Popular Science Monthly/Volume 9/October 1876/Nature of the Invertebrate Brain I
|NATURE OF THE INVERTEBRATE BRAIN.|
NOTHING distinctly answering to a brain is to be found in the lowest animals in which a nervous system exists. It is thus, for instance, with star-fishes and the larger nematoid entozoa, in which what most nearly resembles a brain consists of a mere band of nerve-fibres surrounding the commencement of the œsophagus, and containing a few nerve-cells, partly between its fibres and partly in groups slightly removed therefrom.
The absence of distinct ganglia in the neighborhood of the mouth in star-fishes is doubtless due, in the main, to the form of these animals, and their low type of organization. Each arm or ray presents its own nervous system, and the ring or band round the mouth seems to be little more than a commissure connecting these otherwise distinct parts of the common system.
In the larger parasitic nematoids the nervous system is more concentrated. The œsophageal ring and immediately adjacent parts constitute almost all that is known of the nervous system in these organisms, and it contains, or is in relation with, a larger number of ganglion-cells than the similar part in star-fishes. Thus, in addition to the cells intermixed with the fibres of the ring itself, there are five or six groups adjacent to and in connection with it, which receive fibres from certain large papillae surrounding the mouth and having a rudimentary tactile function. These papillae are, in all probability, the nematoids' principal sensory organs. By means of the connecting nerve-fibres and ganglion-cells they are brought into relation with the nervous ring, and from this other outgoing fibres are, doubtless, given off to the four great longitudinal muscular bands by which the movements of the animal are effected. The distribution of these latter or motor nerve-fibres, however, has not been distinctly traced. The absence of ganglionic swellings on, or in connection with, the œsophageal ring of nematoids is probably dependent upon the comparative simplicity and limited number of impressions capable of being received through these cephalic papillæ.
We turn now to the nervous system, and to those parts of it, more especially, which answer to the brain of higher animals as it occurs in the three sub-kingdoms of the Invertebrata, containing its higher types of life. These sub-kingdoms are Vermes, Arthropoda, and Mollusca.
Among representatives of the sub-kingdom Vermes, the nervous system varies a good deal in minor details, in accordance with the degree of organization, and with the diversity of the sensory and locomotor endowments of the several organisms. The broad features of the nervous system, however, are very similar in all.
The Nemertidæ, a class of marine worms, possess a nervous system of very simple type. They have soft and highly-contractile bodies, covered with cilia, but are otherwise wholly devoid of external appendages or traces of segmentation. On the anterior extremity of the body, a little posterior to the mouth, two, four, or more specks of pigment are met with, which are conjectured to serve the purpose of rudimentary ocelli, and while the animal is moving from place to place this anterior part of its body doubtless acts as its principal tactile surface. Nerve-fibres proceed from these regions, and converge so as to form three or four nerve-trunks on each side, which enter a comparatively large ganglionic mass lying on the lateral aspect of the sheath of the proboscis. Each ganglion is pyriform in shape, and connected with its fellow by means of two commissures, one of which passes over, and the other underneath, the proboscis. It is difficult to trace the ultimate distribution of the nerve-fibres in these creatures; so that, although fibres can be followed nearly up to the pigment-spots, none have been detected in immediate continuity with them.
The inferior commissure between the two ganglionic masses is shorter and broader than .the upper, and, while it serves in part to bring the two ganglia into communication, it is also partly composed of commissural fibres, uniting the two great lateral nerve-trunks. These start from the ganglia, and, proceeding along the sides of the body, give off numerous branches to the longitudinal and circular muscles between which they are situated.
The pyriform ganglia are mostly of a pink or reddish color, and they are crowded with small nerve-cells. They represent the brain as it exists in these animals, and we have here, perhaps, a type of the simplest form which this organ could assume among active creatures possessing a distinct bilateral symmetry. Tactile and possibly gustatory impressions, together with impressions produced by light or darkness, doubtless come from the anterior extremity of the organism to the pyriform ganglia on either side, and are thence reflected along correlated channels in the great efferent bundles, proceeding to the muscles on one or both sides of the body, and also to the muscular proboscis. Other departments of the nervous system may exist in these animals, though as yet none have been detected.
In the common earthworm the nervous system is somewhat differently developed. The lateral ganglia of the Nemertidæ are replaced by two upper ganglia, connected by lateral commissures with a single lower ganglion; and, as a consequence of the coalescence of the two lower halves, we have, instead of the two lateral cords of the Nemertidæ, a double ventral nervous cord traversing the whole length of the body. There are no distinct ocelli in the earthworm. The body is composed of a multitude of ring-like segments, each of which is provided with lateral setæ, which are called into play during the subterranean locomotions of the animal.
