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Animals! Introduction What is an animal? Structure, nutrition and life history define animals… – While there are exceptions to nearly every criterion for distinguishing an animal from other life forms, five criteria, when taken together, create a reasonable definition. 1. 2. 3. 4. 5. Multicellular, heterotrophic eukaryotes No cell wall Nervous and muscle tissue Sexual reproduction, diploid dominant Unique regulatory gene for development 1. Multicellular, heterotrophic eukaryotes • They must take in preformed organic molecules through ingestion, eating other organisms or organic material that is decomposing. – They gotta eat! 2. No cell wall • Animal cells lack the cell walls that provide strong support in the bodies of plants and fungi. • The multicellular bodies of animals are held together with the extracellular proteins, especially collagen. 3. Nerves and Muscles • Animals have two unique types of tissues: – nervous tissue for impulse conduction – muscle tissue for movement 4. Sexual reproduction, diploid stage dominates life cycle • small flagellated sperm fertilizes a nonmotile egg • zygote undergoes cleavage, a succession mitotic cell divisions, leading to the formation of a multicellular, hollow ball of cells called the blastula. • During gastrulation, part of the embryo folds inward, forming the blind pouch characteristic of the gastrula This produces two tissue layers: the endoderm as the inner layer and the ectoderm as the outer layer. 5. Regulatory genes: Hox genes • Hox genes: Regulatory genes that act in the developing embryo to control the transformation of a zygote to an animal of specific form – regulate the expression of other genes – contain common DNA sequences, called homeoboxes – All animals, from sponges to the most complex insects and vertebrates have Hox genes – the number of Hox genes correlated with complexity of the animal’s anatomy. – variation in when and where the Hox genes are expressed in a developing embryo provides the genetic basis for the great diversity of animal forms that have evolved from a common ancestor Animal evolution • The animal kingdom probably evolved from a colonial, flagellated protist • Ancestor lived over 700 million years ago in the Precambrian era. • probably related to choanoflagellates • Modern choanoflagellates are tiny, stalked organisms inhabiting shallow ponds, lakes, and marine environments Phylogenetic Tree of Animals • Phylogeny is changing due discoveries in molecular biology • The traditional view of relationships among animal phyla are based mainly on key characteristics of body plans and embryonic development. • There are four main branches… 1. Parazoa vs. Eumetazoa • Parazoa: lack true tissues – Phylum Pofifera: sponges – unique development and structural simplicity • Eumetazoa: have true tissues – Tissues are a basic feature of all other animal phyla 2. Radial vs. Bilateral Symmetry Eumetazoans divided based on body symmetry Radiata: Radial Symmetry • phylum Cnidaria (hydras, jellies, sea anemones and their relatives) and phylum Ctenophora (comb jellies) • Diploblastic: two germ layers - ectoderm, covers surface of the embryo, endoderm, the innermost layer, lines the developing digestive tube Bilateria: Bilateral Symmetry • Have a dorsal and ventral side, an anterior and posterior end, and a left and right side. • Cephalization: concentration of sensory equipment on the anterior end (a brain!) • Triploblastic: third germ layer, mesoderm lies between the endoderm and ectoderm – develops into the muscles and organs 3. Body Cavity • Acoelomates: triploblastic animals with solid bodies – Do not have a cavity between the digestive tract and outer body wall – phylum Platyhelminthes • Pseudocoelom: there is a body cavity, but it is not completely lined with tissue derived from the mesoderm. – include the rotifers (phylum Rotifera) and the roundworms (phylum Nematoda) • Coelomates: organisms with a true coelom, a fluid-filled body cavity completely lined by tissue derived from the mesoderm. – The inner and outer layers of tissue that surround the cavity connect dorsally and ventrally to form mesenteries, which suspend the internal organs. Functions: • cushions the suspended organs • hydrostatic skeleton in some animals • enables the internal organs to grow and move independently to the outer body wall 4. Protostomes vs. Deuterostomes Coelomate phyla are divided based on differences in their development. Protostomes • spiral cleavage: planes of cell division are diagonal to the vertical axis of the embryo • Coelom formation begins in the gastrula • Blastopore forms the mouth – The mollusks, annelids, arthropods Deuterostomes • radial cleavage: the cleavage planes are parallel or perpendicular to the vertical egg axis • Coelom forms from the mesodermal outpocketings • Blastopore forms the anus – echinoderms, chordates (you!)