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

Nerve Cells And Nerve Strands

December 15th, 2009

The nerve cells in primitive Coelenterata are joined together by their processes to form a nerve network, the most primitive type of nervous system. The next improvement was the emergence of separate clusters of nerve cells, with their subsequent development into more organized and more compact nerve strands. These came into existence wherever the co-ordinated action of many contractile elements was required. Such clusters form the nerve rings encircling the umbrella of a jelly-fish, and cause the whole umbrella to tighten up or come loose, thus enabling the creature to move actively in the water.

In flatworms, the descendants of the Coelenterata, ail the nerve cells are concentrated in the form of strands arranged like braiding around the body in intricate patterns. Numerous constrictions between the strands, as well as the sites where the nerves come into direct contact, ensure the co-ordinated functioning of the entire nervous system. A diffuse network of nerve strands was undoubtedly an improvement compared with the network of randomly scat­tered nerve cells. However, this barrel-like nervous system proved too cumbersome and intricate to control the functions of the animal’s separate parts and organs, and a new organ was required to direct its operation.

Such a central organ first appeared in the higher represen­tatives of flatworms. It consists essentially of nerve strands with numerous nerve cells, aggregated into masses which are known as ganglia. These ganglia not only assumed the most difficult functions, but also influenced the work of other parts of the nervous system. Ganglia are primarily to be found near the sense organs, the eyes, the organ of equilibrium, and also near the gullet with which the flatworms catch their prey, hold it and push it into the intestine.

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December 15th, 2009 13:30:36