FROM this stand-point the types of the nervous system may be arranged into the following ascending series:

             (I) The disseminated type.

          (II) The systemic, or segmental type, which may be subdivided, according to form as well as degree of co-ordination, into the following subdivisions:

            (a) The radial type.

            (b) The bilatero-ventral type.

            (c) The median-dorsal type.

            In the Actinia the neurons are in a disseminated state. The neurons, as yet but little differentiated from the other cells, are found strewn in a more or less disconnected state under the ectoderm or outer layer of the animal. In the Planaria a similar structure is found.

            In the Ascidian and others, the nervous system consists of a group of neurons, or of a single nerve-ganglion with protoplasmic processes radiating all over the body of the animal. Dendrons, axons, collaterals and the branches of terminal arborizations are distributed all over the organism, receiving impressions from the external and internal environments and reacting by muscular activity or glandular secretion. The same neuron receives its sensory impressions, and also sends out motor reactions in response to the stimulations,—that is, it is also motor—the neuron, therefore, is sensory motor in character.

            In the Medusae, such as the Acalephae, there is a series of groups of neurons or of ganglia, but they are not interconnected. The groups are not organically connected by radiating fibres, but their co-ordination is extremely simple, being rather synchronous in nature. (See Fig. 7.) Each ganglion, with its portion, acts by itself, and all work together, because they belong to the same general body system of the particular individual. The co-ordination is established through the general synchronous action of body cells, otherwise the cells could not possibly co-operate. Should the portion with its ganglion be cut off, it would still go on functioning independently. The ganglia are neither functionally nor organically directly connected, but have a co-ordination of a purely synchronic order, and are like separate individuals working in unison and performing their functions at the same given time. In other words, in this stage the co-ordination is due not to internal unifying organizations, but to the mere fact of the synchronous rhythm in the physiological functions.

FIG. 7.—Diagram of the bell of Aurelia Aurita with eight sense-organs. (After Claus.)

            In Hydromedusae the interconnection of the nerve-centres is more organic in character. The groups of neurons or nerve-centres are connected by their protoplasmic processes and are organized into a double ring system. The upper ring consists of a layer of more or less sparsely strewn ganglia connected by delicate protoplasmic processes. The lower ring consists of more ganglia than the upper one. These ganglia are interconnected by more or less thick nerve-fibres. Both rings are brought into relations by nerve-fibres, thus giving rise to a further co-ordination of the two co-ordinated ring systems of neurons. From the ganglia of the two rings, fibres radiate in all directions, supplying every portion of the animal with sensory and motor fibres. (Fig. 8.)

FIG. 8.—HYDROMEDUSA.

a, umbrella; b, manubrium; c, margin of the swimming bell with nerve-ring.

             Similarly in the Radiata, such as the Echinus, or the starfish, the nervous system consists of a series of ganglia co-ordinated into a central ring, localized round the mouth of the animal. This central ring of ganglia sends out nerve-fibres in different directions. Each arm is innervated by its closely situated ganglion or group of neurons. All the ganglia or groups are connected by radiating protoplasmie processes, thus establishing a co-ordination of all the groups of ganglia. (Fig. 9.)

FIG. 9.—NERVOUS SYSTEM OF THE STARFISH.

a, central nerve-ring that surrounds the mouth; b, peripheral nerves of the arms. (After J. Loeb.)

            In the Annelida, representing the bilatero-ventral type, we meet with a more complex structure of the nervous system. (Fig.10.) The ganglia instead of having a radial arrangement become organized in chains. The anterior or central ganglion is the most massive, and is probably probably brought about by a coalescence of many minor ganglia. The rest of the ganglia are much smaller and form a longitudinal series which have a segmentation corresponding to the metameres of the body. The cerebral ganglion gives origin to the nerves of the sense-organs, the other ganglia supply the other organs, and also give rise to a visceral nervous system. Of the sensory organs present, there may be found an auditory vesicle, tactile organs, and a pair of eye-spots.

FIG. 10.—A, THE LEECH (Hirudo medicin). B, THE NERVOUS SYSTEM OF THE LEECH. (After J. Steiner.)

