plified. Some of the simpler forms possess a nervous system very much like that of a short Centipede, and from this condition all degrees of fusion and concentration are found, the maximum being reached where all the ganglia of the ventral cord have united into a single nerve-mass, precisely as in Crabs and Spiders. Three such stages, as exemplified by a Termite, a Water-Beetle, and a Fly, are represented in fig. 1019. In those insects which begin life as larvæ, it commonly happens that in this early stage of life the nervous system is simpler than in the adult, exhibiting less fusion and concentration. This is exemplified by comparison of a cater

Fig. 1019. Central Nervous Systems of a Termite

pillar with the butterfly or moth which it becomes, or a bee-grub with an adult bee. Cases are known, however, where the nervous system is condensed both in larva and adult, e.g. the House-Fly and its allies (Muscida). A curious reversal of the ordinary state of things is found in the Ant-Lion (Myrmeleo), for here the nervous

(Termes, A), a Water-Beetle (Dytiscus, B), and a Blow system of the relatively short

Fly (Musca, c), to illustrate stages in concentration. b, brain; e, eye; v, ventral ganglion.

and squat larva is more concentrated than that of the elon

gated adult. That this should be so is probably not merely due to difference in shape, for the complex habits of the rapacious larva involve elaborate adjustments to the surroundings, which need an efficient and centralized nervous system for their proper performance (see vol. ii, p. 111). So far as we know, the life of the adult is relatively simple.

It remains to be added that all the air-breathing Arthropods possess a visceral nervous system, which may attain considerable complexity, and takes origin from the nerve-ring.

NERVOUS SYSTEMS OF MOLLUSCS (MOLLUSCA). The least concentrated type of nervous system is found, as might be expected, among some of the Primitive Molluscs (Amphineura). The central nervous system of a Mail-Shell (Chiton), for instance, consists of a nerve-ring from which four thick nerves run back (fig. 1020). Two of these are pedal cords, that traverse the substance of the muscular foot, while the others are lateral cords

placed at a higher level, and uniting with one another behind above the intestine. The nerve-cells are distributed pretty uniformly throughout both ring and cords,

in the course of which are no distinct ganglia. The pharynx with its rasping organ receives branches from the nervering, which do swell into small ganglia, and this is also the case with a pair of nerves running from the lateral cords to the under side of the stomach (see figure). In this sluggish animal digestion is the dominant function, and that is possibly why the only distinct ganglia in the nervous system are related to the digestive organs. The visceral nervous system consists in this case of (1) the nerves which run from the nerve-ring to the pharynx, back to pair of gastric ganglia; V., part (2) the lateral cords and their branches.

I.A.

Fig. 1020.-A Mail-Shell (Chiton) dissected from above, to show Central Nervous System

M., Mouth; I.A., intestinal aperture; N.R., nerve-ring; P.C., pedal cord; L.C., lateral cord; St., stomach nerve passing of Visceral nervous system.

Passing from a simple form like the Mail-Shell to those which are more specialized, we shall find that as we ascend the scale to higher and higher types the nervous system becomes more and more centralized, in the same sort of way as in Arthropods. The nerve-cells are no longer scattered throughout the central nervous system, but are collected into definite ganglia, of which the most important are thickenings of the nerve-ring. This is very well seen in Snails and Slugs (Gastropoda), a vast number of which present a similar arrangement to that represented in fig. 1021 for the River-Snail (Paludina). In the middle of the figure is seen the nerve-ring, which is thickened into three distinct pairs of ganglia -(1) brain-ganglia above, (2) side - ganglia laterally (dotted in the figure), and (3) footganglia below. The brain-ganglia, as shown. at the top of the figure, give origin to a cord that supplies the pharynx, and swells into a pair of small ganglia from which nerves run to the pharynx. This is part of the visceral nervous system, the rest of it consisting of a nerve-loop by means of which the

VOL. IV.

Fig. 1021.-Central Nervous System of a River-Snail (Palu

dina), enlarged. See text. The circles shaded in the centre and

"

connected with the pedal ganglia
are the so-called " ears
cysts)

96

(Oto

A

Ph.

B

two side-ganglia are connected together, and in the course of which are three ganglia supplying many of the organs of the body. As in all Gastropods, the upper part of the body of the RiverSnail has been subjected to a sort of twisting process, as the spiral shell suggests, and this has affected the nerve-loop, making it 8-shaped, as shown in the figure. This wellspecialized central nervous system is associated with the presence of a

Br..G

Ped. G.
S.G..