The double ventral cord has a fibrous structure along its upper surface, while below there is an irregular stratum of ganglion-cells. These cells are more abundant about the centre of each body-segment, and their aggregation gives rise to a series of rudimentary ganglia in these situations. From, every one of the ganglionic swellings two nerves are given off on each side, while a third pair of nerves issues from the cord itself just anterior to the swelling, and is distributed along the anterior boundaries of the segment.
The œsophageal ganglia in the earthworm are, proportionately to the rest of the nervous system, much smaller than in the Nemertidæ; and this is perhaps due in great part to the existence of the numerous segmental ganglia in the former, which have no existence in the marine worms. The movements of the Nemertidæ, like those of the nematoids, are probably much more exclusively under the control of the œsophageal ganglia than are those of the segmented earthworm—in which each of the body-ganglia doubtless has much to do with bringing: about the contraction of contiguous muscles. The earthworm has also a more complex visceral structure than is to be met with among the Nemertidæ; and, moreover, it presents more distinct evidences of a nervous interconnection between the different organs of the body and some of the principal nerve-centres. Lockhart Clarke has described a complicated ganglionic network on each side of the œsophagus, starting from the commissures and sending prolongations to the intestine and other parts. By means of this principal visceral system of nerves, the internal organs are brought into relation with one another, and with the nervous system of animal life—that is, with those parts having to do more especially with the relation of the organism to its medium.
The upper or supra-œsophageal ganglia, representing the brain of the earthworm, receive a nerve-trunk on each side, composed of fibres coming from the tactile upper lip, and, as no sensory filaments of a different order are known to be immediately connected therewith, the functions of the brain in this animal must be comparatively simple. This upper lip contains a certain amount of diffused pigment, though there are no signs of the existence of distinct ocelli. I have spoken of the part as a special organ of touch, but it is equally probable that it may be capable of receiving more special impressions representing rudimentary tastes. The separation between these modes of sensibility may in such low organisms be somewhat indefinite.
In the leech we meet with some variations in the arrangement of the nervous system, of a kind analogous to changes subsequently to be spoken of as occurring in higher forms of life. The nervous system becomes more concentrated. There is no longer a ganglion for each segment, but one for every three or four segments of the animal; and the two ventral cords approximate so closely as to be almost fused into one. In the common medicinal leech, for instance, there is a bilobed ganglion (a) above the mouth, which receives fibres from the tactile lips, and also ten distinct filaments from as many pigment-spots (b b) or ocelli, situated round the margin of this upper lip.
From this bilobed ganglion, which corresponds with the brain proper of higher animals, a cord descends on each side of the œsophagus, and the two unite in a heart-shaped supra-œsophageal ganglion (c), from which afferent nerves are given off to the muscles whose business it is to move its three saw-like jaws, as well as to the muscles of the oral sucker. This lower ganglion in part corresponds with the "medulla oblongata" of vertebrate animals. It is continuous with the double ventral cord, on which twenty equidistant rhomboidal ganglia are developed. Each of these ganglia gives off two nerves on either side, whose branches are distributed to the muscles and parietes of adjacent segments.
In this animal also a simple filament is given off from the posterior part of the supra-œsophageal ganglion, and is distributed along the dorsal aspect of the alimentary canal. It foreshadows an important system of nerves corresponding partly with that of the "sympathetic," and partly with the pneumogastric (or lung and stomach) nerves in higher animals. This system is known among invertebrates as the "stomato-gastric system." In other members of the invertebrate series it frequently takes its origin from the commissures connecting the upper and lower ganglia, rather than from the upper ganglion itself. The more complicated stomato-gastric system of the earthworm has an origin of this kind.
The kind of nervous system which pertains to the earthworm and to the leech exists, with only comparatively trivial variations, throughout the whole sub-kingdom Vermes.
The next sub-kingdom—the Arthropoda—comprises centipedes, crabs, spiders, and insects. They are all characterized by the possession of hollow and jointed organs of locomotion, containing distinct muscles, these appendages being represented among Vermes only by lateral setæ or bristles of different kinds. The lowest types of these various classes possess a nervous system closely analogous to that existing among the various kinds of worms. In the more complex types of crabs, spiders, and insects, however, we meet with a great increase in the complexity of animal organization, and this increase of complexity is shared in by the nervous system. Among insects, for instance, the respiratory organs assume a marvelous degree of elaboration, and the development of this system, together with a correlated development of their nervous and muscular systems, contributes greatly to the enormous powers of locomotion for which these denizens of the air are remarkable. The acuteness and structural elaboration of their sense-organs is almost sure to be greatly increased in such active creatures; and, looking to the nature of the intelligence in these lower animals, there is thus afforded an increasing stimulus to brain-development and slightly higher brain-functions.
