Echinoderms and Invertebrate Chordates Ch 29

Echinodermata Endoskeleton, radial symmetry, simple nervous system, varied nutrition, water vascular system – Water vascular system—hydraulic system that operates under water pressure, enables animal to move, exchange gases, capture food, excrete wastes Move along surfaces and pry open food with tube feet – Water enters and leaves through madreporite, a sievelike, disk-shaped opening on the upper surface of body Bilaterally symmetrical, ciliated larvae Deuterostomes (mouth forms 2 nd )

Diversity of Echinoderms ~6000 species exist today, 6 classes – Asteroidea: sea stars (25% of phylum) most species have 5 rays, but some can have more than 40! – Ophiuroidea: brittle stars Extremely fragile, rays can break off (helps survive predator attacks), propel themselves by slithering, tube feet for feeding – Echinoidea: sea urchins and sand dollars Globe-/disk-shaped, covered with spines, no rays, tube feet for eating and modified as gills for respiration – Holothuroidea: sea cucumbers Vegetable-like appearance, moves along ocean floor, expels tangled sticky mass of tubes through anus or ruptures and loses organs, which regenerate in a few weeks (evasion of predators) – Crinoidea: sea lilies and feather stars Resemble plants, sea lilies only sessile echinoderms, feather stars sessile in larval form; use feathery arms to swim around – Concentricycloidea: sea daisies Two species discovered in New Zealand, tube feet around edge of disk

Invertebrate Chordates All chordates have a notochord—a long, semirigid, rodlike structure between digestive system and dorsal hollow nerve cord – Made up of large, fluid-filled cells held within stiff fibrous tissue Invertebrates: may retain notochord to adulthood Vertebrates: notochord replaced with backbone All chordates have dorsal hollow nerve cord – Develops from a plate of ectoderm that rolls into a hollow tube Composed of cells surrounding fluid-filled canal above the notochord

Embryonic invagination of the notochord and neural tube Anterior region with early central nervous system

Commonalities in Chordates All chordates have a notochord All chordates have pharyngeal pouches – Paired openings in pharynx behind mouth All chordates have a postanal tail – At some point in development Development controlled by homeotic genes

Diversity of Invertebrate Chordates Tunicates (sea squirts): chordate features visible in larval stages Lancelets (similar to fish): spend most of their time buried in sand with only heads out to filter food from water

Origins Fossil record is limited because of soft bodies Possibly arose from sessile animals that fed by tentacles (larval stages may be similar)