selves. The structures which I have called the "hemispherical ganglia," with their concentric rows of nuclei and slight connection with the other parts of the ganglion, are so very unlike anything that we see in the brains of mammalia, that we are puzzled to discover their homologues. Nor does their relation to the roots of the cephalic nerves, or the origin of the interganglionic cord, appear at first to explain their nature. Fibres pass into them from the groups of ganglionic cells at the roots of the various cephalic nerves; some of the fibres from those nerves may pass into them directly, but this I was never able to demonstrate; and by means of their peduncles joining the longitudinal cords, they have a direct connection with all the other ganglia. They would therefore seem to have a function different from the ordinary nerve-cells-a general and diffused function, which has no special relation to any part of the animal, or to any of its sensory or motor apparatus, but is supplementary to, and conjoined with, the action of those cells, wherever innervation exists in the body. It is no unjustifiable inference to suppose, that in them reside the higher manifestations of nerve-force which the animal exhibits. The lobster is not a mere machine, that responds to impressions made on its nervous system from without through its organs of special senses, or on the extremities of its afferent nerves. tions which it performs as the result of these may be explained by means of the arrangement of cells which we have seen in the ganglia, and at the roots of the cephalic nerves; but we cannot so account for its great cunning and perseverance in the search for food, its sexual appetites and instincts, its regular migratory habits from deep to shallow water at certain seasons, its strongly developed instinct of self-preservation, &c. These approaches to psychical manifestations doubtless. require special nerve-tissue for their exhibition; just as we know that in the higher animals the psychical functions are connected with the cerebral hemispheres; and the only part of the nervous system in the lobster containing nerve-cells, to which no other function can be assigned, are these hemispherical ganglia. We therefore conclude, that in them originate those manifestations of a higher animal life.

The ac

The terminations of the nerves of special sense in the

cephalic ganglion of this animal are extremely interesting. Some of the fibres end in cells precisely similar to those of the thoracic and abdominal ganglia. And not only are the cells the same in size and appearance, but they are distributed into groups in the same way. Their other fibres run to form part of the interganglionic cord directly. The mechanism of nerve cells for special sensation is therefore, so far as we can ascertain it, the same as for general sensibility through the body. Of course there must be a difference in the mode of activity of those cells, the nature of which will probably for ever remain inappreciable by us; but we thus see that impressions made on the special senses are followed by muscular movements, just in the same excito-motor way as impressions on any other part of the body. The muscles of a lobster's large claw may be thrown into action, either when this claw is touched, or when a foreign body is seen; in the one case the impression being transmitted upwards by the afferent nervefibres of the organ to the groups of cells which control its muscles; and in the other case, the impression being transmitted along the optic fibres, which we have seen to join directly the interganglionic cord, to the same groups of nerve cells, and with the same result,-viz., to cause a combined muscular movement by the nerve-force originated in those cells, and transmitted along the efferent fibres. In both cases it is probable that an impression is also transmitted to the hemispherical ganglia, and that the sensation of pain which the animal is undoubtedly capable of feeling resides there. These ganglia constitute, therefore, a true sensorium in the literal meaning of that term. The animal is endowed with such a high degree of functional activity-far higher in this respect than some members of the vertebrate division-that we must assume the existence of an organ to correspond in function to the ganglia which constitute the brain in fishes. The difference between the brain of a fish and that of a lobster seems to be, that in the former, the cells which minister directly to the excito-motor function-those through which impressions from without are followed directly by action in some form-are mixed and more intimately connected with the cells whose higher function it is to direct and control all

the other nerve-cells in the body, and give the animal its sensational and psychical functions; while in the latter the two kinds of cells are separated. A lobster without the hemispherical ganglia would be a mere excito-motory organism, capable of no sensation, properly so called, and showing no desires or instincts, that would move in answer to impressions on its nerves of common and special sense, and only in answer to those stimuli.

A careful consideration of the minute structure of the nervous system of any invertebrate animal, such as the one we have just been examining, shows us that histologically and physiologically the vertebrate and the invertebrate animals are nearly allied. In every essential point the ganglia and inter-ganglionic cord of the lobster correspond to the spinal cord of the vertebrate, while the cephalic ganglion is analogous both in structure and function to the brain. The tendency to segmentation seen in both sub-kingdoms is most marked in the nervous system of the invertebrate, because in this division the nervous system does not form the centre round which all the other parts are developed, as is the case with the spinal axis of the vertebrate. Such an examination makes us esteem lightly, too, such generalizations of the mere external form of the nervous system, as that made by Audouin and Milne-Edwards in the Crustacea. No doubt they were useful, as the Linnæan classification of plants was useful, as a prelude to a more natural and scientific classification; but that we are to conclude an animal to be high in the scale, merely because its nervous system happens to be compressed into a mass to accord with the external shape of the body, seems as rational as to affirm, that the nervous cord of the earth-worm and nematode is more analogous to the spinal cord of the vertebrate than that of the lobster, because it happens to have ganglionic cells all the way down.

Description of Plates.
PLATE I.*

Fig. 1. Cross section of sheath of interganglionic cord.―a, Outer longitudinal layer of fibres; b, inner circular layer; c, nerve-tubes cut across.

* The original drawings for the Thesis were made by my friend Dr Sibbald, from my own sketches, and with the microscopic preparations before him. NEW SERIES.-VOL. XVII. NO. I.-JAN. 1863.

