Introduction

    This week we will explore the very diverse and successful Echinoderm Phylum.  The five living classes of echinoderms today include the sea stars, brittle stars, sea urchins, crinoids and sea cucumbers, but abundant as these are, they represent just a remnant of an immense group that once included 20 different classes.  The organisms are perhaps best known for their water vascular system, which operates the tube feet as well as governs gas exchange and feeding processes. Echinderms also have an endoskeleton, composed of many little plates that can articulated or fused together.  These plates are called ossicles, and are embedded in the dermal tissue.  The endoskeleton assures a good chance for fossilization, because it is these ossicles that are often preserved in the fossil record.

    Echinoderms have a remarkable feature that is unique in the animal kingdom:  they can reverse the rigidity of their dermal cells and connective tissue.  This is most evident, for example, in the action of a starfish as it attacks a pelecypod.  The  connective tissue stiffens, and provides a framework against which the tube feet can pull as the pelecypod's valves are forced open.  Later, the starfish can soften the connective tissue, return to its flexible condition, and move away.  Apparently, the ability to reversibly affect the rigidity of the dermal and connective tissue is directly related to controlling the concentration of calcium ions in the tissues.  To read more about this interesting feature, I recommend Ruppert et al. (2004) (reference is below).
 
 
 
 

Basic Facts to Know about Echinoderms:  They are
1.	Eukaryotes
2.	Metazoans with organs, true tissues, nervous, muscular, and reproductive systems
3.	characterized by an endoskeleton composed of ossicles (calcite plates) that can be articulated or fused
4.	characterized by unique ability in the Animal Kingdom to reverse the rigidity of their connective tissue
5.	characterized by radial symmetry, often visible as pentameral (5-rayed) in some groups
6.	characterized by an internal water vascular system that operates movement, feeding, gas exchange
7.	limited to marine environments only because of lack of osmotic regulation
8.	Excellent index fossils:  widely distributed, well preserved, easily identified, rapidly evolving
9.	found to filter feed, or to be heribivores, predators, scavengers, and detritivores
10.	gonochoric (males and females present, with a few exceptions) and larvae have a planktonic phase

Crinoids, Blastoids, and other Stalked Echinoderms

Class Crinoidea (Cambrian to Recent)
    The Crinoids represent the only living group of stalked echinoderms still in existance.  They have a fossil history that extends into the Middle Cambrian Burgess Shale.  The crinoid body consists of a calyx or theca, which houses the internal organs and bears the arms which are used for feeding.  Attached to the theca is a stalk or column that consist of disks of calcite columnals, pierced through the center by a fibrous mass of collagen.  The stalk is anchored to the substrate with a holdfast, which looks very similar to plant roots.  Upon death, the collagen breaks down and the columnals are released like beads from a broken necklace.  If wave energy is slight, the column is likely to stay more or less intact, however, any wave action or current action will scatter the columnals, and often in the fossil record, a columnal hash deposit results.  Like all echinoderms, they have endoskeletal ossicles, and a water vascular system that operates the feeding arms.  The arms have ambulacral food grooves that move food towards the mouth, which is in the center of the calyx (theca).  The arms extend into the water current, trap food particles which move via mucus along the ambulacral grooves to the mouth.  The gut is a simple loop that terminates in the anus, which is next to the mouth on the top of the calyx.  The arms were strong, containing extensions of the coelem, and are often preserved.

Figures of Crinoid Terminology and fossil Crinoid shown with permission
from the University of California Museum of Paleontology

Class Blastoidea (Middle Ordovician-Late Permian)

    The Blastoids are the second most common group of stalked echinoderms.  The calyx  of blastoids shows a strong pentameral symmetry, and looks like a nut or the bud of a flower.  The arms were very delicate, with no coelemate extension, and are not usually preserved.  The mouth is a simple opening located in the center of the top of the calyx, and surrounded by typically five circular openings called spiricles that apparently served as the exhalent currents.  These openings can be clearly seen in the photo below, where the center view shows the star-shaped opening that is the mouth, and the surrounding spiricles.  The largest of the spiricles is the anal opening (the anispiricle).  In side view of the calyx, you can see the ambulacra, where the arms (brachioles) would have attached.

Calyx View of Pentremites, a common Blastoid of the Mississippian, shown with permission from
the University of California Museum of Paleontology

Part I, 20 pts.  :  Examine the Blastoids and Crinoids in the Teaching Collection, and draw two of these, labelling the calyx, the spiricles (if visible), the ambulcra, and the columnals (if present).  Label the top (oral side) and the bottom (aboral side) of the calyx.  Label the class and time range, as well.         Examine the teaching collection and pick two non-stalked echinoderms.  Draw them, labelling the madrepore, the ambulacral area, and the mouth.  Label the Class and time range, as well.

