Chordata - Kingdom: Animalia - SS1 Biology Lesson Note
The subphylum of Chordata, Vertebrata, which comprises most chordates, is further divided into several classes based on specific characteristics and evolutionary relationships.
The major divisions or classes of Vertebrata are as follows:
1. Pisces (fishes): The term "Pisces" encompasses all fish species, including both jawless fishes (agnathans) and jawed fishes (gnathostomes). Jawless fishes, such as lampreys and hagfishes, are primitive fish that lack jaws and paired fins. They have a unique circular mouth with tooth-like structures and a cartilaginous skeleton. These fishes are mostly found in freshwater and marine environments and have a unique life cycle that includes a parasitic feeding stage. Jawed fishes, or gnathostomes, comprise most fish species. They are further divided into two major groups: cartilaginous fishes (Chondrichthyes) and bony fishes (Osteichthyes). Cartilaginous fishes, including sharks, rays, and skates, have a skeleton made of cartilage, tough skin covered in tooth-like scales called dermal denticles, and well-developed jaws.
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2. Amphibia: Amphibians are a class of vertebrates that have a dual life cycle, spending their early life stages in water (aquatic larvae) and transitioning to a land-based existence as adults. Examples of amphibians include frogs, toads, salamanders, and newts. They have moist, permeable skin, typically lay eggs in water, and undergo metamorphosis during their development.
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3. Reptilia: Reptiles are a class of vertebrates characterized by their scaly skin, which helps prevent water loss. They lay amniotic eggs, which contain protective membranes to retain moisture. Reptiles include lizards, snakes, turtles, crocodilians, and tuataras. They are primarily terrestrial, although some species inhabit aquatic environments.
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4. Aves (Birds): Birds are warm-blooded vertebrates known for their feathers, beaks, and ability to fly. They have adaptations for flight, including lightweight skeletons, powerful wings, and a highly efficient respiratory system. Birds also lay hard-shelled eggs and exhibit various behaviors and adaptations for diverse ecological niches.
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5. Mammalia (Mammals): Mammals are a class of vertebrates characterized by mammary glands that produce milk for nourishing their young. They have hair or fur on their bodies and possess specialized teeth. Mammals exhibit a wide range of adaptations and behaviors and are further divided into several subclasses, including monotremes (egg-laying mammals), marsupials (pouched mammals), and placental mammals (mammals with a placenta).
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Vertebrata can be further subdivided into two groups: poikilothermic (cold-blooded) and homoiothermic (warm-blooded) animals. Poikilothermic animals, also known as ectothermic animals, are organisms whose body temperature fluctuates with the temperature of their environment. Unlike endothermic (homoiothermic) animals (such as mammals and birds) that maintain a relatively constant internal body temperature, poikilothermic animals rely on external heat sources to regulate their body temperature.
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