G

Fig. 2. a, Nerve-tubes of different sizes from interganglionic cord.

What ap

pear to be minutely striated fibres at one end, are at the other (where there are fewer of them) seen to be merely smaller tubes; b, a large tube split up into fibrilla; c, nerve-cells from caudal ganglion; d, the same, with what appear to be stellate nuclei.

Fig. 3. a, Small cells from cephalic ganglion; b, one of the nuclei from a hemispherical ganglion.

Fig. 4. Cross section of abdominal interganglionic cord.-a, Large nervetubes; b, sheath, which is thickened on the dorsal surface at d; c, septum; e, the motor-nerve, which springs from the cord between the ganglia, lying at this part in apposition to it.

Fig. 5. Section of about one-fourth of a thoracic ganglion.-a, Longitudinal fibres of interganglionic cord; b, fibres of lateral nerve; c, cross fibres from one lateral nerve to the opposite side; d, fibres from longitudinal cord joining lateral nerves; e, sheath; f, a group of cells, with most of their fibres passing towards the head; g, another group, with fibres passing across to opposite side; h and i, large nerve-cells; k and l, scattered bipolar nerve-cells; m, a bundle of nerve fibres cut across. Fig. 6. A section of the same ganglion as that from which fig. 5 was made, but more towards its dorsal surface.-a, Interganglionic cord; b, lateral nerve; c, a group of cells whose " 'pedicle" passes backwards towards the caudal extremity; d, another group, whose "pedicle" passes to the opposite side; e, longitudinal fibres cut somewhat obliquely; f, isolated nerve-cells.

Fig. 7. Diagram of an ideal ganglion, embodying the results of all the sections made.-a, Longitudinal fibres; b, cross fibres; c, fibres from longitudinal cord to lateral nerves; d, e, fibres of "reinforcement;" f, group of cells sending its "pedicle" forwards, but with connections to other groups; g, group of cells sending its "pedicle" to opposite lateral nerve; h, group between the cords, sending two bundles of fibres, m and n, to lateral nerves of opposite sides; k, group of cells sending "pedicle" towards caudal extremity; 1, group of cells whose " pedicle" joins lateral nerve of same side.

PLATE II.

Fig. 1. Cross section of thoracic ganglion, showing a, b, lateral nerve (in outline) cut across; c, a few of the fibres of longitudinal cord cut across ; d, e, groups of cells whose pedicles pass to opposite side; f, group with pedicle passing to lateral nerve of same side; g, group whose connections have been cut away.

Fig. 2. Longitudinal section of an abdominal ganglion.-a, Longitudinal cord ;. b, lateral nerve cut across; c, group of cells, a few of whose fibres pass among the longitudinal fibres, and at d join them; f, other groups of cells, whose fibres converge towards the lateral nerves.

Fig. 3. The two cords connecting the cephalic to the first thoracic ganglion, with the sheath taken off one of them, magnified four times.-a, Ganglionic swelling at root of pneumogastric nerve; b, pneumogastric

The preparations were also examined by Professor Goodsir, before the " Defence of the Thesis." I must here express my great obligations to Dr Sibbald for the manner in which the illustrations were done.

nerve; c, one cord, with its sheath dissected partly off; d, other cord; e, cross nerve from one to the other, whose fibres, g, are dissected away from the cord, so as to show how they join the pneumogastric ganglion and nerve; f, accessory pneumogastric.

Fig. 4. Section of cephalic ganglion in the plane of entrance of longitudinal cords and optic nerves.-a, Optic nerve; b, longitudinal cord; c, fibres from longitudinal cord of opposite side to optic nerve; d, fibres to optic of same side; e, commissural fibres of optics; f, fibres from second cephalic nerve; g, h, groups of ordinary ganglion cells; i, group of ganglion cells in front of optic commissure (the whole space vacant in the drawing had been filled up by those cells, but they had been dislodged); k, a few commissural fibres from one longitudinal cord to the other.

Fig. 5. Cross section of the cephalic ganglion, slightly anterior to the centre.

a," Hemispherical ganglion ;" b, "Peduncle;" c, longitudinal cord cut across; d, oval striated mass below hemispherical ganglion; e, group of small stellate cells; f, h, ordinary ganglion cells; g, another group of cells similar to those at e.

Fig. 6. Vertical section of anterior part of cephalic ganglion, in the line of one of the optic nerves.-a, Optic nerve fibres; b, layer of small stellate cells, which many of the optic nerve fibres join; c, hemispherical ganglion; d, bending of the optic nerve fibres over the hemispherical ganglion at an acute angle.

Reply to some Comments of Mr F. Marcet on the Power of Selection ascribed to the Roots of Plants. By CHARLES DAUBENY, M.D., Professor of Botany, Oxford.*

A friend pointed out to me, a short time ago, in the February Number of the Bibliotheque Universelle, some comments on a paper of mine relative to the power attributed to plants of rejecting abnormal or poisonous substances presented to them by the soil in which they grow, the substance of which paper was communicated to this Section at the Meeting of the British Association for 1861.† These remarks, proceeding as they do from the respectable pen of M. Francis Marcet, seem to call for some notice on my part, and I therefore propose to offer, on the present occasion, a few observations in reply. I do not, however, intend to dwell at any length upon the points of difference between his views

* Read at a Meeting of the British Association, on Tuesday, October 7th, 1862.

↑ See also my paper in the “Journal of the Chemical Society of London," vol. xiv. for 1861.