Animal Biology #7
DEUTEROSTOMES:  ECHINODERMS, etc.

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. Deuterostome characteristics: Embryological similarities shared by chordates, hemichordates, and echinoderms: Radial cleavage: The 8-celled stage has 2 tiers of 4 cells each, with each cell directly below or above another. Indeterminate cleavage: Cells separated in early embryonic stages can develop into an entire embryo. Deuterostome condition: The embryo's blastopore becomes the posterior (tail) end. (Remember that in molluscs, arthropods, and other protostome phyla, the blastopore becomes the mouth.) Phylum Echinodermata: 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 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. Asterozoa: Body star-shaped, with protruding arms. Includes starfishes and brittle stars. Echinozoa: Body globe-shaped, with no protruding arms. Includes sea cucumbers, sea urchins, and sand dollars. Deuterostomes Index             Syllabus Prev rev. Mar. 2016 Next