Among the lower centipedes, such as Iulus and Geophilus, in which the limbs, though very numerous, are feeble and ill-developed, the nervous system exhibits only a slight advance over the forms which it presents among the higher Annelida (Fig. 3). But in the more powerful predatory forms, of which the common centipede may be taken as a type, a distinct advance is met with. This carnivorous animal has a smaller number of well-developed limbs, and its nervous system closely resembles that found among caterpillars or the larvæ of higher insects.
The supra-œsophageal ganglia receive nerves from the two pairs of antennæ, and from the groups of ocelli on each side of the head, and they are connected by œsophageal cords with a bilobed infra-œsophageal ganglion, which distributes nerves to the jaws and other parts about the mouth. This bilobed infra-œsophageal ganglion is the first and largest of a series of ventral ganglia, numbering twenty-two in all, which are connected together by a double ventral cord. Every ganglion sends off nerves on each side to a pair of limbs.
From the posterior part of the brain, or from the œsophageal cords, the stomato-gastric nerves are given off, and distribute themselves over the alimentary canal in the usual manner.
Organs of vision become much more elaborate in crabs, spiders, and insects, than among worms or centipedes. And, while organs of touch and taste are further perfected in these higher arthropods, two new sensory endowments also become manifest. . These organisms, or at least all the higher forms of them, are capable of being impressed by and of discriminating the different odors of some substances anterior to the contact of such substances with their gustatory surfaces. This new power aids them in their search for or recognition of food. Such organisms are, in addition, capable of appreciating those vibrations of the medium they inhabit, which induce in us impressions recognized as sounds or noises. In other words, they acquire a rudimentary power of hearing.
These additional sensory endowments are of high importance to all organisms, but more especially to those possessing active powers of locomotion—serving, as they do on the one hand, to help to bring their possessors into relation with food, and, on the other, to warn them of the approach of enemies, of friends, or of sexual mates.
Among Crustacea great differences are met with in the degree of concentration of the nervous system, the variations being in the main dependent upon differences of external form in the respective members of the class. In some of the lower terms of the series allied to wood-lice (such as talitrus and oniscus), in which the body is elongated and composed of many almost similar segments, the nervous system is not very different from that of the more highly-organized worms.
In slightly higher forms of Crustacea, however, the two divisions of the originally double ventral cord approximate and become fused together, while, at the same time, the equality of its ganglia diminishes. Thus, in such forms as the lobster and the crayfish, the ganglia of the thorax which supply nerves to the limbs are distinctly larger than those of the abdominal segments, though these are also of good size, since the tail-segments are actively called into play during locomotion.
In the prawn a further development and concentration of the nervous system is seen. The thoracic ganglia are fused into a single elliptical mass, though those of the abdominal segments still remain separate.
But in the ordinary edible crab and its allies (Fig. 4), a still more remarkable concentration of the nervous system is met with. All the thoracic and all the abdominal ganglia are here fused into one large perforated mass of nervous matter (c), situated near the middle of the ventral region of the body. From this large compound ganglionic mass nerves are given off to the limbs, to the abortive tail, and to other parts.
The brain of the crab (a) is represented by a rather small bilobed ganglion. It receives nerves from the pedunculated compound eyes, from the two pairs of antennæ, and from the palpi-bearing mandibles. The posterior antennæ (or antennules, as they are sometimes termed) contain in their basal joint a body which is supposed to represent an olfactory organ, though others have regarded it (on very insufficient grounds) as an organ of hearing. The rather small bilobed brain is, indeed, regarded by many naturalists as essentially composed of three pairs of ganglia, completely fused into one another, but in relation with the three pairs of sensory organs—the eyes, the tactile antennæ, and the supposed olfactory antennules. It is connected, by means of a long cord (b, b), on each side of the œsophagus, with the anterior extremity of the great ventral ganglion. These cords are long because of the absence of any separate sub-œsophageal ganglia, and because of the comparative distance of the great ventral nervous mass into the composition of which these ganglia enter. The great length of the œsophageal cords is one of the most notable characteristics of the nervous system of the higher Crustacea.
The "stomato-gastric" system of Crustacea is closely similar to that which exists in centipedes. One part of it is given off from the œsophageal cord on each side, while another median branch proceeds from the posterior part of the united cephalic ganglia, as in Iulus (Fig. 3,f).