            In the Annelida the segmental plan of structure stands out clear and distinct. Each ganglion supplies and innervates its own segment. The individual is formed by associations of segments, and the nervous system consists of an aggregation of ganglia, largely preserving their individual independence. The differentiation of structure and function is at its minimum, the segments are practically similar as to constitution, and form so many repetitions of one and the same fundamental ganglion. In the Radiata the plan of repetition is radial, in the Annelids the plan is longitudinal, and as such it is far more restricted as to co-ordination.

            In the lower animals, in the Medusae, in the Siphonophorae, the neurons and their ganglia are diffused without any plan and co-ordination; in the Radiata, such as the starfish, the neurons and their ganglia become organized on some general plan, but the plan is still diffused, the similar still undifferentiated ganglia become radially organized along many lines of repetitive co-ordination. The further advance of repetitive co-ordination is a restriction to two lines or even to one line, instead of many similar lines of development. In the lower forms of the Annelida, as well as the embryonic stages of the higher forms, two similar chains of ganglia run parallel to each other, and only in the adults of the more advanced forms do we find that the two parallel chains become integrated into one series. The course of evolution is to a greater integration of similarly functioning ganglia.

            In the Molluscoidea, which probably descend from a common form with the Annelids, the nervous system is extremely simple, consisting of one ganglion or group of neurons, sending off their protoplasmic processes to various portions of the organism. In the Bryozoa, usually united to form colonies, the nervous system consists of one oesophageal ganglion placed between the mouth and the arms. In the Laphopoda this ganglion is contained in the concavity of the laphophore, the disk bearing tentacles on which the mouth is placed, and supplies the tentacles and oesophagus with numerous nerves.

            When many individuals go to form a colony or a composite individual, a more or less co-ordinating colonial nervous system is found to take rise.

            If from the Annelida, we pass to the Arthropoda we find the same fundamental type preserved. There is one supraoesophageal ganglion that constitutes the brain and a suboesophageal ganglion and ganglionic chain, or ventral nerve cord. With the rise in the scale of evolution of the Arthropoda the cerebral ganglia become more massive and more complex and give rise to bundles of nerves innervating the different sense-organs which become finer and more complex in structure and function. The suboesophageal ganglion with the chain of ventral ganglia undergo a similar change: they grow and develop quantitatively as well as qualitatively; they become more massive, more complex, more integrated, and more differentiated. Thus in Limulus polyphemus, one of the oldest representatives of the Arthropoda, the nervous system consists of a dorsal or supraoesophageal ganglion, a commissure and suboesophageal ganglion, with a chain of six small abdominal or ventral ganglia.

FIG. 11.—NERVOUS SYSTEM OF LIMULUS POLYPHEMUS.
o, Subraoesophageal ganglion; c, commisure; u, suboesophageal ganglion.
A, b, c, d, abdominal ganglia. (After J. Loeb.)

            The nervous system of the Limulus is highly segmental in character. (Fig. 11.) Each peripheral organ belonging to a segment of the body has its corresponding segment in the nervous system. This is well brought out in one of the experiments made by Professor Loeb. The whole nervous system was removed in a Limulus with the exception of a small portion of the left side of the commissure and the six ventral ganglia. No connection was left between the two portions, still the Limulus went on living; it had to be artificially fed. These ventral ganglia are indispensable, since they innervate the gills requisite in the respiration of the animal. The portion of the commissure left was also necessary, because some of the leg movements were requisite in the reception of food. The animal in such a state received the food and nourished itself like a normal individual. So independent are the groups of neurons, that Patten has demonstrated that in a Limulus each leg receives food in a normal way, when nothing else is left in the process of operation except the portion that directly innervates the leg.

            In the crab the dorsal or brain ganglia are little developed; the ventral ganglia are, however, far more massive and better developed.

            In the house-fly the dorsal ganglion is comparatively massive, and so are also some of the ventral ganglia. (Fig. 12.)

            In the honey-bee the dorsal ganglion, or the brain, is very massive and highly developed, and so are also the ventral ganglia. (Fig. 13.) This is brought out clearly, when we compare the nervous system of the honey-bee with that of the house-fly.

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