General Introduction

Animal Biology examines the diversity, classification, structure, and vital functions of animals and animal-like organisms. Animals are mainly heterotrophic, meaning they obtain food from other organisms. Unlike plants, animal cells lack a rigid cellulose cell wall and are usually capable of movement at some stage of life. Because many organisms do not fit neatly into the old plant-animal division, the five-kingdom system recognizes Prokaryotae, Protoctista, Fungi, Plantae, and Animalia.

Key Definitions

• Heterotrophic nutrition: A mode of nutrition in which an organism depends on other organisms for organic food.
• Autotrophic nutrition: A mode of nutrition in which an organism manufactures its own food, usually by photosynthesis.
• Animalia: The kingdom containing multicellular heterotrophic organisms with cells lacking cellulose walls.
• Protoctista: A kingdom containing diverse eukaryotic organisms including protozoans and algae.

Exam High-Yield Points

• Animals are mainly heterotrophic, motile, and lack cellulose cell walls.
• Protozoans are animal-like but are classified under Protoctista.
• The course covers animal diversity and major physiological processes.

Lecture Unit 1: Classification and Protozoa

Classification organizes organisms into groups based on shared features. The branch of biology concerned with this process is taxonomy. Taxonomy includes identification, nomenclature, and classification. Systematics studies biological diversity and evolutionary relationships among organisms.

Classification Terms

• Taxonomy: The science of identifying, naming, and classifying organisms.
• Classification: The arrangement of organisms into ordered groups based on similarities and differences.
• Nomenclature: The scientific naming of organisms according to accepted rules.
• Taxon: Any named rank or group in classification, such as phylum, class, genus, or species.
• Species: A group of similar organisms capable of interbreeding naturally to produce fertile offspring.

Two major types of classification are recognized. Artificial classification uses easily observed characters such as habitat, color, or mode of movement. Natural classification uses many important characters and reflects possible evolutionary relationships. Phylogenetic classification is based on ancestry and descent, while phenetic classification emphasizes overall similarity in structure, cells, and biochemical features.

The binomial system gives each organism two scientific names. The first is the genus and begins with a capital letter. The second is the specific name and begins with a small letter. Both names are italicized or underlined. Example: Clarias gariepinus.

Taxonomic Hierarchy

Animals and animal-like organisms may be grouped broadly into invertebrates and vertebrates. Invertebrates lack a backbone, while vertebrates possess a backbone. Invertebrates are further treated in four organizational groups: Protozoa, Mesozoa, Parazoa, and Metazoa.

Levels of Organization in Invertebrates

• Protozoa: Mostly unicellular or acellular organisms whose life functions are performed by organelles.
• Mesozoa: Very simple multicellular forms that lack true organs and are considered intermediate in organization.
• Parazoa: Multicellular animals whose cells are not organized into true tissues and organs. Sponges are the main example.
• Metazoa: Multicellular animals with coordinated cells organized into tissues, organs, and systems.

Add Fig 1.1: Protozoan body forms and organelles. Insert this after the paragraph on Protozoa.

Protozoan Phyla

Exam High-Yield Points

• Protozoans are at the protoplasmic level of organization.
• Rhizopods use pseudopodia; ciliates use cilia; many sarcomastigophorans use flagella.
• Apicomplexans are mainly parasitic and include malaria-causing Plasmodium.
• Ciliates have a macronucleus for ordinary cell activities and a micronucleus for reproduction.

Lecture Unit 2: Parazoa and Early Metazoa

Parazoa are simple multicellular animals represented mainly by Phylum Porifera. They have many cells, but the cells do not form true tissues and organs. Metazoa are multicellular animals with cells organized into tissues, organs, and systems.

Phylum Porifera

Porifera means pore-bearing. Sponges are sessile aquatic animals, mostly marine, with bodies perforated by ostia. Water enters through ostia, moves through canals and the spongocoel, and leaves through the osculum. Choanocytes, or collar cells, generate water currents and trap food particles. Sponges lack true nervous, digestive, and circulatory systems.

Add Fig 2.1: Sponge body plan showing ostia, choanocytes, spongocoel, and osculum.

Classes of Porifera

• Calcarea: Sponges with calcareous spicules. Example: Sycon.
• Hexactinellida: Glass sponges with six-rayed siliceous spicules. Example: Euplectella.
• Demospongiae: Largest class; skeleton may contain spongin and non-six-rayed siliceous spicules. Example: Spongilla.
• Sclerospongiae: Reef-associated sponges with calcium carbonate skeleton and spicule-bearing surface layer.

