The majority of animals today are invertebrates. The defining trait of invertebrates is the lack of bones. Other characteristics of invertebrates range from having a complete digestive system or an incomplete digestive system to having special appendages for movement. Evolution of invertebrates began with multicellularity and then on to development of tissues, such as the ectoderm and endoderm. Bilateral symmetry, cephalization, the mesoderm, the pseudocoelom, and other traits evolved later in invertebrates.
Invertebrates: Animals that lack a vertebral column.
Incomplete Digestive System: A system that consists of a digestive cavity with one opening.
Complete Digestive System: A system that consists of a digestive tract with two openings.
Larva: A juvenile, or immature, stage of an animal.
Fission: An asexual reproduction process where an animal simply divides into two parts. Each part then regrows the missing part, and the result is two whole organisms.
Ectoderm: Outer embryonic cell layer that invertebrates use to develop tissues from.
Endoderm: Inner embryonic cell layer that invertebrates use to develop tissues from.
Radial Symmetry: When an organism has a distinct top and bottom but no distinct head and tail ends, the body can be divided into two halves like a pie.
Cephalization: The concentration of nerve tissues at one end of the body, forming a head region.
Bilateral Symmetry: When an organism has distinct
head and tail ends, the body can be divided into two identical right and left halves.
Mesoderm: The third layer of cells between the ectoderm and the endoderm.
Pseudocoelom: A partial body cavity that is filled with fluid.
Hydrostatic Skeleton: The internal support provided by the pressure of the fluid from the coelom.
Coelom: A fluid-filled body cavity completely enclosed by mesoderm.
Segmentation: The division of the body into multiple parts, which allows flexibility and a wider range of motion.
Notochord: A rigid rod that runs down the entire body.
Invertebrates do not have bones-although some have a skeleton, it is not made up of bone.
They can have either:
All invertebrates can move on their own during some part of the life cycle.
Most have a nervous system that can respond to the environment.
Most reproduce sexually.
Some reproduce asexually by fission or budding.
Invertebrates can be divided into protostomes and deuterostomes depending on how they develop as embryos.