Text No. 1 - Vertebrates and Invertebrates
Animals can be classified into two main groups: vertebrates and invertebrates. The main
difference between vertebrates and invertebrates is that invertebrates, like insects and
flatworms, do not have a backbone or a spinal column. Examples of vertebrates include
humans, birds, and snakes.
Differences in Physical Characteristics
Invertebrates have no backbone, while vertebrates have a well-developed internal skeleton
of cartilage and bone and a highly developed brain that is enclosed by a skull. A nerve cord
is enclosed by vertebrae — individual bones that make up a vertebrate's spine. Vertebrates
have well-developed sensory organs, a respiratory system with either gills or lungs, and
a bilateral symmetry with an advanced nervous system that further distinguishes them from
invertebrates.
Differences in Habitat
Both types of animals live in a variety of habitats, but vertebrates can essentially suit
themselves in all habitats easily. The highly developed nervous system and internal
skeletons of vertebrates allow them to adapt to land, sea, and air.
Nevertheless, invertebrates are also found in a vast range of habitats, from forests and
deserts, to caves and seabed mud.
Population of Vertebrates vs. Invertebrates
To date, nearly 2 million species of invertebrates have been identified. These 2 million
species make up about 98% of the total animals identified in the entire animal kingdom, i.e.,
98 out of 100 types of animals in the world today are invertebrates. On the other hand,
vertebrates only form 2% of the animal species. Human beings are vertebrates.
Adaptation to the Environment
In contrast to invertebrates, vertebrates have a highly developed nervous system. With the
help of their specialized nerve-fiber system, they can react very quickly to changes in their
surroundings, giving them a competitive edge. Compared to vertebrates (animals with
backbones), most invertebrates have simple nervous systems, and they behave almost
entirely by instinct. This system works well most of the time, even though these animals are
often incapable of learning from their mistakes. Moths, for example, repeatedly flutter
around bright lights, even at the risk of getting burned. Notable exceptions are octopuses
and their close relatives, which are thought to be among the most intelligent animals in the
invertebrate world.
Similarities Between Vertebrates and Invertebrates
The feature uniting all chordates (all vertebrates and some invertebrates) is that at some
stage in their lives, all have a flexible supporting rod, a notochord, that runs through the
length of their bodies. In a majority of chordates, the notochord is replaced by a series of
interlocking bones called vertebrae which appear during early development. The presence of
these bones is what determines whether an animal is a vertebrate (has vertebrae) or
invertebrate (does not have vertebrae).