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Phylum Echinodermata The “spiny skinned” creatures -Sea stars and sea urchins- General Characteristics – All members of the phylum exhibit 5 part radial symmetry • The larvae are all bilaterally symmetrical • They have arms in multiples of 5 extending from a central disc – This is a large group of more than 7000 species who are all marine – All members have the ability to regenerate lost parts as long as the central disc is unharmed – Most are slow bottom crawlers General Characteristics – They have well developed endoskeletons • Composed of individual plates called ossicles • Covered by a thin layer of skin • In some species spines form from the endoskeleton and project out of the skin – Water Vascular system is used for movement • A water filled system of interconnected canals that connect to tube • • feet Used for moving the arms (rays) Also aides in feeding and gas exchange – diffusion occurs through the thin walls of the tube feet – Coelomic circulation and respiration • Movement of nutrients and gases move freely through a large fluid filled cavity to nourish all the cells Feeding and Digestion • Many members feed by extending or spitting out their stomach to envelop their food – The stomach acids digest the food externally – The nutrients and the stomach are then sucked back into the animal • In sea stars, the intestine is very short or missing, in sea urchins and sea cucumbers the intestine is very long and coiled in the animal to allow for the slower digestion of plant material • Nutrients are passed through the coelomic fluid to the individual cells Nervous System • They have a very rudimentary (simple) nervous system • Nerves coordinate movement Reproduction • Male and female organs are found in different individuals • The fertilization occurs externally through spawning (male releases sperm and females release eggs into the water column and rely on luck) • The larvae are planktonic and rarely resemble the adult • Asexual reproduction of a sort occurs if the animal is cut into two pieces – each piece develops into a new organism (regeneration) Class Asteroidea • This includes the sea stars • Can have between 5 and 50 arms radiating from the central disc • Usually act as predators to bivalves, snail, barnacles and other slow-moving animals Class Ophiuroidea • This includes the brittle stars • The five arms are extremely long and flexible • Eat mostly detritus Class Echinoidea • This class includes the sea urchins and sand dollars • Endoskeleton forms a hard rigid ball • Movement is achieved using the long spines that radiate out of the endoskeleton • Usually herbivores Class Holothuroidea • This class includes the sea cucumbers • Superficially worm like in appearance – They have the basic body plan of a sea urchin but the body has been elongated – Endoskeleton consists of microspicules (like a sea sponge) which gives it its warty appearance • most are deposit feeders • have a novel defense mechanism – when threatened the sea cucumber will throw up 80 percent of its internal organs – the sea cucumber has the ability to regenerate the lost organs – hopefully the predator will eat the organs and allow the animal to live Phylum Hemichordata General Characteristics – Act almost like the missing link between chordates and echinoderms – they have a dorsal nerve cord – allows the movement of nerve messages to the arms and legs of the body – they have a ventral nerve cord that runs along the proboscis – small phylum with only 90 species – usually detritus feeders