The animal Kingdom
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| Animal Kingdom (Source: Google) |
The Animal Kingdom, or Kingdom Animalia, encompasses a vast and diverse group of organisms that share common traits such as being multicellular, heterotrophic (requiring organic substances for food), and having the ability to move at some stage of their life. With over a million species identified, the need for systematic classification becomes paramount to understanding their evolutionary relationships and biological characteristics. This blog provides a comprehensive look into the classification and the foundational features of animals based on the detailed notes.
1. Basis of Classification
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| Classification (Source: Google) |
The classification of animals relies on several fundamental features, including the level of cellular organization, symmetry, coelom, segmentation, and the presence of a notochord. Each characteristic offers insight into the complexity and evolution of the animal group.
Levels of Organization
While all animals are multicellular, they exhibit varying complexities in cell organization:
- Cellular Level: The simplest form, found in sponges, where cells are loosely aggregated with limited specialization.
- Tissue Level: Coelenterates like jellyfish show organization where similar cells form tissues performing specific functions.
- Organ Level: Seen in Platyhelminthes (flatworms), where tissues come together to form organs.
- Organ System Level: Higher organisms such as annelids and chordates have organs that form complex systems, such as the digestive and circulatory systems.
Symmetry
Animals can be classified based on their body symmetry:
- Asymmetrical: Sponges lack symmetry and cannot be divided into equal halves.
- Radial Symmetry: Seen in animals like jellyfish, where any plane passing through the center divides the body into equal halves.
- Bilateral Symmetry: Most animals, including humans, exhibit bilateral symmetry, where the body can only be divided into identical halves along one plane.
Coelom (Body Cavity)
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| Coelom (Source: Goggle) |
The presence or absence of a body cavity (coelom) is another vital feature:
- Acoelomates: Animals without a body cavity, such as flatworms.
- Pseudocoelomates: Animals like roundworms that have a false coelom, not completely lined by mesoderm.
- Coelomates: Animals with a true coelom, such as annelids, arthropods, and chordates.
Segmentation
Segmentation refers to the division of the body into repeated segments. Earthworms are a prime example of segmented animals, where their body shows external and internal segmentation.
Notochord
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| Notochord (Source: Goggle) |
The notochord is a rod-like structure that provides skeletal support in some animals. Chordates, such as humans, have a notochord at some stage of development, while non-chordates, including organisms like sponges and echinoderms, lack this structure.
2. Major Animal Phyla
Phylum Porifera (Sponges)
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| (Source: Goggle) |
Poriferans, commonly known as sponges, are the simplest multicellular animals. They have a porous body structure through which water flows, allowing for feeding, respiration, and waste removal. Sponges have an internal skeleton made of spicules and reproduce both sexually and asexually.
- Found in the ocean, asymmetrical shape, and cellular level of organization.
- They take in food, exchange gases, and excrete waste through water flow.
- Water enters through pores and exits through an opening called osculum.
- The inner cavity, spongocoel, is lined with collar cells or choanocytes.
- They have a skeleton made of spongin fibers or spicules.
- Sponges can reproduce both asexually and sexually, and they’re hermaphrodites.
Phylum Coelenterata (Cnidaria)
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| (Source: Goggle) |
This group includes jellyfish, corals, and sea anemones. Cnidarians have specialized cells called cnidoblasts for capturing prey. They exhibit a tissue level of organization and radial symmetry, and they may alternate between two body forms: sessile polyps and free-swimming medusae.
- Aquatic animals, either stationary or free-swimming, with tissue-level organization and radial symmetry.
- They have a central digestive cavity with a single opening surrounded by sensory tentacles.
- Special cells called cnidoblasts on tentacles contain stinging structures called nematocysts.
- Digestion is both inside and outside the body.
- Some have a hard skeleton made of calcium carbonate.
- They exist in two forms: polyps (sessile) and medusae (free-swimming).
- Some undergo alternation of generations, producing offspring asexually and sexually.
Phylum Platyhelminthes (Comb jellies)
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| (Source: Goggle) |
Also known as flatworms, Platyhelminthes have a flattened body and exhibit bilateral symmetry. They are acoelomates, meaning they lack a true body cavity. Many are parasitic, including the tapeworm and liver fluke, which live in the intestines of other animals.
- Mostly parasitic worms with flattened bodies, bilateral symmetry, and no body cavity.
- Parasites have hooks and suckers for attachment.
- They have flame cells for osmoregulation and excretion.
- Flatworms are hermaphrodites and can regenerate.
- Reproduction is both internal and through larval stages.
Phylum Annelida
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| (Source: Goggle) |
Annelids, or segmented worms, are characterized by a body divided into segments. They have a coelom and exhibit organ-system-level organization. Earthworms and leeches are common examples.
- These creatures have bilateral symmetry, a true coelom, and segmented bodies.
- Longitudinal and circular muscles aid in movement.
- They have closed circulatory systems and nephridia for waste removal.
- Reproduction is usually sexual, with some species being hermaphrodites.
- Examples are Pheretima (earthworm), Nereis, and Hirudinaria (bloodsucking leech).
