Introduction to Animals! • Animals are monophyletic and all of them share three traits: • Multicellularity. • Heterotrophy—they ingest their food. • They move under their own power at some point in their life cycle. • All animals except sponges also have: • Neurons (nerve cells) that transmit electrical signals to other cells. • Muscle cells that can change the shape of the body by contracting.
Introduction • Over 1.2 million animal species have been described and named to date, and biologists predict that tens of millions more have not yet been discovered.
Themes in the diversification of animals • Between 30–35 major animal phyla are recognized. Each animal phylum has synapomorphies that identify it as a monophyletic group. • Groups of phyla are characterized by fundamental aspects of morphology and development that changed as animals diversified. • Animals are incredibly diverse, particularly in morphology.
Analyzing Comparative Morphology • The origin and early evolution of animals was based on four aspects of the fundamental architecture, or bodyplan, of animals: • Number of embryonic tissue layers. • Type of body symmetry and degree of cephalization (formation of a head region). • Presence or absence of a fluid-filled body cavity. • How the earliest events of embryo development proceed.
The Origin and Diversification of Tissues • All animals have tissues—tightly integrated structural and functional units of cells. • With the exception of most of the sponges, all animals have epithelium, a layer of tightly joined cells that covers the body surface. • One group of sponges also has epithelium. • Other animals have an array of other tissue types as well as epithelium.
The Origin and Diversification of Tissues • In animals other than sponges, the number of tissue layers that exist in an embryo varies. • Diploblasts are animals whose embryos have two types of tissues, or germ layers: • The ectoderm (“outside skin”). • The endoderm (“inside skin”). • Triploblasts are animals whose embryos have three germ layers: • The ectoderm. • The endoderm. • The mesoderm (“middle skin”).
The Origin and Diversification of Tissues • Germ layers develop into distinct adult tissues and organs. • In general, ectoderm produces the covering of the animal and endoderm generates the digestive tract. Mesoderm gives rise to the tissues in between. • In a triploblast: • Ectoderm => skin and the nervous system. • Endoderm => the lining of the digestive tract. • Mesoderm => the circulatory system, muscle, and internal structures such as bone and most organs.
The Origin and Diversification of Tissues • Most cnidarians (which include the jellyfish, corals, sea anemones, and more) and all ctenophores (comb jellies) are diploblastic. • All other animals are triploblastic. • The evolution of mesoderm was important because it gave rise to the first complex muscle tissue used in movement.
Nervous Systems and Body Symmetry • Sponges lack neurons, but cnidarians and ctenophores have nerve cells that are organized into a nerve net. All other animals have a centralized nervous system, or CNS. • In a CNS, some neurons are clustered into tracts or cords, and others are clustered in masses called ganglia. • Organisms with nerve nets tend to have radial symmetry while organisms with a CNS tend to have bilateral symmetry.
Body Symmetry • Most sponges are asymmetrical. • Animals with radial symmetry such as cnidarians, ctenophores, and some sponges, have at least two planes of symmetry. • Most other animals exhibit bilateral symmetry, with a single plane of symmetry and long, narrow bodies.
How Are Symmetry and Nervous Systems Related? • Radially symmetric organisms are equally likely to encounter their environment in any direction. • A diffuse nerve net can receive and send signals efficiently. • Bilaterally symmetric organisms tend to encounter their environment at one end. • It is advantageous to have many neurons concentrated at that end, with nerve tracts that carry information from there down the length of the body.
Symmetry and Cephalization • Bilateral symmetry allowed cephalization, the development of a head region where structures for feeding, sensing the environment, and processing information are concentrated. • The large mass of neurons that is located in the head, and that is responsible for sending and receiving information to and from the body, is called the cerebral ganglion or brain. • Locating and capturing food is particularly efficient when movement is directed by a distinctive head region and powered by the rest of the body.
Evolution of a Body Cavity • Animals may or may not have an enclosed, fluid-filled body cavity called a coelom. • Triploblasts with no coelom: acoelomates; those with a coelom: coelomates. • The coelom forms from within the mesoderm and thus is lined on both sides with cells from the mesoderm.
