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Echinoderms

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Phylum: Echinodermata

Reading

Read Phylum Echinodermata in the lecture notes.

Examine a slide showing different stages of development in sea stars. Find the following stages: single cell, blastula, gastrula, and a later embryonic stage. 

  • Name and describe the type of cleavage exhibited by deuterostome embryos.
  • Identify the blastopore. What structure in the adult does this give rise to?

Notice that the embryo shows bilateral symmetry yet the adult is radial. The ancestral echinoderms were bilateral and bilateral symmetry is maintained in the larval stage.

Below: Development (various stages)

Sea Star (Class Asteroidea)

Obtain a preserved sea star for dissection. Identify the oral and aboral surfaces. On the aboral surface, find the sieve plate (madreoporite), a structure that allows water to enter the water vascular system. Notice the spines that protrude from the skin. The word "echinoderm" means spiny-skinned".

On the oral surface, identify the tube feet and mouth.

Below: aboral surface (left) and close-up of the skin (right) of a sea star.

Examine the oral surface of a sea star under a dissecting microscope and observe the spines. Most of the fleshy projections are skin gills and are used to absorb oxygen. Claw-like projections called pedicellariae function to capture small animals and protect the skin gills. A sieve plate (madreporite)  located on the aboral surface allows water to enter the water vascular system.

To examine the internal structures, you will remove the skin from the aboral surface (see the diagram below). With scissors or a scalpel, cut along the side of one arm the entire length of the arm from the tip of the arm to the point where the arm attaches to the central disk. As you cut you will notice that there is a layer of calcium carbonate plates just below the surface of the skin. It will be necessary to cut through these plates.

Cut around the tip and then cut along the opposite side of the arm. When you reach the base of the arm, continue cutting to the next arm and repeat this cutting procedure for this arm and the rest of the arms. 

At this point, you have cut the animal's skin into two halves, an upper surface and a lower surface.

Carefully, peel the skin back from the tip of one arm. It will be necessary to use a dissecting needle or a blunt probe to help you lift the skin while keeping the internal structures from being lifted away and damaged. Repeat this procedure for the remaining 4 arms. Use extra care to peel the skin away from the central disk of the animal. If your initial cuts along the sides of the arms were not sufficient, you may need to cut the side as you peel the skin back. 

The stomach of the animal has two parts. The upper part is called the pyloric stomach and the lower part is called the cardiac stomach. When feeding, it extends the cardiac stomach out through the mouth and into the shell of a bivalve mollusk. It secretes digestive enzymes into the mantle cavity of the bivalve. The enzymes digest the bivalve's tissue. Digestion is completed within the pyloric stomach and the digestive glands in the arms (discussed below). 

Identify the stomach and try to discern the cardiac and pyloric portions.

The digestive gland is the soft, brown material that fills most of the space in the arms. Remove or lift the digestive gland from one of the arms. The gonads are lighter in color and located underneath the digestive gland. They may be much smaller than the digestive gland. The size of the gonads depends on the stage of the reproductive cycle of the animal and may not be evident in some animals.

Observe the rows of ampullae along each side of the ambulacral ridge. The muscle surrounding these bulb-like structures can contract and extend the tube feet underneath.

Below:
1. Internal structures
2. Tube feet
3. Ampullae

 

Other Echinoderms

Observe representatives of other classes of echinoderms on display.

Below:
1.  Sea Urchin (Preserved) Oral Surface
2.  Sea Urchin Shell, Oral Surface
3.  Sea Urchin Shell, Aboral Surface
4.  Sea Cucumber, Preserved
5.  Brittle Star - aboral surface
6.  Brittle Star - oral surface