Brittle Stars, Starfish, Edrioasteroids

Class Asteroidea (Early Ordovician-Recent) (Starfish)
The "starfish" appears a early as the Ordovician, and became an important predator through the Phanerozoic.  They are not typically well preserved, because the ossicles are very tiny and not articulated together.  Often star fish have five arms, with the tube feet running along the ambulacral arms.  The mouth is located on the underside of the starfish, and the anus and madreporite (the main water entry control for the water vascular system) is located on the top.  The starfish are able to evert their stomachs and digest their victim within its own shell.  There is quite a bit of discussion in the literature concerning the role starfish played in the development of complex hinges and commisures in the mollusks, in the collapse of the brachiopods in the Mesozoic, and their role as prolific predators in the Mesozoic marine revolution.

Image of fossil starfish shown with permission from the University of California Museum of Paleontology, live starfish showing underside with tube feet visible, shown courtesy of NOAA NURP, Image ID: reef0206, The Coral Kingdom Collection Credit: Photo Collection of Dr. James P. McVey, NOAA Sea Grant Program
 

Class Ophiuroidea (Early Ordovician - Recent) (Brittlestars)
Similar to starfish, ophiuroids (brittlestars) typically have five arms, but these tend to be very thin arms, radiating from a central disk.  The mouth is located on the underside of the disk-body, and the anus and madreporite on the top.  However, there is no coelomate extension through the arms, as in the starfish, and so the primary use of the arms is in wriggling about to provide locomotion for the brittlestar.  They are a major component of the deep ocean floor benthos-very few species live in shallow waters today.

Image of fossil brittlestar shown with permission from the University of California Museum of Paleontology, image of live brittlestars on the floor of the deep ocean shown courtesy of NOAA NURP.Image ID: nur01504, Photographer: S. Stancyk  Credit: OAR/National Undersea Research Program (NURP); University of South Carolina

Class Edrioasteroidea (Early Cambrian-Late Pennsylvanian)
This interesting group consisted of small (about 1 cm diameter) circular shaped animals that attached to hard surfaces.  On the top surface is an arrangement of five loosely coiled or curving ambulacral arms.  The tube feet caught food particles and transported them to the mouth.  They are fairly common in the Cincinattian limestones of the Ordovician, so you may have collected some in your own samples.  They are often found cemented onto brachiopods, particularly strophomenid brachiopod valves.

Image of fossil Edrioasteroid shown with permission from the University of California Museum of Paleontology

Sea Urchins, Sand Dollars, Sea Biscuits, Heart Urchins

Class Echinoidea (Late Ordovician-Recent)

As a group, echinoids tend to be found as fossils more commonly than the other echinoderms.  This is because the ossicles are fused together to make a strong structure that is often preserved in the burrows in which the animal lived.  The mouth is on the underside, the anus is located on the top, near the madreporite plate, and there are typically five ambulacral grooves where the tube feet are located.  The tube feet vary considerably.  The tube feet on the bottom are designed for locomotion, whereas those on the top are for respiration, and there are also long stalks with grasping claws called pedicellaria that remove debris and protect the echinoid from smaller organisms.  All echinoids possess spines, which are composed of single crystalline masses of calcite.  The spines can pivot at the base, and thus move in different orientations.  There are two major groups to consider:

The Regular Echinoids (Late Ordovician-Recent)
    This group includes the the sea urchins, which have a radial symmetry, feed on algae by scraping it off rocks with a feeding structure called Aristotle's lantern, and have a fossil record that extends back to the Ordovician.

Regular Echinoid sea urchin, photo courtesy of NOAA NURP Credit: Photo Collection of Dr. James P. McVey, NOAA Sea Grant Program

The Irregular Echinoids (Jurassic - Recent)
    This group includes the sand dollars, sea biscuits, and heart urchins.  The irregular echinoids are marked by a secondary bilateral symmetry, as the whole body is flattened, and consequently the anus moved towards the posterior of the shell, and the mouth stayed central or moved towards the anterior.  Most irregular echinoids burrow, and thus have limited ambulacral areas (just on the top of the shell).  Food grooves with tube feet move food particles along on the underside of the shell towards the mouth.   An extreme body design of this group is the flat sand dollar, which first appeared in the early Tertiary, and which has slits or lunules in the shell to prevent it from being lifted out of the sediment by wave action while it is feeding.

Sand Dollar Image shown with permission from the University of California Museum of Paleontology
 
 
 

For More Information...
For more on the fascinating biology of this group, I recommend
Ruppert, Fox and Barnes, 2004 Invertebrate Zoology, , 7th edition, Chapter 28, Echinodermata, pp. 872-929.