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Deuterostomes are animals that share such embryological similarities as
radial, indeterminate cleavage and a blastopore that becomes the tail end. Echinoderms (starfishes, crinoids, sea urchins, and their relatives) are often radially symmetrical as adults, but their embryonic stages show similarities to the chordates. The Chordata include animals with a notochord, a dorsal hollow nerve cord, gill slits, and many embryological similarities linking them with echinoderms. Chordates include tunicates, sea lancets, fishes, amphibians, reptiles, birds, and mammals. Deuterostome characteristics: Embryological similarities shared by chordates, hemichordates, and echinoderms:
Animals with a unique water-vascular system, using sea water as a circulatory fluid. Several embryonic similarities to chordates, including a true coelom, which develops as an enterocoel. Change of symmetry in many cases, from a bilateral larva to a radial adult, typically in a 5-fold pattern. Protective plates or shells frequently made of calcium carbonate and armed with bumps or spines. High ability to regenerate lost parts. Sessile (attached) echinoderms (Homalozoa and Crinozoa): Echinoderms that grow attached include crinoids (sea lilies) and a variety of extinct groups (blastoids, cystoids, carpoids, etc.). Many grow on stalks attached to the bottom. Body cup-shaped, open toward the top, with a mouth in the center of the top surface. Arm-like rays, in multiples of five, grow out and upward from the margins of the mouth. Each ray has a ciliated groove (the ambulacrum) that traps food particles and brings them to the mouth. The earliest fossil forms were irregular and lacked symmetry, but radial symmetry developed early, generally in a 5-fold pattern. Biologists believe that echinoderm ancestors were bilaterally symmetrical and that filter-feeding (filtering small particles of food from the water) made radial symmetry selectively advantageous. Attached echinoderms flourished mainly during Paleozoic times. Today, only a few crinoids remain; other attached echinoderms are extinct. Free-moving echinoderms (Echinozoa and Asterozoa): Mostly bottom-feeding scavengers and predators that attack other invertebrates. The mouth, on the lower surface, faces downward. Branches of the water-vascular system may form foot-like podia, used in locomotion. Each of these podia has a suction-cup extension that can hold on and a water-filled bulb that controls water pressure (or suction).
Chordata and Hemichordata: Notochord: A stiff, flexible rod, forming the body axis. When muscles contract, it allows bending but prevents the body from collapsing like an accordion. In embryos, it induces the nervous system to form above it. Gill slits: Openings from pharynx to either side, just behind mouth. Hemichordata: Acorn worms and their relatives. All of them filter feed. Some use gill slits; others use tentacle-like feeding structures. Related to Chordata, but now usually treated as a separate phylum. Phylum Chordata: Animals with a notochord, a series of gill slits, and a dorsal, hollow nerve cord developing from a neural tube. These traits may occur in larval stages, not always in adults. Urochordata (tunicates or "sea squirts"): An actively swimming larva with well-developed notochord and nerve cord undergoes metamorphosis into a filter-feeding adult. The adult usually passes large amounts of water through a large gill basket. Cephalochordata (sea lancets or amphioxus): Small, thin animals that filter feed by passing water through many gill slits. A notochord extends the entire length of the animal, including the head. Vertebrata (vertebrates): Animals with a vertebral column or backbone that functionally replaces the notochord in adults, and a braincase that encloses and protects the brain. Examples: fishes, amphibians, reptiles, birds, and mammals. |
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