ECHINODERMATA

From Eldredge (1991)


INTRODUCTION

The phylum Echinodermata consists of several types of complex organisms which show a general pentameral symmetry and have a well developed water vascular system. Echinoderms are also characterized by their mesodermal skeleton. Echinoderms occur in a variety of morphologies including free-living forms such as starfish and sand dollars as well stalked forms such as sea lilies which are attached to the sea floor.

The first part of the lab will concentrate on the stalked echinoderms also called pelmatozoans. Pelmatozoans are exclusively marine and live in a variety of habitats of normal salinity. They are all filter-feeders. As a group, the pelmatozoans have been quite abundant in the geologic past especially the Paleozoic, yet since the close of the Mesozoic they have mainly relegated to deep-water, cryptic environments.

The second part of this lab will concentrate on the very diverse non-stalked echinoderms belonging to the subphyla Asterozoa and Echinozoa, and collectively referred to as free-living echinoderms. Similar to the pelmatozoans, the free-living echinoderms all have a mesodermal skeleton comprised of calcite plates and a complex water-vascular system including tube-feed. Unlike the pelmatozoans, the groups of this lab are quite abundant in the modern seas, occupying a large number environments by an equally large number of life-habits. Because of their abundance and diversity, the echinozoans will be the largest part of the lab.


CLASSIFICATION & GEOLOGIC RANGES

 Phylum Echinodermata (Precambrian -Recent)

Subphylum Crinozoa (Cambrian-Recent)

Class Crinoidea (Cambrian-Recent)

Subphylum Blastozoa (Cambrian-Permian)

Class Blastoidea (Silurian-Permian)

Class Rhombifera (Ordovician-Devonian)

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)

 


PELMATOZOAN GENERAL MORPHOLOGY

In general, the pelmatozoan skeleton can be dived into two main parts: The stem and the calyx. The stalk or stem is composed of numerous disks called columnals which have a central hole called the lumen. Stems are often secured to the substrate by means of a holdfast or root system. The calyx (sometimes called theca), a cup-like structure as shown in these specimens, which may or may not support a variety of arms. As outlined below, the calyx is usually composed of a number of different kinds of plates, grooves, and pores; some of which are quite specialized


Appendage and Calyx Morphology

Ambulacral groove: one of the 5 radially arranged regions specialized for food gathering.

Brachial plates: arm plates; several different types depending on position relative to arm branch e.g., primibrachials and secundibrachials. See these fossil specimens.

Brachioles: small erect food gathering appendages surrounding the edge of the ambulacrual area in blastoids.

Pinnules: small linear hair-like branches that may occur on each arm plate and also aid in food gathering.

Basal plates: the circlet of plates below and off-set to, and joining, the radial plates.

Infrabasal plates: secondary plates below basals.

Radial plates: below lowest brachial and above basal plates where ray terminates.

Interray plates: plates that are added between rays.

Lancet: ambulacral plate of blastoid, usually site of brachiole attachment.

Deltoid: small triangular or rhombahedral plate above the radial plates in blastoids.

Pores: openings for tube feet.

Tegmen: Part of calyx occasionally with plates that resides above the attachment points of arms. Sometimes elevated into a anal pyramid.

Ray: trace of plates from arm through calyx; there are usually five (or multiples thereof) rays in pelmatozoans; ray terminates with radial plates (note the larger arrows in the figure below).

 


Figure 1 - Pelmatazoan Plate Arrangements

Modified from McRoberts (1991)


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Subphyla CRINOZOA & BLASTOZOA

Subphyla ASTEROZOA & ECHINOZOA


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