350 NERVOUS SYSTEM OF RADIATA. ideas; keeping in check the passions and emotions, or, on the other hand, promoting their healthful activity by directing the attention to the objects of them; and determining the movements which the reason prompts :-and the acquirement and right direction of such regulating power is the highest object of all Education. 433. It will be recollected that every form of Nervous System essentially consists of two kinds of nervous tissue,— the tubular or fibrous, whose functions seem to be purely conductive (§§ 60, 62),-and the vesicular or ganglionic, which seems to be the seat of all the changes to which this apparatus ministers, and the source of all its peculiar powers (§§ 61, 63). -The principal forms under which this apparatus presents itself in the several divisions of the Animal Kingdom, and the general nature of the functions to which it is subservient in each, will now be successively described in the ascending series, from Zoophytes up to Man. Structure and Actions of the Nervous System in the principal Classes of Animals. 434. In most of the RADIATED classes, it is difficult to discover any distinct traces of a Nervous System; the general softness of their tissues being such, that it cannot be certainly distinguished amongst them. It clearly exists, however, in the highest group, the ECHINODERMATA; and it presents an Fig. 180.-NERVOUS SYSTEM OF a, position of the mouth. extremely simple form, which partakes of the general arrangement of parts in these animals. In the Star-fish, for instance, it forms a ring which surrounds the opening into the stomach (fig. 180); this ring consists of a nervous cord that forms communications between five ganglia, one of which is placed at the base of each ray. These ganglia appear to be all similar to each other in function. Every one of them sends a large trunk along its own ray; and two small branches to the organs in the central disk. The rays being all similar to each other in structure, it would appear that no one of these NERVOUS SYSTEM OF RADIATA AND TUNICATA. 351 ganglia can have any controlling power over the rest. All the rays have at their extremities what seem to be very imperfect eyes; and so far as these can aid in directing the movements of the animal, it is obvious that they will do so towards all sides alike. Hence there is no one part which corresponds to the head of higher animals; and the ganglia of the nervous system, like the parts they supply, are but repetitions of one another, and act independently of one another. Each would perform its own individual functions if separated from the rest; but, in the entire animal, they are brought into mutual relation by the circular cord, which passes from every one of the five ganglia to those on either side of it.-In Man, as well as in all the Vertebrated and Articulated animals, and in some of the Mollusca, there is a like repetition of the parts of the Nervous System on the two sides of the central line of the body; but the organs are only double, instead of being repeated five times. Still the two hemispheres of the brain, and the two halves of the spinal cord, in the Vertebrated animal,—and the two halves of the chain of ganglia, in the Articulated animal, are as independent of one another as are the five separate ganglia of the Star-fish; and they are made to act in mutual harmony by similar uniting bands of nervous fibres, which are termed commissures. 435. In the nervous system of MOLLUSCA, we do not meet with any such repetition of parts; the body itself not presenting this character. In the lowest and simplest animals of this group, there exists only a single ganglion, which may be regarded as analogous to any one of the ganglia of the Star-fish; but in the higher, we find the number of ganglia increased, in accordance with the increase of the functions which they have to perform. The simplest form of the nervous system in this class is seen in the accompanying figure (fig. 181), which represents one of the solitary TUNICATA, the Ascidia. At a is seen the orifice by which the water enters for sup- Fig. 181.-NERVOUS SYSplying the stomach with food, and for aerat TEM OF ASCIDIA. ing the blood (§ 114); and at b is the orifice by which the current of water passes out again, after it has served these purposes. 352 NERVOUS SYSTEM OF TUNICATA AND CONCHIFERA. Between these orifices is the single ganglion c, which sends filaments to both of them, and other branches which spread over the surface of the mantle d. These animals are for the most part fixed to one spot during nearly the whole of their existence; and they show but little sign of life, beyond the continual entrance and exit of the currents already adverted to. When any substance is drawn-in by the current, however, the entrance of which would be injurious, it excites a general contraction of the mantle; and this causes a jet of water to issue from both orifices, which carries the offending body to a distance. And in the same manner, if the exterior of the body be touched, the mantle suddenly and violently contracts. 