Most zoologists believe that this group of animals arose from bilaterally symmetrical ancestors even though the animals show a pentaradial symmetry. Many of them have a bilateral larval stage and hence the radial feature may be secondarily acquired. Further, they are definitely triploblastic and eucoelomate. Echinoderms are marine animals and include sea stars, sea urchins, sea cucumbers, and sea lilies. The body parts are arranged in "fives" around the oral/aboral axis. Radial symmetry is an adaptation to sedentary life-styles. Echinoderms are dioecious with external fertilization.
Major classes of Echinodermata include:
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! Place a specimen of Asterias (starfish) on a tray preferably immersing it completely underwater, and observe the following:
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) is the opening, or intake, of the water vascular system. (The term bivium refers to the two arms on either side of the madreporite, and trivium refers to the remaining three rays.) 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.
Place the 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 centimeter 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. The intestine is a short tube coming from the aboral surface of the stomach. The pyloric ceca are long, greenish finger-like 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 medially 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 mid-region of the arms on the oral side.
Starfish are dioecious. The reproductive system includes the gonads which are branched structures occurring in pairs at the base of each arm.
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