Phylum Arthropoda
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| (Source: Goggle) |
This is the largest animal phylum, including insects, spiders, and crustaceans. Arthropods are characterized by an exoskeleton made of chitin, jointed appendages, and a segmented body. They have a wide variety of respiratory systems, including gills, tracheae, and book lungs, depending on the species.
- This is the largest phylum, containing insects.
- They have bilateral symmetry, a true coelom, and jointed legs.
- Their bodies are covered by a hard exoskeleton made of chitin.
- Respiration occurs through various methods like trachea, gills, or book lungs.
- They have open circulatory systems and compound or simple eyes.
- Examples include economically important species like Bombyx (silkworm) and disease vectors like mosquitoes.
Phylum Mollusca
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| (Source: Goggle) |
Mollusks include snails, clams, and octopuses. They have a soft body, often protected by a hard shell. Mollusks have a coelom and exhibit an organ-system-level of organization, with specialized organs like the radula for feeding and gills for respiration.
- These animals have bilateral symmetry, a true coelom, and a shell.
- They have distinct body parts like a head, foot, and visceral hump.
- Respiration and waste removal happen through feather-like gills.
- They have a rasping organ called a radula for feeding.
- Reproduction is usually sexual, with indirect development.
- Examples include Pila (apple snail), Octopus, and Pearl oyster.
Phylum Echinodermata
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| (Source: Goggle) |
Echinoderms, such as sea stars and sea urchins, have a unique water vascular system that aids in locomotion and feeding. They exhibit radial symmetry as adults but have bilateral symmetry in their larval stages.
- Adults show radial symmetry, while larvae have bilateral symmetry.
- They have a true coelom and an endoskeleton made of calcareous ossicles.
- Echinoderms have a unique water vascular system for various functions.
- Reproduction is usually sexual with indirect development.
- Examples include Asterias (starfish), Ophiura (brittle star), and Echinus (sea urchin).
Phylum Chordata
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| (Source: Goggle) |
The phylum Chordata includes all vertebrates (animals with backbones) and some invertebrates. The key characteristic of chordates is the presence of a notochord, a dorsal hollow nerve cord, and pharyngeal gill slits at some stage of development. Vertebrates are further divided into classes such as fishes, amphibians, reptiles, birds, and mammals.
- Chordates are characterized by a hollow nerve cord, notochord, and gill slits.
- They have bilateral symmetry, a true coelom, and a closed circulatory system.
- Chordata is divided into three subphyla: Urochordata, Cephalochordata, and Vertebrata.
- Vertebrata is further divided into Agnatha (jawless) and Gnathostomata (jawed), which includes Pisces and Tetrapoda.
- Examples include various fish species, amphibians, reptiles, birds, and mammals.
Classes
Class I – Cyclostomata (Circular Mouthed Fishes)
- These fishes have a circular and sucking mouth without jaws.
- They are ectoparasites on other fishes.
- Cyclostomes have 6-15 pairs of gill slits, with no scales or fins.
- They have a cartilaginous vertebral column and closed circulation.
- Examples include Petromyzon (Lamprey) and Myxine (Hagfish).
Class II – Chondrichthyes (Cartilaginous fishes)
- These fishes have a cartilaginous endoskeleton and their mouth is on the ventral side.
- They lack operculum covering the gills.
- Chondrichthyes have placoid scales on their skin.
- They maintain buoyancy by constant swimming as they lack air bladders.
- Examples include Scoliodon (Dogfish), Trygon (Stingray), and Carcharodon (Great white shark).
Class III – Osteichthyes (Bony fishes)
- These fishes have a streamlined body with a bony endoskeleton.
- They possess cycloid scales on their skin.
- Osteichthyes have a two-chambered heart and an air bladder for buoyancy.
- Examples include marine species like Hippocampus (Sea horse) and freshwater species like Labeo (Rohu).
Following these classes, we have the Super Class Tetrapoda, which includes four classes:
Class IV – Amphibia
- Amphibians can live both in water and on land.
- They typically have moist skin and lay eggs in water.
- Examples include frogs, salamanders, and limbless salamanders like Ichtthyophis.
Class V – Reptilia
- Reptiles are mostly terrestrial and have dry, cornified skin with scales or scutes.
- They lay eggs on land and have internal fertilization.
- Examples include crocodiles, alligators, chameleons, and snakes like Naja and Viper.
Class VI – Aves
- Birds have feathers for flying and their forelimbs are modified into wings.
- They have dry skin with oil glands and pneumatic bones for flight.
- Birds lay eggs with hard shells and exhibit internal fertilization.
- Examples include crows, pigeons, parrots, and flightless birds like ostriches.
Class VII – Mammalia
- Mammals are mostly terrestrial but some can fly or live in water.
- They have skin covered in hair or fur and mammary glands to feed their young.
- Mammals have a four-chambered heart and are warm-blooded.
- They give birth to live young and exhibit internal fertilization.
- Examples include kangaroos, cats, lions, and marine mammals like dolphins.
The classification of animals is based on fundamental characteristics that reveal their evolutionary relationships and biological functions. Understanding the diversity of the Animal Kingdom helps scientists to categorize and study the vast array of species, each with unique adaptations that allow them to survive and thrive in different environments. From the simple sponges to the complex chordates, the Animal Kingdom reflects the remarkable evolutionary journey of life on Earth.