In the second place a tendency to conjunc tion by a motion transversely.
Lastly, a second motion in the line of the axis of the body, the effect of which is the concentration definitively of a greater or smaller number of nervous centres primarily indepen dent of one another.
The Talitrus exhibits in the most striking manner the first of the three dispositions which we have mentioned from the mo ment at which the nervous sys tem appears. In this genus, in fact, we perceive on either side of the median line a ganglionic chain, formed by the conjunc tion of the nervous centres, extremely simple in their struc ture, and flattened and some what lozenge-shaped in their outline.* There are thirteen pairs thus constituted, corres ponding to the thirteen seg ments which enter into the com position of the whole body. The two nuclei of each pair com municate together, in the same manner as each pair is con nected with that which succeeds, and with that which precedes it, by means of medullary cords in the first instance and longitu dinal cords in the second. In all essential particulars each pair is a counterpart of any and every other pair, without even excepting the cephalic ganglion, and it is with difficulty that the thoracic pairs are seen to be in a slight degree larger than the others. At a somewhat greater distance forward from the cesophagus, too, than usual, we observe the cephalic ganglion, which sends branches to the antennm and. eyes,. and the nervous cords by means of which it commu nicates with the ganglions.of the first thoracic rings. These cords, having the cesophagus inter posed between them, are held a little farther apart than the other branches, which establish com munications between the different succeeding pairs of ganglions in the longitudinal direction.
Already in the Oniscus asellus* and in the Cy amus cetit we find the ganglionic cord, double in its middle portions, simplified at its opposite extremities in such wise that the ganglions of the first and of the last pairs are single. This commencement of approximation coincides in other respects with an incipient approximation in the longitudinal direction for, to the four teen segments of which the whole body consists, we find no more than ten pairs of ganglions apportioned.
This tendency to centralization is still more conspicuous in the PhyllosOma.t Here vve discover the two cephalic nuclei united by their internal angle, without, however, their state of doubleness being thereby obscured. It is the same with the first pair of thoracic ganglions, from which they are separated by the whole length of the great oval lamina which supports the cephalic appendages and is traversed lengthwise by the nervous filaments which embrace the cesophagus. The ganglions of the second pair, although rudimentary, are still united immediately, as are those of the third pair also. Those of the six suc ceeding pairs, on the contrary, only communicate by means of a transverse but thick and short commissure, so that it gives to the connexion established be tween the nuclei of the several pairs, the appearance of a more immediate conjunction than ac tually exists. To conclude the abdominal ganglions are perfectly distinct, and those of the several pairs are only connected by means of extremely slender fila ments.
In the Cymothoa the union of the medullary nuclei in the trans verse direction is complete, and all we perceive is a single series extended along the median line through the whole length of the body. This is similar to the nervous system of the Talitrus conjoined longitudinally; with this difference, that the longitudinal filaments uniting the ganglion have continued distinct, as if to testify, by their doubleness, to the mode of formation of the single ganglionic cord.
But it is more especially in the types which still ask our attention, that we perceive the system of centralization pushed yet farther by the actual conjunction of the nuclei, which we have hitherto only seen approximated to one another, in consequence of their gliding or encroaching, as it were, upon the median line.