Vertebrates and Invertebrates
“A Backbone Makes All the Difference”
Animal classification is a matter of sorting out similarities and
differences, of placing animals in groups and then breaking those
groups apart into subgroups. The whole endeavor creates a
structure—a hierarchy in which the large high-level groups sort out
bold and obvious differences, while the low-level groups tease apart subtle, almost imperceptible,
variations. This sorting process enables scientists to describe evolutionary relationships, identify shared
traits, and highlight unique characteristics down through the various levels of animal groups and
subgroups.
Among the most basic criteria by which animals are sorted is whether or not they possess a backbone.
This single trait places an animal into one of just two groups: the vertebrates or the invertebrates and
represents a fundamental division among all animals alive today as well as those that have long ago
disappeared. If we are to know anything about an animal, we should first aim to determine whether it
is an invertebrate or a vertebrate. We'll then be on our way to understanding its place within the
animal world.
What are Invertebrates?
Invertebrates are a broad collection of animal groups (they do not belong to a single subphylum like
the vertebrates) all of which lack a backbone. Some (not all) of the animal groups that are
invertebrates include:
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Sponges (Phylum Porifera)
Jellyfish, hydras, sea anemones, corals (Phylum Cnidaria)
Flatworms (Phylum Platyhelminthes)
Snails, Octopus (Phylum Mollusca)
Insects, Spiders (Phylum Arthropoda)
Earthworms (Phylum Annelida)
Echinoderms (Phylum Echinodermata)
What are Vertebrates?
Vertebrates (Subphylum Vertebrata) are animals that possess an internal skeleton (endoskeleton) that
includes a backbone made up of a column of vertebrae (Keeton, 1986). The Subphylum Vertebrata is a
group within the Phylum Chordata (commonly called the 'chordates') and as such inherits the
characteristics of all chordates:
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bilateral symmetry
body segmentation
endoskeleton (bony or cartilaginous)
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complete digestive system
ventral heart
closed blood system
tail (at some stage of development)
In addition to the traits listed above, vertebrates possess one additional trait that makes them unique
among chordates: the presence of a backbone.
Some animal classes that are vertebrates include:
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Jawless fish (Class Agnatha)
Cartilaginous fish (Class Chondrichthyes)
Bony fish (Class Osteichthyes)
Amphibians (Class Amphibia)
Reptiles (Class Reptilia)
Birds (Class Aves)
Mammals (Class Mammalia)
Vertebrates are the most advanced organisms on Earth. The traits that make all of the animals in this
section special are their spinal cords, vertebrae, and notochords. It's all about having a series of nerves
along your back (dorsal side). If you are an organism, you can't just have the nerves sitting there. You
need to give those nerves support and protection. That need brings us to the backbones and a rod of
cartilage called the notochord.
Not so many species… Fifty thousand species might seem like a lot. Compared to the invertebrates,
there are not that many species of vertebrates. You might be asking why. One reason is that
vertebrates are usually larger than invertebrates. They need more space. Another reason is that, even
though they are more advanced, there are many limitations on the environments that are available to
them.
Think about it this way. If you are smart mammal, would you rather live near the ocean or in the frozen
tundra of the arctic? Many land animals can make that decision and move to more desirable areas for
living. Those nicer areas can only support so many species of animals.
They have got the brains… Vertebrates are smart. Some of them are very smart. We mean you. Most
vertebrates have very advanced nervous systems. While a goldfish might not compare to your
intelligence, when you compare a goldfish to a sea anemone, a goldfish is like Einstein. Octopi are
probably the smartest invertebrates and may equal or be smarter than some vertebrates. Octopi are
the exception in the invertebrate category.