Metazoan Grades

• Diploblastic animals: Possess ectoderm and endoderm, with mesoglea between them. Example: Cnidaria.
• Triploblastic animals: Possess ectoderm, mesoderm, and endoderm. Most higher invertebrates are triploblastic.

Phylum Cnidaria

Cnidarians are diploblastic, radially symmetrical animals with tissue-level organization. Their body cavity is the gastrovascular cavity or enteron. It has a single opening that serves as both mouth and exit. Their diagnostic structures are nematocysts, stinging capsules used for food capture and defense. Cnidarians occur as polyps, medusae, or both.

Add Fig 2.2: Cnidarian polyp and medusa body forms.

• Hydrozoa: Polyp usually dominant; examples include Hydra, Obelia, and Physalia.
• Scyphozoa: Medusa usually dominant; true jellyfish such as Aurelia.
• Anthozoa: Only polyp stage; sea anemones and corals such as Actinia and Astrangia.

Phylum Platyhelminthes

Platyhelminthes are flatworms. They are triploblastic, bilaterally symmetrical, acoelomate animals. Important higher-animal features first clearly appear here: cephalization, bilateral symmetry, a central nervous system, and organ-system organization. They lack circulatory and respiratory systems; exchange occurs mainly through the body surface. Excretion and osmoregulation occur through protonephridia ending in flame cells.

• Turbellaria: Mostly free-living flatworms such as Dugesia.
• Monogenea: Mostly ectoparasites with a posterior attachment organ called haptor.
• Trematoda: Parasitic flukes with suckers and complex life cycles. Examples include Fasciola and Schistosoma.
• Cestoidea: Tapeworms; adult forms live in vertebrate intestines, lack a digestive tract, and absorb nutrients through the body surface.

Phylum Nematoda

Nematodes are roundworms. They are triploblastic, bilaterally symmetrical, pseudocoelomate, cylindrical, unsegmented animals. They have a complete digestive tract with mouth and anus. Their elastic cuticle protects the body, and their muscles are mainly longitudinal. Sexes are separate and many species show sexual dimorphism. Some are free-living, while others are important plant and animal parasites.

Add Fig 2.3: Flatworm, tapeworm, and nematode body plan comparison.

Exam High-Yield Points

• Porifera are at the cellular level of organization.
• Cnidarians are diploblastic and possess nematocysts.
• Platyhelminthes are acoelomate; Nematoda are pseudocoelomate.
• Tapeworms feed by absorbing digested food through their body surface.

Lecture Unit 3: Annelida and Mollusca

Phylum Annelida

Annelids are segmented, coelomate worms. They show metamerism, meaning the serial repetition of body segments. They are triploblastic and bilaterally symmetrical. The coelom is a fluid-filled cavity between mesodermal layers. Annelids possess a closed circulatory system, segmental nephridia for excretion, a ventral nerve cord, and often chitinous chaetae.

• Polychaeta: Mostly marine bristle worms with many chaetae and parapodia. Examples: Nereis, Arenicola.
• Oligochaeta: Earthworms with few chaetae, no parapodia, and a clitellum. Example: Lumbricus.
• Hirudinea: Leeches; lack chaetae and parapodia, possess anterior and posterior suckers. Example: Hirudo.

Phylum Mollusca

Molluscs are soft-bodied, triploblastic, coelomate animals. The typical molluscan body has a head, muscular foot, visceral mass, and mantle. The mantle often secretes a shell. Most molluscs possess a radula, a toothed feeding ribbon, except bivalves. Their circulatory system is usually open, but cephalopods have a closed system.

Add Fig 3.1: Basic annelid and mollusc body plan, including earthworm segmentation and mollusc mantle, foot, and visceral mass.

Exam High-Yield Points

• Annelids are segmented coelomate worms with closed circulation.
• Leeches have suckers and lack chaetae.
• The molluscan mantle secretes the shell.
• Bivalves lack a radula; cephalopods are the most advanced molluscs.

Lecture Unit 4: Arthropoda

Arthropoda is the largest animal phylum. Arthropods are segmented, bilaterally symmetrical, triploblastic animals with jointed appendages and a chitinous exoskeleton. Their success is linked to the exoskeleton, specialization of body regions, efficient sensory organs, diverse feeding habits, and the ability to moult.

Add Fig 4.1: Insect or arthropod body plan showing head, thorax, abdomen, appendages, and internal systems.

