Cognitive ability is defined as a mechanism that animals acquire,
process, store, and act based on the information they have received from
the environment. The process includes perception, acquisition,
memorization, and decision making. In other words, the brain is the most
important center of cognitive processing, which has undergone many
changes in structure, size, neuronal pathways, effective hormones, etc.
In this paper, we reviewed data on the role and effect of these factors
on different groups of invertebrates and vertebrates. Also, it has been
reviewed how they are involved in cognitive processes. The brain size
and the density of the neurons vary noticeably across invertebrates and
vertebrates clades. It seems the evolution of brain size is firmly
correlated to the evolution of cognition. Generally, species with larger
brains and more neurons demonstrated more complex cognitive behaviors
except in certain taxa. Besides, hormonal-mediated events have a crucial
role in the central nervous system. It shows the changes in cognitive
function correlate with different kinds of hormones and chemicals. We
provided an insight into the interrelationship between different
telencephalon regions and cognitive functions. We compared how different
areas in the brain change the cognitive abilities in different clades of
animals, including invertebrates and vertebrates taxa. Also, we reviewed
invertebrates' nervous systems in a separate section because of the
substantial discrepancy.