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PHYLUM ECHINODERMATA (spiny skin). 1. ECHINODERM CHARACTERISTICS They have pentaradial symmetry with the body divided into 5 (or multiples of 5) around the oral-aboral axis. The advantage is they have sensory and feeding structures in all directions.
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1. ECHINODERM CHARACTERISTICS They have pentaradial symmetry with the body divided into 5 (or multiples of 5) around the oral-aboral axis.
The advantage is they have sensory and feeding structures in all directions.
2. They have a skeleton made up of a series of plates called ossicles. The ossicles are held in place by connective tissue.
3. They have a water-vascular system consisting of water filled canals and the tube feet. It also contains a ring canal that surrounds the mouth.
4. The ring canal connects to the outside by a stone canal or a madreporite. The madreporite is used as an inlet to replace water. Polian vesicles are sac-like extensions of the ring canal that store fluid for use in the water-vascular system.
5. The radial canals branch off the ring canal and go out into the arms. The tube feet usually end in a bulb-like ampulla. When the ampulla contracts it squeezes water into the tube foot allowing it to move.
6. The tube feet have suckers which they use to attach themselves to the substrate. Some gas exchange and diffusion happens across the tube feet.
7. Class Asteroidea (seastars, star fish) Usually have five arms that radiate out from the central disk.
8. The mouth is located on the bottom of the disk, often surrounded by oral spines. Dermal branchiae are extensions of the body wall which extend out through the ossicles. They function in gas exchange (respiration).
9. A row of ossicles running the length of each arm on the oral surface (underside) is called the ambulacral groove. The groove consists of two tube feet with the radial canal and radial nerve in between.
10. Maintenance Functions. They feed on snails, bivalves, polycheates, coral, and detritus.
11. The stomach is very large and takes up most of the coelom. The stomach is divided into two parts. The cardiac stomach is the largest and receives the food first.
12. The aboral or pyloric stomach is used to distribute the nutrients to the rest of the body.
13. Most feed on bivalves. It does this by wrapping its arms around the bivalve and applying pressure. They use different arms as they tire until the bivalve weakens.
14. When the shells are about 0.1mm apart the starfish everts part of its cardiac stomach into the shell. It will secrete enzymes which begin to digest the bivalve inside its own shell. 14.
When it weakens enough and can’t keep the shell closed the starfish finishes opening it and eats.
15. Gas exchange and removal of metabolic waste occurs by diffusion across the dermal branchiae, tube feet, and other membranes. The principle nitrogenous waste is ammonia.
16. The nervous system is composed of a nerve ring around the mouth and radial nerves that extend out to the arms and help control the tube feet.
17. They have sensory receptors located all over that help them respond to changes in light, temperature, chemicals, etc. Some have special tube feet on the ends of the arms that have ocelli (light sensitive cells).
18. Regeneration, Reproduction, and Development. If a starfish is damaged it is capable of regenerating the damaged parts.
19. Some perform asexual reproduction by dividing their central disks. • Most are dioecious. • External fertilization occurs when conditions are right.
20. The ciliated embryos float around eating plankton and become bipinnaria larva. When the arms start to grow it settles to the ocean floor and becomes a brachiolaria larva which develops into an adult starfish.
21. Sea Daisies Recently moved into class Asteroidea. They have no internal digestive system but a thin membrane called a velum is used to cover the substrate and digest and absorb all available nutrients.
22. CLASS OPHIUROIDEA (basket stars and brittle stars) a. Brittle stars have unbranched arms, basket stars have branched arms.
23. Most have no dermal branchiae and the tube feet have no suction cups. Tube feet are moved by muscles at their bases. Madreporites are located on the oral side.
24. They move about by using their arms to crawl over the substrate.
25. Maintenance Functions They are scavengers and predators feeding on plankton. Basket stars have mucus covering their tube feet which they wave around collecting plankton.
26. Regeneration, Reproduction, and Development. They have the capability to generate missing parts. In a process called autotomy they can use muscles to sever one of its own arms in an effort to escape.
27. They are dioecious. Fertilization is external. The larval stage is called an ophiopluteus. It swims and feeds on plankton.
28. CLASS ECHINODEA (sea urchins, sand dollars, heart urchins)
Heart Urchin
29. Sea urchins live on hard surfaces, sand dollars and heart urchins live on sand or mud.
30. Sea urchins are rounded, they have a skeleton called a test made up of ten plates that arch between the oral and aboral ends. Some are venomous.
31. Maintenance Functions They feed on algae, coral polyps, and scavenge dead animals.
32. The Aristotle’s lantern is a structure used for chewing. It’s function is to cut food into smaller pieces that can be eaten. They have a large coelom and nutrients are circulated in the coelomic fluids.
