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Embryonic Development & Life Cycles of Invertebrates & Vertebrates

Instructor: Jayne Yenko

Jayne has taught health/nutrition and education at the college level and has a master's degree in education.

In this lesson we will compare and contrast the embryonic development and life cycles of vertebrates and invertebrates. You'll learn how they are similar and how they are different, and view examples.

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.

Life Cycle of the Mosquito
Mosquito Life Cycle

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.

Laying eggs in water is common to some vertebrates and many invertebrates, whether their natural habitat is fresh or marine water. The species that lay eggs in water lay huge numbers, partly because fertilization often occurs in the water itself, but also because predation reduces the odds of many eggs of hatching. It is more advantageous for these eggs to be jelly-like, with the thin, translucent covering that is permeable to water and gas exchange.

Although vertebrates may give birth to live young, these babies, especially those born to mammals and birds, may need additional time for further development before they can take care of themselves. Some reptiles and fish give birth to live young, which are simply small versions of their parents. They are independent at birth and no parenting is involved.

Once born, vertebrates may be quite different from each other, particularly as adults. For example, arm buds in different species look pretty much the same, but may develop into wings, arms, or flippers. Gill slits in mammals will become the ears and pharynx.

Embryonic Development in Invertebrates

Embryos of invertebrates develop much the same as vertebrates, although they often are born into a larva stage, which is very different from the adult of the same species. The larva need to undergo metamorphosis at least once in order to develop further into adulthood. While larva often look quite similar between species, the changes as they develop into adults can be quite dramatic.

Spiders hatch from eggs as small versions of their parents, ready to take care of themselves, similar to reptiles and fish. In another example, butterflies lay eggs that hatch into caterpillars. Caterpillars spend most of their time eating and growing larger. They look nothing like their parents at this stage, prior to metamorphosis. This is similar to the vertebrate amphibians, like frogs that hatch as tadpoles, for example.

Vertebrate Life Cycle

Many vertebrates have three stages in their life cycle, but some, like amphibians, undergo a metamorphosis stage between the juvenile and adult stages.

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