Here, then, is the beginning of a nervous system, one cell placed on the surface so modi fied as to be fitted to receive and transmit an external impression to another cell, deeply placed, modified so as to contract on the receipt of the stimulus, a process of the surface-cell being the channel of communication.
In the medosie the anatomical structure is more elaborate. Here there are special cells on the surface, fitted to receive external im pressions, and cells placed deeply, modified into fibres, fitted to contract when stimulated, as in the polyp, but the connection between the two is less direct. Delicate filaments connect the outer cell with the deeper fibre. But in the course of the filaments is a mass of nucleated protoplasm, a nerve-cell. So that the external impression is made on a surface-cell, transmitted • from it to a nerve-cell by a fine filament, and then by means of another filament the impulse is passed on to the muscle-fibre, resulting in a contraction. Here the nerve-cell is the inter ' mediary between the outer perceiving cell and the deep coetract ing fibre, and there can be no doubt that the intermediary nerve-cell plays a more important part than merely transmitting the impulse inwards. The filaments connect ing the nerve-cell, on the one side with the surface, on the other side with the deep parts, we may now call nerve-filaments. Moreover, the nerve-filaments are so numerous as to form I a marked ring of fibres running in the margin of the bell, and the cells lie among them, and there cannot be a doubt that the arrangement is not simply that of one cell on the surface being connected through the medium of nerve filaments and cell with one muscular fibre deeply placed, but a more complex one by which intercommunicating nerve - filaments connect nerve-cells with one another. As a result of this intercommunication it is possible to have not only definite limited movements of simple muscle-fibres, but complex, co-ordinated movements, in which numerous fibres take their appropriate part. For, corresponding to the increased complexity of the anatomical structure, there is increased complexity of function, the medusa responding to an external stimulus, not merely by a single simple move ment, but by complex movements, for example, of locomotion, or by discharge of stinging cells for defence, or by modification of the three, or by some movement indicating not merely the fact that a stimulus has been perceived, but even that the position of the stimulated part of the surface has been appreciated.
Now, when in the light of such facts an animal higher in the scale is examined, the character of the nervous system is more easily understood. The nerve-cells are found col lected into more well-defined groups and the nerve-filaments into more well-defined strands. The groups of cells with the fibres connecting them together are called ganglia. The strands of nerve-fibres obviously connect the ganglia with well-defined areas of the body, certain nerve-fibres connecting the ganglion with the surface of the body and others with the deeper parts. The essence of the functions they per
form is the same. Some stimulus, applied to the surface, will cause an impression to be con veyed to some cells of the ganglion, and then from these cells an impulse will pass by other fibres to deeper parts, leading to movement or some other action. Moreover, one ganglion is visibly connected with another, so that the impression received by the cells of one gan glion, which takes note of stimuli affecting one part of the body and regulates the actions of that part, may be communicated to the gan glion of another part of the body, leading to co-ordinated action of both parts. In the crab there are two main ganglia (Fig. 89), connected together, with numerous nerve bundles radiating from them.
In animals still higher in the scale of organization the increase in the de velopment of parts and in the com plexity of functions necessitates more numerous ganglia and more numerous fibres, connecting them with the region of the body over which each ganglion presides, and with one another. Thus in the ant (Fig. 90) there is a regular chain of ganglia and intercommunicating fibres.
Movement, in response to an external stimulus, bus/ been taken as the illustration of nervous action, because it is the most obvious one. So increased variety of possible movements, the appearance of rhythmical movements, and the adjustment of limited movements for parti cular purposes, have been referred to as illus trations of increased complexity of function necessitating increased development of nerve fibres and cells. But lest the exclusive use of this illustration should mislead the reader, it is necessary to say, what is of course obvious, that the gradual increase in the development of parts and complexity of functions, observed as one ascends in the scale of animal life, may be illustrated in a multitude of other ways, for instance in the appearance and development of digestive organs, of a heart and blood-circn biting apparatus, of secreting glands, of organs for the removal of waste substances from the body, such as lungs and kidneys. The func tions which all these parts perform are main tained and regulated by nervous action. J ust as the action of a muscular fibre is performed in response to a stimulus from a nerve-cell, and just as that nerve-cell discharges its stimulus down a nerve to the muscle in spouse to some impression received from with out, so all the organs referred to are caused to perform their functions by impressions from nerve-cells conveyed to them along nerve fibres. So also the nerve-cells do not act auto matically or spontaneously in controlling the action of the various organs, but are roused to action by impressions reaching them from with out. The increase in nerve-ganglia, therefore, and in the nerve-fibres connecting them with different parts of the body and with one another, which is observed as one passes from lower to higher animals, is necessitated by development in a great variety of ways.