the ganglia which peculiarly appertain to the suckers, and which are connected with them by distinct filaments; whilst in the other there is nothing but fibrous structure, forming a direct communication between these and the cephalic ganglia; so that each sucker has a separate relation with a ganglion of its own, whilst all are alike connected with the cephalic ganglia, and are placed under their control. We see the results of this arrangement, in the modes in which the contractile power of the suckers may be called into operation. When the animal embraces any substance with its arm (being directed to this action by its sight or other sensation) it can bring all the suckers simultaneously to bear upon it; evidently by a voluntary or instinctive impulse transmitted along the connecting cords that proceed from the cephalic ganglia to the ganglia of the suckers. On the other hand, any individual sucker may be made to contract and attach itself by placing a substance in contact with it alone; and this action will take place equally well when the arm is separated from the body, or even in a small piece of the arm when recently severed from the rest, thus proving that, when it is directly excited by an impression made upon itself, it is a reflex act, quite independent of the cephalic ganglia, not involving sensation, and taking place through the medium of its own ganglion alone.

855. In the Molluscous classes, generally speaking, the Nervous system bears but a small proportion to the whole mass of the body; and the part of it which ministers to the general movements of the fabric, is often small in proportion to those which serve some special purpose, such as the actions of respiration. This is what we should expect from the general inertness of their character, and from the small amount of muscular structure which they possess. On the other hand, in the Articulated classes, in which the locomotive apparatus is highly developed, and its actions of the most energetic kind, we find the Nervous system almost entirely subservient to this function. In its usual form, it consists of a chain of ganglia, connected by a double cord; commencing in the head, and passing backwards through the body (Plate II., Fig. 2). The ganglia, though they usually appear single, are really double; being composed of two equal halves, sometimes closely united on the median line, but occasionally remaining separate, like the cephalic ganglia of the Solen (Fig. 1, a, a), and being united together by a transverse commissural trunk. In like manner, the longitudinal cord, though really double (as seen in the upper part of Fig. 2), often appears to be single, in consequence of the close approximation of its lateral halves (as in the lower part of Fig. 2). In general we find a ganglion in each segment; giving off nerves to the muscles of the legs, as in Insects, Centipedes, &c.; or to the muscles that move the rings of the body, where no extremities are developed, as in the leech, worm, &c. In the lower Vermiform (or worm-like) tribes, especially in the marine species, the number of segments is frequently very great, amounting even to several hundreds; and the number of ganglia follows the same proportion. Whatever be their degree of multiplication, they seem but repetitions of one another; the functions of each segment being the same with those of the rest. The cephalic ganglia, however, are always larger and more important; they are connected with the organs of

special sense; and they evidently possess a power of directing and controlling the movements of the entire body; whilst the power of each ganglion of the trunk is confined to its own segment. The longitudinal ganglionic cord of Articulata occupies a position which seems at first sight altogether different from that of the nervous system of Vertebrated animals; being found in the neighborhood of the ventral or inferior surface of their bodies; instead of lying just beneath their dorsal or upper surface. There is reason, however, for regarding the whole of the body of these animals as having an inverted position; so that they may be considered as really crawling upon their backs. On this view, their longitudinal nervous tract corresponds with the spinal cord of Vertebrata in position, as we shall find that it does in function.

856. We shall draw our chief illustrations of the structure of the nervous system in the Articulated series, from the class of Insects; in which it has been particularly examined. In these animals the number of segments never exceeds twelve (exclusive of the head), either in their larva, pupa, or imago states; and the total number of pairs of ganglia, therefore, never exceeds thirteen, including the cephalic ganglia. These, in the larva, are nearly equal in size, one to another (Plate II., Fig. 2, a, and 1-12); the functions of the different segments of the body being almost uniform; and the development of the organs of special sense not being such as to involve any considerable predominance in the size of the cephalic ganglia. We observe, at the anterior extremity, the pair of cephalic ganglia (a); from which proceeds, on each side, a cord of communication to the first ganglion (1) of the trunk. This double cord, with the ganglia above and below, thus forms a ring which embraces the oesophagus; the cephalic ganglia being situated on the upper side of it, whilst the ganglionic column of the trunk lies beneath the alimentary canal along its whole length. In the Sphinx ligustri, or Privet Hawkmoth, the nervous system of whose larva is here represented, the last two segments of the body are drawn together as it were, into one; and instead of distinct 11th and 12th ganglia, we find but a single mass nearly double the size of the rest, and obviously formed of the elements that would have otherwise gone to form the two.