Major Arthropod Groups

• Trilobita: Extinct marine arthropods with three-lobed bodies.
• Onychophora: Caterpillar-like forms such as Peripatus, showing annelid-like and arthropod-like features.
• Crustacea: Mostly aquatic; two pairs of antennae; gills; examples include crabs, prawns, and crayfish.
• Diplopoda: Millipedes; two pairs of legs per apparent segment; mostly herbivorous.
• Chilopoda: Centipedes; one pair of legs per segment; carnivorous with poison claws.
• Arachnida: Spiders, scorpions, ticks, and mites; no antennae; four pairs of walking legs.
• Insecta: Body divided into head, thorax, and abdomen; three pairs of legs; many have wings.

Insect Metamorphosis

• Incomplete metamorphosis: Egg to nymph to adult; wings develop externally.
• Complete metamorphosis: Egg to larva to pupa to adult; wings develop internally.

Economic Importance of Insects

• Beneficial roles: Pollination, honey production, silk production, decomposition, biological control, and research use.
• Harmful roles: Disease transmission, crop destruction, food spoilage, parasitism, and structural damage by termites.

Exam High-Yield Points

• Jointed appendages and chitinous exoskeleton define arthropods.
• Growth requires moulting because the exoskeleton is rigid.
• Insects have three body regions and three pairs of legs.

Lecture Unit 5: Echinodermata

Echinoderms are exclusively marine deuterostome invertebrates. They are important because their embryological pattern links them more closely to chordates than to many other invertebrates. Adults usually show pentaradial symmetry, while larvae are bilaterally symmetrical.

Add Fig 5.1: Sea star water vascular system showing madreporite, ring canal, radial canal, ampullae, and tube feet.

General Features

• Calcareous endoskeleton made of ossicles.
• Spiny skin and absence of true head.
• Unique water vascular system used in locomotion, feeding, attachment, and respiration.
• Tube feet controlled by hydraulic pressure.
• No special excretory organs.

Classes

• Crinoidea: Sea lilies and feather stars; oral surface faces upward.
• Asteroidea: Starfish; arms continuous with central disc; tube feet with suckers.
• Ophiuroidea: Brittle stars; arms sharply separated from central disc.
• Echinoidea: Sea urchins and sand dollars; arms absent; test present.
• Holothuroidea: Sea cucumbers; elongated body; reduced ossicles.

Exam High-Yield Points

• Echinoderms are marine deuterostomes.
• The water vascular system is diagnostic.
• Adult symmetry is usually pentaradial; larval symmetry is bilateral.

Lecture Units 6 and 7: Chordata and Tetrapods

Chordates possess a notochord, dorsal hollow nerve cord, pharyngeal clefts or pouches, and post-anal tail at some stage of development. Vertebrates are chordates in which a cranium is present and the notochord is partly or fully replaced by a vertebral column.

Add Fig 6.1: Chordate features and major vertebrate groups.

Subphyla of Chordata

• Hemichordata: Marine worm-like forms with pharyngeal slits.
• Urochordata: Tunicates; notochord usually present in larval tail.
• Cephalochordata: Lancelets; notochord extends along the body.
• Vertebrata: Cranium and vertebral column or equivalent support.

Vertebrate Groups

• Agnatha: Jawless vertebrates such as lampreys and hagfishes.
• Chondrichthyes: Cartilaginous fishes such as sharks and rays; no swim bladder; internal fertilization.
• Osteichthyes: Bony fishes; operculum covers gills; swim bladder often present.
• Amphibia: Moist skin, aquatic larvae, adults breathe by lungs, skin, and buccal cavity.
• Reptilia: Dry scaly skin, internal fertilization, amniotic eggs; first fully terrestrial vertebrates.
• Aves: Birds; feathers, wings, beak, air sacs, and high metabolic activity.
• Mammalia: Hair, mammary glands, diaphragm, four-chambered heart, and parental care.

Exam High-Yield Points

• Notochord, dorsal hollow nerve cord, pharyngeal clefts, and post-anal tail define chordates.
• Agnathans lack jaws; gnathostomes possess jaws.
• Reptiles, birds, and mammals are amniotes.
• Mammals are recognized by mammary glands and hair.

Lecture Unit 8: Nutrition in Animals

Nutrition is the intake and use of food materials for energy, growth, repair, and maintenance. Animals are heterotrophs and may feed holozoically, saprotrophically, or through symbiotic relationships.

