KINGDOM ANIMALIA

PHYLUM ECHINODERMATA – SPINY SKIN AND A WATER VASCULAR SYSTEM

 

 

INTRODUCTION

 

         Echinoderms are strictly marine animals and include sea stars, sea urchins, sea cucumbers, and sea lilies.  The phylum name (echino = spine, derm = skin) refers to the calcareous spines that project from the skin of many members.  Echinoderms are triploblastic eucoelomates with a complete gut and a deuterostome pattern of embryological development.  As adults, they exhibit pentamerous (5-part) radial symmetry, but this radial symmetry is secondary and echinoderm larvae typically exhibit bilateral symmetry.  Echinoderms are unique in having a water-vascular system, a network of water-filled canals that extends throughout the body and gives rise to tube feet, which are used for locomotion and feeding.

 

Major classes of Echinodermata include:

Asteroidea - starfish

Ophiuroidea - brittle stars, basket stars

Echinoidea - sea urchins, sand dollars

Holothuroidea - sea cucumbers

Crinoidea - sea lilies, feather stars

 

 

 

PROCEDURE

 

Starfish are found in relatively shallow waters, and range in size from less than an inch to nearly three feet in diameter.  They feed primarily on bivalves, forcing the shell to open with their tube feet, everting their stomach into the victim's body cavity, and digesting it.  The larvae are known as bipinnaria and have bilateral symmetry.  Starfish can perform autotomy (self-amputation) of their arms.  However, if a small portion of the central disc remains attached to it, the amputated arm can then regenerate and form a new individual!

 

1.  Bipinnaria larvae:  Obtain a prepared slide of starfish bipinnaria larvae.  Examine the slide under a compound microscope and note that the larvae exhibit bilateral, not radial, symmetry.  You can see a developing digestive tract inside the larvae, and if you look closely you may be able to see bands of cilia that are used for locomotion and feeding.

 

 

 

Figure 1.  Bipinnaria larva of starfish showing bilateral symmetry and the developing digestive tract (from Biodidac).

2.  External features:  Place a specimen of Asterias (starfish) on a tray preferably immersing it completely under water, and identify the oral and aboral (lower and upper) surfaces.  Observe the five arms, noting their spiny texture (from which they get the name echinoderm - spiny skin).  Remove a half inch square of the skin from the aboral surface and examine it under water using a dissecting scope.  Note the calcareous spines, dermal branchiae (skin gills - little sac-like structures on the skin) and pedicellariae (claws - tiny pincer-like structures).

 

         The madreporite (a light colored, circular, slightly raised structure located on the aboral surface near the base of two arms) may function to allow water into the water vascular system.  The anus is seen as a minute opening at the center of the aboral surface.  Ambulacral grooves are the deep grooves that extend from the oral surface along the midline of each arm.  The tube feet are seen as double rows of soft tubular "feet" on each arm, lying along and just inside the ambulacral groove.

 

 

3.  Internal features:  Place your specimen with the aboral side up and make a transverse cut along the length of one arm to get a cross section.  The ossicles are calcareous plates buried in the fleshy region beneath the outer skin.  This represents the endoskeleton which gives the body rigidity and support.

 

         Cut off about a cm from the tip of the arm.  Then cut the sides of the arm and carefully remove the aboral surface, both the skin and the ossicles.  Separate the internal organs from the skin, leaving the organs in place.  Cut around the aboral disk without injuring the madreporite.  Now the entire body contents should be visible.  Note that the coelom is surrounded by the digestive system.

 

         The stomach is the central, pouch-like structure, and the intestine is a short tube coming from the aboral surface of the stomach.  The pyloric ceca are long, greenish fingerlike bodies on each ray - these are the enzyme-producing digestive glands.

 

         Also visible are components of the water vascular system.  The stone canal is a short, bent tube coming from the bottom of the madreporite.  The ring canal is a circular canal around the disk to which the stone canal is connected.  The radial canal runs radially through each ray.  The ampullae are bulb-like structures above the tube feet.  The tube feet are sucker devices for attachment, connected to the ampullae and located all along the midregion of the arms on the oral side.

 

         Starfish are dioecious with external fertilization.  The reproductive system includes the gonads, which are branched structures occurring in pairs at the base of each arm.

 

 

Figures from Invertebrate Zoology by Brusca and Brusca.
 
4.  Demonstrations:  On demonstration are examples of several other echinoderm classes, including sea urchins and sand dollars (Echinoidea) and sea cucumbers (Holothuroidea).

 

 

Figure 2.  Diagram of the water vascular system of an echinoderm.  (From Brusca & Brusca, 2003, Invertebrate Zoology, 2nd ed.).

 

 

 

 

 

 

 

Figure 3.  Longitudinal section through an echinoderm showing internal structure.  (From Brusca & Brusca, 2003, Invertebrate Zoology, 2nd ed.).


Here is a student video of a starfish dissection

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