Evolution of a Body Cavity • The coelom creates a container for circulation of oxygen and nutrients, and acts as an efficient hydrostatic skeleton that allows soft-bodied animals to move even without fins or limbs. • The evolution of the coelom and the resulting hydrostatic skeleton gave bilaterally symmetric organisms the ability to move efficiently in search of food.
Protostome and Deuterostome Development Patterns • Except for adult echinoderms, all coelomates (including juvenile echinoderms) are bilaterally symmetric and have three embryonic tissue layers. • This group, called the Bilateria, can be divided into: • Protostomes, in which the mouth develops before the anus, and blocks of mesoderm hollow out to form the coelom. • Includes arthropods, mollusks, and segmented worms. • Deuterostomes, in which the anus develops before the mouth, and pockets of mesoderm pinch off to form the coelom. • Includes chordates and echinoderms.
Protostome and Deuterostome Development Patterns • Events in embryonic development—gastrulation and coelom formation—differ in protostomes and deuterostomes and give them their names. • Gastrulation is a series of cell movements that results in the embryonic tissue layers; as gastrulation proceeds, the coelom forms.
The Tube-within-a-Tube Design • The basic animal body plan is a tube-within-a-tube design in which the outer tube forms the body wall and the inner tube forms the gut. • This is easy to see in such animals as worms, but the body plan of animals with limbs is also the same; the tubes are just mounted on limbs.
Evaluating Molecular Phylogenies • The phylogenetic tree based on DNA sequence data lets us draw several possible conclusions: • A group of protists called choanoflagellates are the closest living relatives of animals. • Sponges are the sister group to all other animals. • Ctenophora and cnidaria are a monophyletic group. • Endoderm and ectoderm were the first embryonic tissue types to evolve, and that radial symmetry evolved before bilateral symmetry.
Evaluating Molecular Phylogenies • The evolution of mesoderm preceded the evolution of the coelom. • The split between the protostomes and the deuterostomes was a major event in the evolution of Bilateria. • A fundamental split occurred within protostomes: • The Lophotrochozoa grow by extending the size of their skeletons and includes the mollusks and the annelids. • The Ecdysozoa grow by shedding their external skeletons or outer coverings and expanding their bodies and includes the arthropods and the nematodes.
Evaluating Molecular Phylogenies • The coelom was lost during flatworm (Platyhelminthes) development. • During bilaterian evolution, segmentation, the presence of repeated body structures, arose several times independently. • Vertebrates, those animals with skulls and usually backbones, are a monophyletic lineage. • Invertebrates, all animals that are not vertebrates, are a paraphyletic group.
Sponges Are the Sister Group to All Other Animals • Porifera (sponges) are the most basal animal phylum and share several key characteristics with choanoflagellates. • Both taxa are sessile (attached to a substrate) as adults. • Sponges feed using cells called choanocytes, which are similar to choanoflagellates.
Themes in the Diversification of Animals • Within each animal phylum, the basic features of the body plan do not vary from species to species. • The diversification of species within each lineage was, in most cases, triggered by the evolution of innovative methods for sensing the environment, feeding, and moving.
Sensory Organs • A concentration of sensory organs in the head region (incl. mouth & brain) is a key aspect of cephalization. There is a great deal of diversity of sensory abilities and structures among the animals. • Most animals have the common senses of touch, balance, smell, taste, and hearing. Most animals can also detect light—and many have a well-developed sense of sight. • As animals diversified, a wide array of more specialized sensory abilities evolved.
Specialized Sensory Abilities • Magnetism—some animals can detect magnetic fields and use them as a navigation aid. (e.g., pigeons) • Electric fields—some aquatic predators can detect electrical activity in the muscles of passing prey. (e.g., sharks, stingrays) • Barometric pressure—some birds can avoid storms by sensing changes in air pressure. (many birds can do this) • In addition to different abilities, there is great diversity in sensory structures, such as the eye, among different animal groups.
Feeding • The feeding tactics observed in animals can be broken into four general types: • Suspension feeders. • Deposit feeders. • Fluid feeders. • Mass feeders. • The structure of an animal’s mouthparts correlates closely with its method of feeding.
How Animals Feed: Four General Tactics • Suspension feeders, or filter feeders, capture food by filtering out particles suspended in water or air. • Suspension feeders employ a wide array of structures to trap suspended particles and bring them to their mouths. • For example, clams trap food particles in their gills, while baleen whales filter food through the horny baleen plates that line their mouths. • Suspension feeders are commonly aquatic, and many are sessile.