857. When the structure of the chain of ganglia is more particularly inquired into, it is found to consist of two distinct tracts; one of which is composed of nervous fibres only, and passes backwards from the cephalic ganglia, over the surface of all the ganglia of the trunk; whilst the other includes the ganglia themselves. Hence every part of the body has two sets of nervous connexions; a direct one with the ganglion of its own segment, and an indirect with the cephalic ganglia. Impressions made upon the afferent fibres, which proceed from any part of the body to the cephalic ganglia, become sensations when conveyed to the latter: whilst in respondence to these, the influence of sensations received by the cephalic ganglia, and operating through them, harmonizes and directs the general movements of the body, by means of the communicating cords proceeding from them. For the reflex operations on the other hand, the ganglia of the ventral cord are sufficient; each one ministering to the actions of its own segment, and, to a certain extent also, to those of other segments. It has been ascer

tained by the careful dissections of Mr. Newport, that of the fibres constituting the roots, by which the nerves are implanted in the ganglia, some pass into the vesicular matter of the ganglion, and, after coming into relation with its vesicular substance, pass out again on the same side (Fig. 152, f, k); whilst a second set, after traversing the vesicular

Portion of the ganglionic tract of Polydesmus maculatus:-b, inter-ganglionic cord; c, anterior nerves; d, posterior nerves; f, k, fibres of reflex action; g, h, commissural fibres; i, longitudinal fibres, softened and enlarged, as they pass through ganglionic matter.

matter, passes out by the trunks proceeding from the opposite side of the same ganglion; and a third set runs along the portion of the cord which connects the ganglia of different segments, and enters the nervous trunks that issue from them, at a distance of one or more ganglia above or below. Thus it appears, that an impression conveyed by an afferent fibre to any ganglion, may excite a motion in the muscles of the same side of its own segment; or in those of the opposite side; or in those of segments at a greater or less distance, according to the point at which the efferent fibres leave the cord.

858. The general conformation of Articulated animals, and the arrangement of the parts of their nervous system, render them peculiarly favorable subjects for the study of the reflex actions; some of the principal phenomena of which will now be described. If the head of a Centipede be cut off, whilst it is in motion, the body will continue to move onwards by the action of its legs; and the same will take place in the separate parts, if the body be divided into several distinct portions. After these actions have come to an end, they may be excited again, by irritating any part of the nervous centres, or the cut extremity of the nervous cord. The body is moved forwards by the regular and successive action of the legs, as in the natural state; but its movements are always forwards, never backwards, and are only directed to one side when the forward movement is checked by an interposed obstacle. Hence, although they might seem to indicate consciousness and a guiding will, they do not so in reality; for they are carried on, as it were, mechanically; and show no direction of object, no avoidance of danger. If the body be opposed in its progress by an obstacle of not more than half of its own height, it mounts over it, and moves directly onwards, as in its natural state; but if the obstacle be equal to its own height, its progress is arrested, and the cut extremity of the body re

mains forced up against the opposing substance, the legs still continuing to move. If, again, the nervous cord of a Centipede be divided in the middle of the trunk, so that the hinder legs are cut off from connexion with the cephalic ganglia, they will continue to move, but not in harmony with those of the fore part of the body; being completely paralysed, as far as the animal's controlling power is concerned; though still capable of performing reflex movements, by the influence of their own ganglia, which may thus continue to propel the body, in opposition to the determination of the animal itself.-The case is still more remarkable, when the nervous cord is not merely divided, but a portion of it is entirely removed from the middle of the trunk; for the anterior legs still remain obedient to the animal's control; the legs of the segments from which the nervous cord has been removed, are altogether motionless; whilst those of the posterior segments continue to act, through the reflex powers of their own ganglia, in a manner which shows that the animal has no power of checking or directing them.