Fig. 1022.-Central Nervous Systems of Pond

Snail (Limnæus, A) and Garden-Snail (Helix, B), clearly defined head, while just the

diagrammatic and enlarged

Br.G., Brain ganglia; Ped.G., pedal ganglia; S.G., side ganglia (dotted). Above are seen the small buccal ganglia connected by pharyngeal nerves (Ph.) with the brain ganglia, while below,

contrary is the case in a Mail-Shell. Centralization has taken place to a still greater extent in some of the the short nerve-loop with its ganglia is represented Gastropods, e.g. in the Pond-Snail (Limnæus) and Garden-Snail (Helix aspersa, fig. 1022), where the nerve-loop, which here has not been inflenced by the twisting of the body, is so short that its

(darkly shaded).

Osph N.

Gill N.

Mantle G.

Mantle N.

Nerve-loop

Gullet

Side G.--

Foot G:

Optic N.

Ph.G.

Pharynx

Brain

Fig. 1023-Central Nervous System of Cuttle-Fish (Sepia), seen from the right side

G., Ganglia; N., nerve; Ph. G., pharyngeal or buccal ganglia; Osph. N., osphradial nerve of watertesting organ Osphradium.

three ganglia are closely approximated to one another, and also to the foot-ganglia and side-ganglia of the nerve-ring.

Both in Bivalve Molluscs (Lamellibranchia) and Tusk - Shells (Scaphopoda) the nervous system follows the type described for Gastropods, but is less concentrated, and the brain-ganglia are relatively small, which may be correlated with the absence of any definite head in the former group, and its imperfect development in the latter.

Among the Head-Footed Molluscs (Cephalopoda) various degrees of concentration are found, there being the least amount of

this in the Pearly Nautilus, which is a primitive and isolated type, while in the active Cuttle-Fishes, Squids, and Octopi cen

tralization is at a maximum. The Common Cuttle-Fish (Sepia officinalis, fig. 1023) possesses a nerve-ring of which the ganglia are exceedingly large and closely connected. In one respect the nerve-ring is less complex than that of the Garden-Snail, for it here includes two only of the three ganglia of the nerve-loop, which is long, distinct, and, like the body, not twisted. The nerve-ring of Cephalopods is more or less enclosed in a gristly case, serving as a sort of skull.

NERVOUS SYSTEMS OF BACKBONED ANIMALS
(VERTEBRATA)

The nervous system attains its maximum complexity in backboned animals, especially in the highest Mammals. The chief part of the central organs consists of a tube, which is placed near the upper side of the body, and in all but the lowest members of the group is sheltered within a skull and backbone. The front end of this nerve-tube is usually swollen into a brain, which is the chief organ of correlation and adjustment, while the rest of it is known as the spinal cord or spinal marrow. The central structures also include a visceral, or, as it is here usually called, a sympathetic nervous system, which where best developed consists of a couple of cords running longitudinally near the under side of the backbone, and swelling at intervals into sympathetic ganglia. Besides these there are outlying ganglia of similar nature in close connection with some of the internal organs, and connected with the cords just mentioned.

The body of a Vertebrate is undoubtedly made up of rings or segments, and although this is not at first sight apparent, the serial arrangement of certain structures shows it to be the case. We find, for example, that a regular succession of spinal nerves is given off from the spinal cord, one pair to each segment. From the brain arise from 10 to 12 pairs of cranial nerves, the number of which, however, does not tell us how many segments are fused to form the head. The number would be a guide if cranial nerves were precisely equivalent to spinal nerves, but this does not appear to be the case. While on the one hand some of them are complex, and equivalent to more than one pair of spinal nerves, others are only comparable to bits of such nerves, so to speak. The sympathetic system is closely

connected with the brain and spinal cord, to which it is subordinate, and its nerves branch out in the organs of digestion, circulation, &c. A few further details have already been given with regard to the nervous system of Man (see vol. i, p. 49).

It was stated at the commencement of this section that the essential elements of the nervous system, i.e. the neurons, are derived from the ectoderm or outer cell-layer. outer cell-layer. Considering that brain and spinal cord are far removed from the surface, while the body is traversed in all directions by nerves, it seems

very difficult to believe such a statement, but the study of development shows that there is no doubt at all about the matter.

At a comparatively early stage in the development of an embryo part of the the ectoderm covering the upper surface thickens into a nerveplate, which sinks below the surface, and at the same time folds up to constitute the nerve-tube. The details for the Lance

let have already been given (vol. iii, p. 345), but in that animal the nerve-plate sinks below the surface before it is completely folded into a tube, while in average cases the two processes go on simultaneously, as will be gathered from fig. 1024.

The walls of the nerve-tube thicken, and by a process of unequal growth the spinal cord and the various regions of the brain come into existence. The rest of the nervous system grows out from the nerve-tube, e.g. the spinal nerves grow out from the spinal cord to the parts of the body which they supply. It therefore follows that these and the other nerves, as well as the sympathetic system, are really ingrowths from the ectoderm or outer cell-layer, although in the adult they are far removed from the surface.

THE BRAIN OF Vertebrates.-At first sight the brains of Fishes,