Starfish Dissection
Background Information
Starfish are actually considered “sea stars” by today’s marine biologists because these creatures are not actually fish, but echinoderms. They are related to sand dollars, sea urchins, and sea cucumbers, and all of these organisms have five-point radial symmetry. There are five main sections to their anatomy that are arranged around a central disk. Thus, the most common type of sea star is one with five arms. Some varieties come with numbers of arms in multiples of five, with maybe ten, twenty, thirty arms or more. The thousands of varieties of sea stars are found in all types of ocean environments-from the deep ocean floor to rocky land bordering the water.
Sea stars eat mussels, clams, small fish, snails, and barnacles. The way they consume their prey is interesting: multiple arms of a sea star will work to pull bivalves open, then the stomach is pushed through the sea star’s mouth to envelop the other organism before it retreat back into the sea star’s body. Sea stars are not oxygenated and do not possess blood. Instead, sea water is taken in through their sieve plates to extend the tube feet.
Some fun facts about sea stars concern their unique limbs. If an arm is cut off by a predator, the vital organs in the lost arm and some parts of the central disk may be enough to regrow an entirely new sea star. The individual arms also possess “eye” on the very tips. These cannot see visually are humans do, but they can feel around the sea floor and tell the difference between light and dark.
Major Internal and External Anatomy
Eye spots: sensory organs of the starfish, sense between dark and light and feel the ocean floor
Madreporite: where water enters the water vascular system
Spines: protect the starfish from predators
Podia (tube feet): organ of locomotion of a starfish; expands with intake of water and retracts on its own
Ambulcaral groove: contains the tube feet on the oral side and is used to pry open the shells of bivalves
Stomach: digest food into usable nutrients
Ring canal: canal in which filtered water enters through the madreporite and branches out into the radiated canals
Ossicles: provide structure to each of the five arms
Lateral canal: canal running the length of the arm; receives water from annular canal and passes it into the tube feet
Incision Guide
First, cut off the tip of a ray. Take note of the tube feet which are attached to the long, zipper-like ridge that extends the length of the ray. The top of the tube feet is the ampulla, which helps to create suction that makes the starfish stay in one place or adjust its position.
Using scissors, then cuts along either side of the skeletal structure extending almost to the central portion of the starfish. Cut across these lines to remove a rectangular flap of skin from the center of that arm.
Remove the somewhat circular flap of skin above the ring canal. This connects the five lateral canals from the five arms. The flap may need to be extended beyond a circular shape to be removed.
Carefully remove the crumbly brown material on either side of the skeletal plates, or ossicles. These are the pyloric caeca. These are the digestive glands that secrete enzymes for digestion. The gonads, sexual reproductive structures of testes or ovaries, look similar to the pyloric caeca but are somewhat ridged in appearance.
No comments:
Post a Comment