Types of Nutrition

• Holozoic nutrition: Solid food is ingested, digested, absorbed, assimilated, and undigested residue is egested.
• Saprotrophic nutrition: Soluble organic materials are obtained from dead or decaying matter.
• Mutualism: Both partners benefit, as in ruminants and cellulose-digesting microorganisms.
• Parasitism: One organism benefits while the host is harmed, as in tapeworms or ticks.
• Commensalism: One organism benefits while the other is not significantly affected.

Feeding Mechanisms

• Particle feeding occurs in sponges and many aquatic organisms.
• Fluid feeding occurs in mosquitoes, aphids, and nectar-feeding insects.
• Scraping occurs in snails using the radula.
• Bulk feeding occurs in many vertebrates.

Exam High-Yield Points

• Holozoic nutrition involves ingestion, digestion, absorption, assimilation, and egestion.
• Saprotrophs are ecologically important in decomposition.
• Parasites depend on hosts and often possess attachment or absorptive adaptations.

Lecture Unit 9: Digestion of Food in Animals

Digestion is the mechanical and chemical breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed. Intracellular digestion occurs inside cells, as in Amoeba. Extracellular digestion occurs in a digestive cavity or alimentary canal, as in mammals.

Add Fig 8.1: Mammalian digestive system or comparison of gastrovascular cavity and alimentary canal.

Mammalian Digestion

• Mouth: Teeth mechanically break food; saliva moistens food and salivary amylase begins starch digestion.
• Oesophagus: Peristalsis moves food to the stomach.
• Stomach: Food is stored and mixed; hydrochloric acid kills many microbes and activates pepsin for protein digestion.
• Small intestine: Major site of enzymatic digestion and absorption. Bile emulsifies fats. Pancreatic enzymes digest carbohydrates, proteins, and fats.
• Large intestine: Water is reabsorbed and faeces are formed for egestion.

Exam High-Yield Points

• Digestion is enzyme-catalyzed hydrolysis of food molecules.
• Bile is not an enzyme; it emulsifies fats.
• Villi increase surface area for absorption.
• Glucose and amino acids enter blood capillaries; many lipid products enter lacteals.

Lecture Unit 10: Excretion

Excretion is the removal of metabolic waste products. It differs from egestion, which removes undigested food. Excretion maintains homeostasis by regulating water, salts, nitrogenous wastes, and pH.

Add Fig 10.1: Nephron structure showing glomerulus, Bowman capsule, proximal tubule, loop of Henle, distal tubule, and collecting duct.

Excretory Systems

• Protozoa: Diffusion and contractile vacuoles.
• Flatworms: Protonephridia and flame cells.
• Annelids: Segmental nephridia.
• Insects: Malpighian tubules; conserve water by excreting uric acid.
• Vertebrates: Kidneys are the main excretory organs.

Human Kidney Function

• Ultrafiltration: Blood pressure forces water and small solutes from the glomerulus into Bowman capsule.
• Selective reabsorption: Useful substances such as glucose, amino acids, salts, and water are returned to the blood.
• Tubular secretion: Additional wastes and ions move from blood into the tubule.
• Water balance: ADH increases water reabsorption in the distal tubule and collecting duct.

Exam High-Yield Points

• Urea is formed in the liver from excess amino acids.
• The nephron is the functional unit of the kidney.
• Freshwater protozoans need contractile vacuoles because water enters by osmosis.
• Insects conserve water by excreting uric acid.

Lecture Units 11 and 12: Reproduction and Development

Reproduction is the production of new individuals of the same species. It may be asexual or sexual. Asexual reproduction involves one parent and does not involve gamete fusion. Sexual reproduction involves gamete formation, fertilization, and genetic recombination.

Asexual Reproduction

• Binary fission: One organism divides into two, as in Amoeba and Paramecium.
• Multiple fission: Many daughter cells are produced, as in Plasmodium.
• Budding: A new individual grows from the parent, as in Hydra.
• Fragmentation and regeneration: Body pieces develop into new individuals in some simple animals.
• Gemmule formation: Resistant internal buds in freshwater sponges.

Sexual Reproduction

Sexual reproduction involves haploid gametes formed by meiosis. Fertilization restores the diploid condition by forming a zygote. External fertilization is common in many aquatic animals, while internal fertilization is important in terrestrial animals. Reptiles solved terrestrial reproduction by internal fertilization and the amniotic egg.

Developmental Types

• Oviparous: Eggs are laid outside the body. Examples: birds, many reptiles, amphibians, and insects.
• Ovoviviparous: Eggs are retained inside the female, but the embryo feeds mainly on yolk. Example: some sharks.
• Viviparous: Embryo develops inside the female and receives nourishment from the mother. Example: most mammals.

