A study of brains : an exercise in the understanding of evolutionary trends at work

Abstract

It is perhaps important to point out that not an animals have a ·'bruin;'. In fact, being unicellular or colonial conglomerations of cells,Protozoa have not yet acquired the complexities of differentiation found in higher animals. Although these simple animals do show sensitivity and respond to environmental stimuli they still lack. nerve cells. In the Coelenterates (jellyfish, hydras and sea-anemones) one .gets differentiation of the body into different cell types ... some of these being directly concerned with the transmission of nerve impulses. The multipolar neurons arid their axons ramify through the bodies of the these diploblastic animals and the systems are so diffuse that the setup is usually referred to as a "nerve-net". However :this tends to become aggregated to form specialized nerve tracts in animals which are higher up in the evolutionary tree.·Since . many .sense organs become concentrated·· in the head (cephalization) concommitantly. the nerve tracts in these regions become further elaborated and nerve masses O'r ganglia are formed. These infact become the rudiments from which higher and more complex brains arise.