Lecture 7 - Animal Diversity

Lecture 7-Animal Diversity Animal are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers Nutrition • Animals are heterotrophs that ingest their food Cell Structure and Specialization • Animals are multicellular eukaryotes • Their cells lack cell walls • Their bodies are held together by structural proteins such as collagen • Nervous tissue and muscle tissue are unique, defining characteristics of animals • Tissues are groups of cells that have a common structure, function, or both Reproduction and Development • Most animals reproduce sexually, with the diploid stage usually dominating the life cycle • After a sperm fertilizes an egg, the zygote undergoes rapid cell division called cleavage • Cleavage leads to formation of a multicellular, hollow blastula • The blastula undergoes gastrulation, forming agastrula with different layers of embryonic tissues • Many animals have at least one larval stage • A larva is sexually immature and morphologically distinct from the adult; it eventually undergoes metamorphosis • A juvenile resembles an adult, but is not yet sexually mature History Most zoologists agree that the common ancestor of living animals may have lived between 675 and 800 million years ago • This ancestor may have resembled modern choanoflagellates, protists that are the closest living relatives of animals Symmetry • Animals can be categorized according to the symmetry of their bodies, or lack of it • Some animals have radial symmetry, with no front and back, or left and right • Two-sided symmetry is called bilateral symmetry • Bilaterally symmetrical animals have • A dorsal (top) side and a ventral (bottom) side • A right and left side • • • Anterior (head) andposterior (tail) ends • Cephalization, the development of a head Radial animals are often sessile or planktonic (drifting or weakly swimming) Bilateral animals often move actively and have a central nervous system Tissues • Animal body plans also vary according to the organization of the animal’s tissues • Tissues are collections of specialized cells isolated from other tissues by membranous layers • During development, three germ layers give rise to the tissues and organs of the animal embryo • Ectoderm is the germ layer covering the embryo’s surface • Endoderm is the innermost germ layer and lines the developing digestive tube, called the archenteron • Sponges and a few other groups lack true tissues • Diploblastic animals have ectoderm and endoderm • These include cnidarians and comb jellies • Triploblastic animals also have an intervening mesoderm layer; these include all bilaterians • These include flatworms, arthropods, vertebrates, and others Body Cavities • Most triploblastic animals possessbody a cavity • A true body cavity is called coelom a and is derived from mesoderm • Coelomates are animals that possess a true coelom • A pseudocoelom is a body cavity derived from the mesoderm and endoderm • Triploblastic animals that possess a pseudocoelom are pseudocoelomates called • Triploblastic animals that lack a body cavity are called acoelomates Protostome and Deuterostome Development • Based on early development, many animals can be categorized as having protostome development or deuterostome development Cleavage • In protostome development, cleavage spiral is and determinate • In deuterostome development, cleavage radial is andindeterminate • With indeterminate cleavage, each cell in the early stages of cleavage retains the capacity to develop into a complete embryo • Indeterminate cleavage makes possible identical twins, and embryonic stem cells Coelom Formation • In protostome development, the splitting of solid masses of mesoderm forms the coelom • In deuterostome development, the mesoderm buds from the wall of the archenteron to form the coelom Fate of the Blastopore • The blastopore forms during gastrulation and connects the archenteron to the exterior of the gastrula • In protostome development, the blastopore becomes the mouth • In deuterostome development, the blastopore becomes the anus