Introduction to Kingdom Animalia

• Animals belong to Kingdom Animalia within the domain Eukaryota.
• They are multicellular, eukaryotic organisms, meaning their cells have a nucleus and other complex organelles.
• Animals are heterotrophic, obtaining nutrients by consuming other organisms.
• Most animals reproduce sexually, with a diploid dominant life cycle.
• Animal cells lack cell walls, unlike plants and fungi.
• Animals are capable of movement at some point in their life cycle.
• Most animals have specialized tissues, such as nervous, muscle, and connective tissues.

Animal Diversity

• Animal diversity is vast, with over 1.3 million identified species and estimates of millions more.
• Animals are classified based on body plan, symmetry, presence or absence of tissues, and developmental patterns.
• Major animal phyla include Porifera (sponges), Cnidaria (jellyfish, corals), Platyhelminthes (flatworms), Nematoda (roundworms), Mollusca (snails, clams, squids), Annelida (segmented worms), Arthropoda (insects, spiders, crustaceans), Echinodermata (starfish, sea urchins), and Chordata (vertebrates and related groups).
• Arthropoda is the most diverse phylum, comprising over 80% of known animal species.
• Chordata includes vertebrates (animals with a backbone) such as fish, amphibians, reptiles, birds, and mammals.

General Characteristics

• Animals exhibit diverse body plans.
• Body plan refers to the overall structure, symmetry, and organization of an organism.
• Animals can have radial symmetry (e.g., jellyfish) or bilateral symmetry (e.g., humans).
• Bilaterally symmetrical animals have a distinct head (anterior) and tail (posterior), and a dorsal (back) and ventral (belly) side.
• Cephalization, the concentration of sensory organs and nervous tissue at the anterior end, is common in bilaterally symmetrical animals.
• Animals can be diploblastic (having two germ layers: ectoderm and endoderm) or triploblastic (having three germ layers: ectoderm, mesoderm, and endoderm).
• Triploblastic animals can be acoelomate (lacking a body cavity), pseudocoelomate (having a body cavity incompletely lined with mesoderm), or coelomate (having a true body cavity completely lined with mesoderm).
• The body cavity (coelom) provides space for organ development and allows for greater flexibility and movement.

Reproduction and Development

• Most animals reproduce sexually, involving the fusion of haploid gametes (sperm and egg) to form a diploid zygote.
• Some animals can also reproduce asexually through mechanisms like budding, fragmentation, or parthenogenesis.
• After fertilization, the zygote undergoes cleavage, a series of rapid cell divisions.
• Cleavage leads to the formation of a blastula, a hollow ball of cells.
• The blastula undergoes gastrulation, where cells migrate inward to form the germ layers (ectoderm, mesoderm, and endoderm).
• The ectoderm gives rise to the outer covering of the animal and the nervous system.
• The mesoderm gives rise to muscles, connective tissues, and most organs.
• The endoderm gives rise to the lining of the digestive tract and associated organs.
• In deuterostomes (echinoderms and chordates), the blastopore (the opening formed during gastrulation) becomes the anus, while in protostomes (most other invertebrates), the blastopore becomes the mouth.

Animal Nutrition

• Animals are heterotrophs that obtain nutrients by ingestion (eating).
• Digestion can occur intracellularly (within cells, as in sponges) or extracellularly (in a digestive cavity, as in most animals).
• Animals have diverse feeding strategies, including herbivores (eating plants), carnivores (eating animals), omnivores (eating both plants and animals), and detritivores (eating dead organic matter).
• Adaptations for feeding vary widely among animals, including specialized mouthparts, digestive enzymes, and symbiotic relationships with microorganisms.

Respiration and Circulation

• Animals require oxygen for cellular respiration and must eliminate carbon dioxide.
• Gas exchange occurs across respiratory surfaces, such as gills, lungs, or the skin.
• Small animals can rely on diffusion for gas exchange, while larger animals require specialized respiratory systems.
• Circulation transports oxygen and nutrients to cells and removes waste products.
• Animals can have open circulatory systems (where hemolymph bathes the organs directly) or closed circulatory systems (where blood is confined to vessels).
• Vertebrates have closed circulatory systems with a heart that pumps blood through arteries, veins, and capillaries.

Excretion

• Animals must eliminate nitrogenous waste products, which are produced from the breakdown of proteins and nucleic acids.
• Nitrogenous wastes can be excreted as ammonia, urea, or uric acid.
• Ammonia is highly toxic but requires little energy to produce and is common in aquatic animals.
• Urea is less toxic than ammonia and is common in mammals and amphibians.
• Uric acid is relatively nontoxic and requires more energy to produce and is common in birds, reptiles, and insects.
• Excretion is carried out by specialized organs, such as kidneys (in vertebrates) or nephridia (in invertebrates).

Nervous System and Sensory Organs

• Animals have nervous systems that allow them to detect and respond to stimuli.
• Nervous systems can range from simple nerve nets (in cnidarians) to complex brains (in vertebrates).
• Sensory organs allow animals to gather information about their environment, including light, sound, chemicals, and mechanical stimuli.
• Sensory organs include eyes, ears, taste receptors, olfactory receptors, and touch receptors.

Movement

• Animals are capable of movement, which allows them to find food, escape predators, and reproduce.
• Movement is achieved through the coordinated action of muscles and skeletal systems.
• Skeletal systems can be hydrostatic skeletons (fluid-filled cavities), exoskeletons (external skeletons), or endoskeletons (internal skeletons).
• Muscles contract to produce movement, and different types of muscles (e.g., skeletal, smooth, cardiac) have different functions.

Animal Behavior

• Animal behavior is the way animals interact with their environment and other organisms.
• Behavior can be innate (genetically determined) or learned (modified by experience).
• Behaviors include foraging, mating, communication, and social interactions.
• Ethology is the study of animal behavior, focusing on the evolutionary and ecological context of behavior.