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Chapter 17 The Evolution of Animals What Is an Animal? – Animals are: • • • • Eukaryotic Multicellular Heterotrophic organisms that obtain nutrients by ingestion Able to digest their food within their bodies – Animal cells lack the cell walls that provide strong support in the bodies of plants and fungi. Sponges No true tissues Cnidarians Radial symmetry Ancestral protist Molluscs Flatworms Tissues Annelids Roundworms Bilateral symmetry Arthropods Echinoderms Chordates Figure 17.5 – A second major evolutionary split is based on body symmetry. • Radial symmetry refers to animals that are identical all around a central axis. • Bilateral symmetry exists where there is only one way to split the animal into equal halves. Radial symmetry. Parts radiate from the center, so any slice through the central axis divides into mirror images. Bilateral symmetry. Only one slice can divide left and right sides into mirror-image halves. Figure 17.6 – Animals also vary according to the presence and type of body cavity, a fluid-filled space separating the digestive tract from the outer body wall. – There are differences in how the body cavity forms. • If the body cavity is not completely lined by tissue derived from mesoderm, it is a pseudocoelom. • A true coelom is completely lined by tissue derived from mesoderm. Body covering (from ectoderm) (a) No body cavity Tissue-filled region (from mesoderm) Digestive tract (from endoderm) Body covering (from ectoderm) (b) Pseudocoelom Muscle layer (from mesoderm) Pseudocoelom Digestive tract (from endoderm) (c) True coelom Coelom Digestive tract (from endoderm) Body covering (from ectoderm) Tissue layer lining coelom and suspending internal organs (from mesoderm) Figure 17.7 MAJOR INVERTEBRATE PHYLA – Invertebrates: • Are animals without backbones • Represent 95% of the animal kingdom Sponges – Sponges include sessile animals that lack true tissues and that were once believed to be plants. Figure 17.8b Cnidarians – Cnidarians (phylum Cnidaria) are characterized by: • The presence of body tissues • Radial symmetry • Tentacles with stinging cells – The basic body plan of a cnidarian is a sac with a gastrovascular cavity, a central digestive compartment with only one opening. – The body plan has two variations: • The sessile polyp • The floating medusa – Cnidarians are carnivores that use tentacles, armed with nematocysts (or cnidocytes) (“stinging cells”), to capture prey. Mouth/anus Tentacle Gastrovascular cavity Polyp form Coral Hydra Sea anemone Gastrovascular cavity Mouth/anus Tentacle Medusa form Jelly Figure 17.9 Tentacle Coiled thread Capsule Trigger Discharge of thread Prey Cnidocyte Figure 17.10 Molluscs – Molluscs (phylum Mollusca) are represented by softbodied animals, usually protected by a hard shell. – Many molluscs feed by using a file-like organ called a radula to scrape up food. – The body of a mollusc has three main parts: • A muscular foot used for movement • A visceral mass housing most of the internal organs • A mantle, which secretes the shell if present Visceral mass Coelom Kidney Heart Mantle Mantle cavity Reproductive organs Digestive tract Shell Radula Anus Gill Mouth Foot Digestive tract Radula Nerve cords Mouth Figure 17.11 – The three major groups of molluscs are: • Gastropods, protected by a single, spiraled shell Gastropods Snail (spiraled shell) Sea slug (no shell) Figure 17.12a • Bivalves, with a shell divided into two halves hinged together Bivalves (hinged shell) Scallop Figure 17.12b • And cephalopods – Typically lacking an external shell – Built for speed and agility Cephalopods (large brain and tentacles) Octopus Squid Figure 17.12c MAJOR GROUPS OF MOLLUSCS Gastropods Bivalves (hinged shell) Cephalopods (large brain and tentacles) Snail (spiraled shell) Scallop Octopus Squid Sea slug (no shell) Figure 17.12 Flatworms – Flatworms (phylum Platyhelminthes) are the simplest bilateral animals. – Flatworms include forms that are: • Parasites or • Free-living in marine, freshwater, or damp habitats Digestive tract (gastrovascular cavity) Nerve cords Mouth Eyespots (detect light) Planarian Nervous tissue clusters (simple brain) Blood fluke Bilateral symmetry Reproductive unit with skin removed Head Suckers Tapeworm Figure 17.13 Hooks Annelids – Annelids (phylum Annelida) have: • Body segmentation, a subdivision of the body along its length into a series of repeated parts • A coelom • A complete