436. These are the only actions, which, so far as we know, the nervous system of these animals is destined to perform. They scarcely exhibit any traces of eyes or other organs of special sense; and the only parts that appear peculiarly sensitive, are the small tentacula which guard the orifice a. It would seem as if the irritation caused by the contact of any hard substance with these, or with the general surface of the animal, caused a reflex contraction of the mantle, having for its result the getting-rid of the source of the irritation. Such a movement could only be performed by the aid of a Nervous system, which has the power of receiving impressions, and of immediately exciting even the most distant parts of the body to act in accordance with them. In the Venus's Fly-trap and Sensitive Plant (VEGET. PHYS., §§ 214, 391), an irritation applied to one part is the occasion of a movement in another; but this takes place slowly, and in a manner very different from the energetic and immediate contraction of the mantle of the Tunicata. 437. In the CONCHIFERA, or animals inhabiting bivalve shells, there are invariably at least two ganglia, having different functions. The larger of these, corresponding to the single ganglion of the Tunicata, is situated towards the posterior end of the body (B, fig. 182), in the neighbourhood of the posterior muscle; and its branches are distributed to that muscle, the mantle, the gills, and the siphons. But we find another ganglion, or rather pair of ganglia (AA), situated near the front of the body, either upon or at the sides of the œsophagus, and connected by a commissural band that arches over it; these ganglia receive nerves from very sensitive tentacula which NERVOUS SYSTEM OF MOLLUSKS. 353 guard the mouth; and they evidently correspond, both in position and functions, to the sensory ganglia of higher animals, whilst the posterior ganglion has for its office to regulate the respiratory movements. In the Pecten, however, as in other Conchifera which possess a foot (fig. 62), we find an additional ganglion (c), the pedal, connected with the cephalic ganglia, and sending nervetrunks to that organ. There is good reason to believe that, whilst the cephalic ganglia alone are the instruments of sensation, so that they exert a general B control and direction over the Fig. 182.-NERVOUS SYSTEM OF PECTEN. (§ 433) of the organs which they supply. A A Fig. 183.-NERVOUS SYSTEM OF 354 NERVOUS SYSTEM OF MOLLUSKS. side. Of these, one passes through the ganglionic masses cc, to communicate with the ganglion B, which is the one connected with the respiratory movements. The others are distributed with the branches of the ganglia co, the function of which is double; for one set of branches from each is distributed to the mantle in general, every part of which (in these shell-less Mollusks) is capable of contracting and giving motion to the body; whilst another set is distributed to that thick and fleshy part of it which is called its foot, and on which the animal crawls (§ 107). There is another ganglion, D, lying in front of the cephalic ganglion, and also receiving branches of communication from it; this ganglion is specially connected with the actions of mastication and swallowing, and is called the pharyngeal ganglion. 439. Thus we see that the cephalic ganglion sends branches to all the other ganglia, though these having different functions, do not communicate with each other; and thus every part has two sets of nervous connexions, one with the cephalic ganglia, and the other with its own ganglion. By the former, the animal becomes conscious of impressions made upon it, these impressions being converted in the cephalic ganglia into sensations; and the influence of its conscious power is exerted through them upon the several parts of its body, causing spontaneous motion. By the latter are produced those reflex actions of the several organs, which do not require sensation, but which depend upon the simple conveyance of an impression to the ganglion, and the transmission of the resultant motor impulse from it to the muscles supplied by its nerves. A small part only of the Nervous System of Mollusks ministers to the general movements of the body; and this corresponds with what has been elsewhere stated (§ 107) of the inertness which is their general characteristic, and of the small amount of muscular structure which they possess. 440. On the other hand, in the ARTICULATED classes, in which the apparatus of movement is so highly developed, and whose actions are so energetic, we find the Nervous System almost entirely subservient to this function. Its usual form has been already described (§ 94) as a chain of ganglia connected by a double cord, which commences in the head, and passes backwards through the body. In general, we find a ganglion (or rather a pair of ganglia united on the middle |