Among Arachnida forms of the nervous system exist which agree in many respects with those belonging to members of the class last described—these resemblances being in the main associated with certain general similarities of external form or configuration of body. Thus in scorpions the arrangement of the nervous system is not very dissimilar from that belonging to the prawn and its allies, since the thoracic ganglia have coalesced with one another and with the anterior abdominal ganglia, so as to form a large stellate nervous mass, which supplies the limbs and the anterior part of the abdomen. The ventral cord throughout the remainder of the abdomen and its caudal prolongation is marked at intervals by a series of small ganglionic swellings.
In spiders proper the nervous system attains its maximum of concentration. In addition to the abdominal and thoracic ganglia having all fused into one another and with the sub-œsophageal ganglion, we
|Fig. 4.—Nervous System of a Crab(Pulinurus).||Fig. 5.—Head and Nervous System of a Spider (Mygale).|
find the large mass thus composed (Fig. 5, s) brought into extremely close relation with the cerebral ganglia or brain (c). They are connected by means of two stout commissures, one on each side of the very narrow œsophagus, whose small size is attributable to the suctorial habits of these carnivorous and predatory creatures. The captured fly is not eaten, its juices are sucked by the fierce spider by whom its life has been taken.
The bilobed brain of the spider receives nerves on each side (o), corresponding in number with the ocelli which the animal may possess. It also receives two large nerves (m) from the so-called mandibles, which are organs presumably developed from modified antennas. These large nerves probably contain outgoing as well as ingoing fibres.
The sub-œsophageal ganglia correspond in the main, as we have already stated, with the medulla oblongata of vertebrate animals, and their fusion with the thoracic ganglia in the Arachnida, as well as in the Crustacea and Myriapoda, confirms the view held by some anatomists, that the medulla should be regarded as a prolongation of the spinal cord, rather than as an integral part of the brain.
The nervous system of insects varies not only among different classes and orders, but even in the same individual, in different stages of its development. The larva, or caterpillar, of a butterfly, for instance, presents a nervous system not very different from that met with in the centipede; while in the imago stage, or perfected insect, the same system has undergone some remarkable changes, leading to increased size of the cerebral ganglia, and also to further development of some of the ganglia pertaining to the ventral cord, with concentration or even suppression of others.
In such insects as butterflies, bees, and dragon-flies, in which the visual organs are enormously developed, and in which the power of vigorous and sustained flight is correspondingly increased, the nervous system attains its maximum of development among the Arthropoda. The brain of these creatures differs from that existing in all other members of the class by. reason of the great development of those portions of it in relation with the visual organs. A ganglionic swelling is frequently found where the nerve joins the brain (Fig. 6, B,)and in some insects there are also small ganglionic swellings at the corresponding parts of the antennal nerves.
As in spiders, the œsophageal ring is very narrow, owing to the greatly-diminished size of the œsophagus in the imago forms of higher insects. The double upper or cerebral ganglion is, however, connected in all insects with a separate sub-œsophageal ganglion, from which nerves are given off to the mandibles, the maxilæ, and the labium, though in spiders, crustaceans, and myriapods, as I have before stated, this part has no existence separate from the thoracic ganglia.
In insects the three thoracic ganglia also often preserve a separate existence (Fig. 6), though in such higher types as I have named above
these ganglia are more frequently fused into a single, lobed mass. The eight abdominal ganglia, which are always much smaller than the thoracic, continue to have a separate existence among some of the less developed types of insects, though it is more frequent for some, or even all, of them to disappear.
The stomato-gastric system also attains considerable complexity in these animals. It is often connected anteriorly with a median frontal ganglion (Fig. 7, E), lying anterior to and below the brain, which supplies branches to the mouth and adjacent parts. This oral or frontal ganglion, besides being connected with the brain, also gives origin to a median recurrent nerve (e). This nerve is connected with other branches, proceeding from one or two pairs of lateral ganglia (c), near to, and taking origin from, the œsophageal cords. The system of nerves thus derived furnishes branches to the stomach, the intestines, and other viscera. In addition, we meet in insects with another well-developed set of visceral nerves, taking origin from a chain of minute ganglia, which lie upon and are connected with the large ventral ganglionated cord. These nerves are distributed to the extensive and greatly multiplied air-tubes, or respiratory organs. They are known to anatomists as "uervi transversi," and are much more developed in insects than are its representatives among any other class of arthropods.
- An artery passes through the perforation in this ganglion.