859. The stimulus to the reflex movements of the legs, in the foregoing cases, appears to be given by the contact of the extremities with the solid surface on which they rest. In other cases, the appropriate impression can only be made by the contact of liquid; thus a Dytiscus (a kind of water-beetle) having had its cephalic ganglia removed, remained motionless, so long as it rested upon a dry surface; but when cast into water, it executed the usual swimming motions with great energy and rapidity, striking all its comrades to one side by its violence, and persisting in these for more than half an hour. Other movements, again, may be excited through the respiratory surface. Thus, if the head of a Centipede be cut off, and, while it remains at rest, some irritating vapor (such as that of ammonia or muriatic acid) be caused to enter the air-tubes on one side of the trunk, the body will be immediately bent in the opposite direction, so as to withdraw itself as much as possible from the influence of the vapor; if the same irritation be then applied on the other side, the reverse movement will take place; and the body may be caused to bend in two or three different curves, by bringing the irritating vapor into the neighborhood of different parts of either side. This movement is evidently a reflex one, and serves to withdraw the entrances of the air-tubes from the source of irritation; in the same manner as the acts of coughing and sneezing in the higher animals cause the expulsion, from their air-passages, of solid, liquid, or gaseous irritating matters, which may have found their way into them.

860. From these and similar facts it appears, that the ordinary movements of the legs and wings of Articulated animals are of a reflex nature, and may be effected solely through the ganglia with which these organs are severally connected; whilst in the perfect being, they are harmonized, controlled, and directed by the instinctive guidance, which depends upon sensations acting through the cephalic ganglia and the fibres proceeding from them. There is strong reason to believe, that the operations to which these ganglia are subservient, are almost entirely of a consensual nature; being immediately prompted by sensations, chiefly those of sight, and seldom involving any processes of a

truly rational character. When we attentively consider the habits of these animals, we find that their actions, though evidently directed to the attainment of certain ends, are very far from being of the same spontaneous nature, or from possessing the same designed adaptation of means to ends, as those performed by ourselves, or by the more intelligent Vertebrata, under like circumstances. We judge of this by their unvarying character, the different individuals of the same species executing precisely the same movements when the circumstances are the same; and by the very elaborate nature of the mental operations which would be required, in many instances, to arrive at the same results by an effort of reason. Of such we cannot have a more remark

able example, than is to be found in the operations of Bees, Wasps, and other social Insects; which construct habitations for themselves, upon a plan which the most enlightened human intelligence, working according to the most refined geometrical principles, could not surpass; but which yet do so without education communicated by their parents, or progressive attempts of their own, and with no trace of hesitation, confusion, or interruption,-the different individuals of a community all laboring effectively for one common purpose, because their instinctive or consensual impulses are the same.

861. It is interesting to remark that, in the change from the Larva to the perfect or Imago state of the Insect, the Cephalic ganglia undergo a great increase in size. (Plate II., Fig. 3, a, a.) This evidently has reference to the increased development of the organs of special sense in the latter; the eyes being much more perfectly formed; antennæ and other appendages used for feeling being evolved; and rudimentary organs of hearing and smell being added. In respondence to the new sensations, which the animal must thus acquire, a great number of new instinctive actions are manifested; indeed it may be said, that the instincts of the perfect Insect have frequently nothing in common with those of the Larva. The latter have reference to the acquirement of food; the former chiefly relate to the acts of reproduction, and to the provisions requisite for the deposit and protection of the eggs and the early nutrition of the young. -We find another important change in the nervous system of the adult or perfect Insect; namely, the concentration of the ganglionic matter of the ventral cord in the thoracic region (e, f); with the three segments of which, the three pairs of legs and the two pairs of wings are connected. The nine segments of the abdomen, in the perfect Insect, give attachment to no organs of motions, and are seldom themselves very movable; and we find that the ganglia which correspond with them have undergone no increase in size, but have rather diminished, and have sometimes almost completely disappeared. Where the last segment, however, is furnished with a particularly movable appendage, such as a sting, or an ovipositor, we always find a large ganglion in connexion with it.

862. These ganglia of the ventral cord evidently correspond in function with the pedal ganglion of the Mollusca; being so many repetitions of it; in accordance with the number of members. We have now to speak of a system of respiratory ganglia, which also are