Regular echinoid

Irregular echinoid

Spine: attached to a tubercle on the test, in the form of a "ball joint". Muscles surrounding the spine enable it to be moved in any direction. Spines are particularly important in locomotion but are also used for defense

Pedicellaria: pincers mounted on small spines that keep the surface of the test clean and free from settling organisms

Madreporite: highly perforated plate through which sea water is brought into the water vascular system

Anus: opening for the anus is called the periproct

Madreporite: highly perforated plate through which sea water is brought into the water vascular system

Anus: opening for the anus is called the periproct

Water Vascular System

Tube feet: emerge from pore pairs in the ambulacral

Mouth Aristotle's lantern:

movable skeletal plates that act like jaws

Tube feet: emerge from pore pairs in the ambulacral

Gonad: gametes are released through holes in the genital plates

Mouth Aristotle's lantern:

movable skeletal plates that act like jaws

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Responses

  • kiros daniel
    What makes an echinoid regular or irregular?
    8 years ago
  • alma
    What is the genital plate in sea urchin?
    2 years ago

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