Invertebrates and Vertebrates
More than 95% of all animals are invertebrates, those who do not have a backbone. Insects, worms, mollusks, crustaceans, jellyfish, octopus, squid, sea urchins, and arachnids are all invertebrates.The rest of all other animals are vertebrates, those with a backbone. Sharks and rays, bony fish, amphibians, reptiles, mammals, and birds are all vertebrates. Embryonic development refers to what happens in the stage before an animal is born, regardless of whether that animal is an invertebrate or vertebrate. Life cycle refers to the changes that occur over the lifespan of the animal.
Invertebrates vary tremendously in their body composition. It's common for invertebrate species to have four distinct stages to their life cycle: egg, larva, pupa, and adult, although this life cycle can vary. For example, octopus and squid hatch and grow into adults without a larva or pupa stage.
Vertebrates tend to have a similar, bilateral body composition, where one side is a mirror image of the other, because their bodies are built around their backbones. Vertebrates typically have three stages to their life cycle: egg, baby or juvenile, and adult, although a few, like some amphibians, undergo a metamorphosis stage prior to becoming an adult.
Embryonic Development in Vertebrates
Vertebrates, during the early stages of embryonic development, are all quite similar due to common ancestry. The genes that direct embryonic development are the same among vertebrates. Growth of the embryo eventually diverges into species-specific appearance and traits, generally during the late fetal, newborn or adult states, depending on the species.
Many vertebrates develop their fertilized eggs within the female's body, and giving birth to live young, like cattle, horse, and sheep. Others, like birds and some reptiles, lay eggs. These egg-layers produce eggs with hard shells (birds) or leathery shells (reptiles). These shell types allow these vertebrates to lay their eggs on land; embryonic development is completed within the egg. The shell prevents dehydration and provides protection for the developing embryo.