Embryogenesis

After fertilization, the zygote undergoes cleavage to form a blastula. Gastrulation rearranges cells to form germ layers. Neurulation begins formation of the nerve cord. Many animals hatch as larvae and later undergo metamorphosis.

Exam High-Yield Points

• Asexual reproduction is rapid but produces little genetic variation.
• Sexual reproduction increases variation and adaptability.
• The zygote is diploid; gametes are haploid.
• Amnion, chorion, allantois, and yolk sac are key amniote membranes.

Lecture Unit 13: Circulatory Systems

Circulatory systems transport nutrients, gases, hormones, and wastes. Small animals may rely on diffusion, but larger animals require bulk flow systems. A circulatory system normally includes a circulatory fluid, a pumping organ, and channels or spaces through which the fluid moves.

Types of Circulation

• Open circulatory system: Blood or haemolymph leaves vessels and bathes tissues in a haemocoel. It occurs in arthropods and most molluscs.
• Closed circulatory system: Blood remains within vessels. It occurs in annelids, cephalopods, and vertebrates.
• Single circulation: Blood passes through the heart once per circuit, as in fish.
• Incomplete double circulation: Blood passes through the heart twice but partly mixes, as in amphibians.
• Complete double circulation: Oxygenated and deoxygenated blood are fully separated, as in birds and mammals.

Exam High-Yield Points

• Closed circulation is faster and more controlled than open circulation.
• Fish have single circulation.
• Birds and mammals have complete double circulation with a four-chambered heart.

Lecture Unit 14: Hormonal Coordination

Coordination is the regulation and integration of body activities. Hormonal coordination uses chemical messengers called hormones. Hormones are produced by endocrine glands or neurosecretory cells and carried to target organs by body fluids. Hormonal responses are usually slower but longer-lasting than nervous responses.

Invertebrate Hormonal Control

• Hydra and planarians use neurosecretions in growth and regeneration.
• In insects, ecdysone stimulates moulting, while juvenile hormone helps maintain larval features.
• Reduction in juvenile hormone permits metamorphosis.

Major Vertebrate Endocrine Glands

Exam High-Yield Points

• Hormones are chemical messengers.
• Insulin lowers blood glucose; ADH increases water reabsorption.
• Thyroxine controls metabolic rate.
• Ecdysone and juvenile hormone control insect moulting and metamorphosis.

Lecture Unit 15: Nervous Coordination

The nervous system provides rapid, precise control through electrical impulses. Its basic functional cell is the neuron. Neurons receive stimuli, conduct impulses, and communicate with other cells at synapses.

Add Fig 15.1: Neuron structure and action potential or synapse diagram.

Neuron Types

• Sensory neuron: Carries impulses from receptors to the central nervous system.
• Motor neuron: Carries impulses from the central nervous system to muscles or glands.
• Intermediate neuron: Connects neurons within the central nervous system.

Nerve Impulse and Synapse

A nerve impulse is a rapid electrical change along a neuron. At rest, the neuron is polarized. During an action potential, sodium ions enter and the membrane depolarizes. Potassium movement restores the resting state. At a synapse, neurotransmitters carry the signal across the synaptic cleft.

Reflex Action

A reflex action is a rapid, automatic response to a stimulus. A reflex arc consists of receptor, sensory neuron, intermediate neuron, motor neuron, and effector.

Exam High-Yield Points

• The neuron is the structural and functional unit of the nervous system.
• Action potentials obey the all-or-nothing principle.
• Synapses use neurotransmitters.
• Reflex actions protect the body and do not require conscious decision before the response occurs.

Final Integrated Exam Revision

Likely Exam Questions

• Define taxonomy, systematics, nomenclature, species, coelom, and metamerism.
• Compare Protozoa, Parazoa, and Metazoa using levels of organization.
• State the diagnostic features of Porifera, Cnidaria, Platyhelminthes, and Nematoda.
• Compare Annelida and Mollusca.
• Explain why arthropods, especially insects, are highly successful.
• Discuss the significance of the water vascular system in echinoderms.
• List the four main chordate features and classify major vertebrate groups.
• Distinguish ingestion, digestion, absorption, assimilation, and egestion.
• Explain urine formation in the nephron.
• Compare asexual and sexual reproduction.
• Compare open, closed, single, and double circulation.
• Explain the difference between hormonal and nervous coordination.

Suggested Open-License Figure Sources

Important: Check the license statement on each linked page before final publication. Wikimedia Commons and OpenStax pages normally show reuse information and attribution requirements on the page.