How Animals Feed: Four General Tactics • Deposit feeders eat their way through a substrate. • Many deposit feeders digest organic matter in the soil or on the seafloor. • Deposit feeders usually have simple mouthparts and their body shape is wormlike.
How Animals Feed: Four General Tactics • Fluid feeders suck or mop up liquids like nectar, plant sap, blood, or fruit juice. • They often have mouthparts that allow them to pierce a structure to withdraw the fluids inside.
How Animals Feed: Four General Tactics • Mass feeders take chunks of food into their mouths. • The structure of the mouthparts correlates with the type of food pieces eaten.
What Animals Eat: Three General Sources • The three general sources of food for animals are • Plants or algae. • Other animals. • Detritus. • Animals that feed on plants or algae: herbivores. • Animals that feed on other animals: carnivores. • Animals that feed on dead organic matter: detritivores. • Omnivores, such as humans, eat both plants and animals.
What Animals Eat: Three General Sources • The difference between predators and parasites • Predators kill other organisms for food using an array of mouthparts and hunting strategies. Predators are generally larger than their prey and kill them quickly. • Parasitesare usually much smaller than their victims and often harvest nutrients without causing death. • Endoparasites live inside their hosts. • Ectoparasites live outside their hosts.
Movement • Movement has three functions in adult animals: • Finding food. • Finding mates. • Escaping from predators. • The ways in which animals move are highly variable. • The major innovation of limbs made highly controlled, rapid movement possible.
Types of Limbs: Unjointed and Jointed • Unjointed limbs, such as those found in the velvet worm, are sac-like. • Jointed limbs, such as those found in crabs and other arthropods, make fast, precise movements possible and are prominent in vertebrates and arthropods.
Are All Animal Appendages Homologous? • Animal appendages are exceedingly diverse, and biologists have long suspected that different genes are responsible for the different types of appendages. • Recent analyses, however, indicate that at least a few of the same genes are involved in the development of all animal appendages.
Is the Dll Gene Involved in All Limb Formation? • The hypothesis is that all animal appendages have some degree of genetic homology and that they are all derived from appendages that were present in a common ancestor. • Research suggests that the Dll gene is involved in limb formation in diverse species. • The conclusion is that the protein product of the same gene marks the initial site of appendage growth in most if not all animals.
Reproduction • Animals exhibit a high degree of variation in how they reproduce. • At least some species in most animal phyla can reproduce asexually (via mitosis), as well as sexually (via meiosis). • When sexual reproduction does occur, fertilization may be internal or external.
Where Do Embryos Develop? • Viviparous species retain the eggs or embryos in the female's body during development. • In oviparous species eggs are laid outside to develop independently of the mother. The vast majority of animal species are oviparous. • In ovoviviparous species, the female retains eggs inside her body during early development, but the growing embryos are nourished by yolk inside the egg and not by nutrients transferred directly from the mother.
Reproduction and Life Cycles • Animal life cycles vary widely. • Biologists distinguish three types of life stages: • Larvae look radically different from adults, live in different habitats, eat different foods, and are sexually immature. • Juveniles look and behave like adults, but are sexually immature. • Adults are the reproductive stage in the life cycle. • Perhaps the most spectacular innovation in animal life cycles involves the phenomenon known as metamorphosis—a change from an immature body type to an adult body type.
Two Types of Insect Metamorphosis • In hemimetabolousmetamorphosis, the juvenile form is called a nymph and looks like a miniature adult. • In holometabolousmetamorphosis, also known as complete metamorphosis, the juvenile individual is called a larva and looks quite different from the adult form. • When the larva has grown enough, it encases itself and becomes a pupa, the stage in which it is remodeled into an adult. • Complete metamorphosis is common in insects and marine animals.
A Complex Life Cycle in Cnidaria • Some cnidarians have three distinct body types during their life cycle: • A largely sessile form called a polyp, which reproduces asexually. • A sexually reproducing, free-floating stage called a medusa. • A larval stage. • Polyps and medusae live in different habitats and the two stages of the life cycle exploit different food sources.