An Introduction to Animal Diversity Overview: Welcome to Your Kingdom The animal kingdom extends far beyond humans and other animals we may encounter 1.3 million living species of animals have been identified Concept 32.1: Animal are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers There are exceptions to nearly every
criterion for distinguishing animals from other life-forms Several characteristics, taken together, sufficiently define the group Nutritional Mode 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 to animals 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 blastula The blastula undergoes gastrulation, forming a gastrula with different layers of embryonic tissues Fig. 32-2-3 Embryonic Development in Animals
Cross section of blastula Gastrula Blastopore Archenteron Many animals have at least one larval stage A larva is sexually immature and morphologically distinct from the adult; it eventually undergoes metamorphosis All animals, and only animals, have Hox genes that regulate the development of
body form Although the Hox family of genes has been highly conserved, it can produce a wide diversity of animal morphology Concept 32.2: The history of animals spans more than half a billion years The animal kingdom includes a great diversity of living species and an even greater diversity of extinct ones The common ancestor of living animals may have lived between 675 and 875 million years ago This ancestor may have resembled modern choanoflagellates, protists that
are the closest living relatives of animals Fig. 32-3 Individual choanoflagellate Choanoflagellates OTHER EUKARYOTES Sponges Animals Collar cell
(choanocyte) Other animals Neoproterozoic Era (1 Billion524 Million Years Ago) Early members of the animal fossil record include the Ediacaran biota, which dates from 565 to 550 million years ago radial symetry many body segments Paleozoic Era (542251 Million Years Ago)
The Cambrian explosion (535 to 525 million years ago) marks the earliest fossil appearance of many major groups of living animals There are several hypotheses regarding the cause of the Cambrian explosion New predator-prey relationships A rise in atmospheric oxygen Hox gene evolution Animal diversity continued to increase mass extinctions vertebrates appeared 460 mya (fishes) made the transition to land circa 360 mya Mesozoic Era (25165.5 Million Years Ago)
Coral reefs emerged, becoming important marine ecological niches for other organisms During the Mesozoic era, dinosaurs were the dominant terrestrial vertebrates The first mammals emerged Cenozoic Era (65.5 Million Years Ago to the Present) The beginning of the Cenozoic era followed mass extinctions of both terrestrial and marine animals These extinctions included the large, nonflying dinosaurs and the marine
reptiles Modern mammal orders and insects diversified during the Cenozoic Concept 32.3: Animals can be characterized by body plans Zoologists sometimes categorize animals according to a body plan, a set of morphological and developmental traits A grade is a group whose members share key biological features A grade is not necessarily a clade, or group that share a same ancestral species Symmetry
Animals can be categorized according to the symmetry of their bodies, or lack of it Some animals have radial symmetry Two-sided symmetry is called bilateral symmetry radial symmetry bilateral symmetry Bilateral Symmetry A dorsal (top) side and a ventral (bottom) side A right and left side Anterior (head) and posterior (tail)
ends Cephalization, the development of a head Tissues Animal body plans also vary according to the organization of the animals 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
embryos surface Endoderm is the innermost germ layer and lines the developing digestive tube, called the archenteron Diploblastic animals have ectoderm and endoderm Triploblastic animals also have an intervening mesoderm layer; these include all bilaterians Body Cavities Most triploblastic animals possess a body cavity A true body cavity is called a coelom and is derived from mesoderm
Coelomates are animals that possess a true coelom coelom body covering (from ectoderm) tissue layer lining coelom and suspending internal organs (from mesoderm) digestive tract (from endoderm) A pseudocoelom is a body cavity derived from the mesoderm and endoderm Triploblastic animals that possess a pseudocoelom are called
pseudocoelomates body covering (from ectoderm) coelom Muscle layer (from mesoderm) digestive tract (from endoderm) Triploblastic animals that lack a body cavity are called acoelomates body covering (from ectoderm) tissue filled region (from mesoderm) wall of digestive cavity (from endoderm)
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 is spiral and determinate In deuterostome development, cleavage is radial and indeterminate 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 Eight-cell stage Eight-cell stage mollusca, annelids, arthropods echinoderms, chordates Spiral and determinate Radial and indeterminate
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 Eight-cell stage Eight-cell stage mollusca, annelids, arthropods echinoderms, chordates Archenteron
Protostome development (examples: molluscs, annelids) Deuterostome development (examples: echinoderms, chordates) Anus Mouth (c) Fate of the blastopore Key
Digestive tube Anus Mouth Mouth develops from blastopore. Anus develops from blastopore. Ectoderm Mesoderm Endoderm Concept 32.4: New views of animal phylogeny are emerging from molecular data Zoologists recognize about three dozen animal phyla
Current debate in animal systematics has led to the development of two phylogenetic hypotheses, but others exist as well Fig. 32-10 Porifera Eumetazoa Metazoa ANCESTRAL COLONIAL FLAGELLATE
Cnidaria Ctenophora Deuterostomia Bilateria Brachiopoda Echinodermata Chordata Platyhelminthes Protostomia One hypothesis is based on the morphological and developmental
Another hyopothesis views animal phelogeny based on molecular data Cnidaria Echinodermata Chordata Platyhelminthes Lophotrochozoa Rotifera Ectoprocta Brachiopoda Mollusca
Annelida Ecdysozoa Nematoda Arthropoda Points of Agreement All animals share a common ancestor Sponges are basal animals Eumetazoa is a clade of animals (eumetazoans) with true tissues Most animal phyla belong to the clade Bilateria, and are called bilaterians Chordates and some other phyla belong
to the clade Deuterostomia Progress in Resolving Bilaterian Relationships The morphology-based tree divides bilaterians into two clades: deuterostomes and protostomes In contrast, recent molecular studies indicate three bilaterian clades: Deuterostomia, Ecdysozoa, and Lophotrochozoa Ecdysozoans shed their exoskeletons through a process called ecdysis nematodes and arthropodes
Some lophotrochozoans have a feeding structure called a lophophore apical tuft of cilia lophos= crest pherein= to carry plathyhelminthes rotifers molluscs annelids Trochophore
larva mout h anu s Other phyla go through a distinct developmental stage called the trochophore larva Phylogenetic studies based on larger databases will likely provide further insights into animal evolutionary history
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