Nervous System
Development and functions

Ganglionated Type Of Nervous System

December 17th, 2009

The nervous system of the ganglionated type proved to be very convenient. In Annelida, which must have descended from the flatworms, all the nerve cells are concentrated in the ganglia, while the nerve strands connecting them hold only the long processes of these cells. Practically every segment of the worm has a pair of ganglia connected to each other. Besides, each ganglion is linked through the nerve strands with the corresponding ganglia of the preceding and following segments. This nervous system bears a close resemblance to a ladder. The anterior pairs of ganglia are the largest. They carry out the most important functions and have command over the rest of the nervous system.

In higher worms the ganglia come closer together, making up a single, compact formation. Their nervous system has some features characteristic of that of contemporary vertebrates.

We do not know what the brain of the first vertebrates was like. The lancelet, one of the most primitive represen­tatives of the chordates, has only a nerve cord, but as yet no cerebrum. This part of the brain first appears in the cyclostomes (lampreys and hagfish) and in fishes.

In these primitive animals the brain is divided up into the same sections as the brain in human beings. These sections are the same, but their structures and, what is more important, their functions essentially difler. The forebram is the main organ controlling the mental processes in a human being. All it does in lampreys and fishes is to analyze olfactory stimulations. In amphibians the functions of the forebrain are somewhat more complicated.

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December 17th, 2009 13:35:34