Invertebrates: Overview

Big Picture

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.

Key Terms

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.

Characteristics of Invertebrates

Invertebrates do not have bones-although some have a skeleton, it is not made up of bone.
They can have either:

  • Incomplete digestive system, so the one opening in the digestive cavity is both a mouth and an anus
  • Complete digestive system, so one opening of the digestive tract is the mouth and the other opening is an anus
  • Food would move through the body in just one direction. This made digestion more efficient. An animal could keep eating while digesting food and getting rid of waste.

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.

  • The embryo may undergo larval stages (stages of immaturity) before becoming mature
  • The larva can differ in characteristics from their adult version - for example, the larva may be able to swim freely but the adult may not.

Some reproduce asexually by fission or budding.

  • Fission is when one animal divides into two parts, and each part regrows what’s missing
  • Budding is when a small bump forms on the parent and remains there while it develops
Image Credit: CK-12 Foundation, CC-BY-NC-SA 3.0


Invertebrates: Overview cont.

Invertebrate Evolution

  • The first invertebrates, which likely resembled modern day sponges, had multiple cells with specialized jobs.
  • Tissues evolved next. Early invertebrates had two types of cell layers that allowed tissues to form: ectoderm (outer) and endoderm (inner).
Image Credit: The cat,
Figure: The components of the ectoderm.
Image Credit: J. Steinbock,
Public Domain
Figure: The components of the endoderm.
  • Radial symmetry was the first type of symmetry to evolve in invertebrates. Animals with radial symmetry have no sense of directions such as forward, backward, left, or right. Controlled movement in these directions is impossible.
Image Credit:
Coral, CK-12 Foundation, CC-BY-NC-SA 3.0. Beetle copyright Yuran, 2010, and sponge copyright Martina Orlich, 2010, used under licenses from
  • Cephalization, the first step toward the evolution of a brain, occurred next. Having a head region where nerve tissues are concentrated allows central control of the entire organism. These organisms were likely ancestors of flatworms.
  • Bilateral symmetry is an outcome of cephalization. It gives organisms a sense of direction and controlled movements to go in a direction.
  • Ancestors of flatworms evolved the mesoderm, which allowed animals to develop new types of tissues, such as muscle.
  • Ancestors of roundworms were the first to evolve complete digestive systems. Specialization in different digestive functions led to the evolution of digestive organs.
  • Ancestors of roundworms also evolved pseudocoelom, which was filled with fluid that allows room for internal organs to develop. This fluid cushions the internal organs and provides stiffness with its pressure.
  • The pressure from the fluid of the pseudocoelom forms a hydrostatic skeleton that produces internal support for the body. The hydrostatic skeleton explains why roundworms are round and why flatworms are flat.
  • The coelom evolved to have the fluid-filled body cavity completely enclosed by mesoderm. In pseudocoelom, only one side of the cavity is lined with mesoderm. The coelom lies between the digestive cavity and a body wall. Mollusks and annelids are invertebrates with a true coelom.
  • Segmentation evolved next and allowed flexibility and a wider range of motion. All annelids and anthropods are segmented. Anthropods also evolved jointed appendages, such as jointed legs for walking and antennae for sensing.


Invertebrates: Overview cont.

Image Credit:
Earthworm copyright Pakhnyushcha, 2010, and ant copyright Fong Kam Yee, 2010, used under licenses from
    • Some invertebrated evolved a notochord. They were probably similar to modern chordates. Some chordates evolved into vertebrates.

    Classification of Invertebrates

    Invertebrates can be divided into protostomes and deuterostomes depending on how they develop as embryos.

    • Protostomes: One of the two groups of categorization based on how invertebrates(and higher animals)develop as embryos. The coelom forms in the mesoderm in a protostome.
    • Deuterostomes: One of the two groups of categorization based on how invertebrates(and higher animals)develop as embryos. The coelom forms from a pouch of endoderm in a deuterostome.
    Image Credit: YassineMrabet, CC-BY-SA 3.

    Major Invertebrate Phyla

    Notable Characteristics
    multicellularity, specialized cells but no tissues, asymmetry, incomplete digestive system
    radial symmetry, true tissues, incomplete digestive system
    jellyfish, corals
    cephalization, bilateral symmetry, mesoderm, complete digestive system
    flatworms, tapeworms,
    pseudocoelom, complete digestive system
    true coelom, organ systems, some with primitive brain
    snails, clams, squids
    segmented body, primitive brain
    earthworms, leeches,
    marine worms
    segmented body, jointed appendages, exoskeleton, brain
    insects, spiders, crustaceans, centipedes
    complete digestive system, coelom, spiny, internal skeleton
    sea stars, sea urchins, sand dollars, sea cucumbers