digestive tract with – Two openings, a mouth and anus – One-way movement of food Anus Brain Main heart Coelom Digestive tract Segment walls Mouth Accessory hearts Nerve cord Waste disposal organ Blood vessels Figure 17.15 – The three main groups of annelids are: • Earthworms, which eat their way through soil • Polychaetes, marine worms with segmental appendages for movement and gas exchange • Leeches, typically free-living carnivores but with some bloodsucking forms MAJOR GROUPS OF ANNELIDS Earthworms Polychaetes Leeches Giant Australian earthworm Christmas tree worm European freshwater leech Figure 17.14 Roundworms – Roundworms (phylum Nematoda) are: • Cylindrical in shape, tapered at both ends • The most diverse and widespread of all animals – Roundworms (also called nematodes) are: • Important decomposers • Dangerous parasites in plants, humans, and other animals (a) A free-living roundworm (b) Parasitic roundworms in pork (c) Canine heart Infected with parasitic roundworms Figure 17.16 Arthropods – Arthropods (phylum Arthropoda) are named for their jointed appendages. – There are about one million arthropod species identified, mostly insects. – Arthropods are a very diverse and successful group, occurring in nearly all habitats in the biosphere. – There are four main groups of arthropods. MAJOR GROUPS OF ARTHROPODS Arachnids Crustaceans Millipedes and Centipedes Insects Figure 17.17 General Characteristics of Arthropods – Arthropods are segmented animals with specialized segments and appendages for an efficient division of labor among body regions. – The body of arthropods is completely covered by an exoskeleton, an external skeleton that provides: • Protection • Points of attachment for the muscles that move appendages Abdomen Cephalothorax (head and thorax) Antenna (sensory reception) Eyes on movable stalks Mouthparts (feeding) Walking leg Swimming appendage Walking legs Figure 17.18 Arachnids – Arachnids: • Live on land • Usually have four pairs of walking legs and a specialized pair of feeding appendages • Include spiders, scorpions, ticks, and mites Two feeding appendages Leg (four pairs) Scorpion Black widow spider Dust mite Wood tick Figure 17.19 Crustaceans – Crustaceans: • Are nearly all aquatic • Have multiple pairs of specialized appendages • Include crabs, lobsters, crayfish, shrimps, and barnacles Two feeding appendages Leg (three or more pairs) Antennae Crab Pill bug Shrimp Crayfish Barnacles Figure 17.20 Millipedes and Centipedes – Millipedes and centipedes have similar segments over most of the body. – Millipedes: • Eat decaying plant matter • Have two pairs of short legs per body segment – Centipedes: • Are terrestrial carnivores with poison claws • Have one pair of short legs per body segment One pair of legs per segment Two pairs of legs per segment Millipede Centipede Figure 17.21 Insect Anatomy – Insects typically have a three-part body: • Head • Thorax • Abdomen – The insect head usually bears: • A pair of sensory antennae • A pair of eyes – The mouthparts are adapted for particular kinds of eating. – Flight is one key to the great success of insects. Head Thorax Abdomen Antenna Eye Mouthparts Figure 17.22 Insect Diversity – Insects outnumber all other forms of life combined. – Insects live in: • Almost every terrestrial habitat • Freshwater • The air Leaf roller Banded Orange Heliconian Giraffe weevil Yellow jacket wasp Peacock katydid Praying mantis Leaf beetle Longhorn beetle Figure 17.23 – Many insects undergo metamorphosis in their development. – Young insects may: • Appear to be smaller forms of the adult or • Change from a larval form to something much different as an adult The larva (caterpillar) spends its time eating and growing, molting as it grows. Finally, the adult emerges from the cocoon. After several molts, the larva becomes a pupa encased in a cocoon. Within the pupa, the larval organs break down and adult organs develop from cells that were dormant in the larva. The butterfly flies off and reproduces, nourished mainly by calories stored when it was a caterpillar. Figure 17.24-5 Monarch butterflies Figure 17.24a Echinoderms – Echinoderms (phylum Echinodermata): • Lack body segments • Typically show radial symmetry as adults but bilateral symmetry as larvae • Have an endoskeleton • Have a water vascular system that facilitates movement and gas exchange – Echinoderms are a very diverse group. Sea star Tube feet Sea urchin Sea cucumber Sand dollar Figure 17.25