as., esophagus; tr. br., transverse bridge behind asophagus; gan. c., central n ass of ganglion cells; musc., nerve ending on muscle; Ant. II, ganglion of Antenna II; Th. I-XI, thoracic ganglia; I-VI form anterior thoracic ganglion of adul; A-J, individual nerve elements. For description of each, see text.

II. Elements which consist of a ganglion cell in the cord and a fibre which runs out at a lateral nerve root. Some, at least, of these elements, possibly all, are connected with muscles, and are motor elements.

III. Elements which consist of a cell lying outside the central ganglionic chain, and a fibre running from it to a ganglion. These must be regarded as sensory elements.

I. Elements of the first group, co-ordinating elements, are of four kinds.

A. Elements made up of a cell in the brain or one of the ganglia, and a fibre which runs posteriorly to the end of the cord, giving off collateral branches to the neuropile in each ganglion through which it passes. For reasons to be explained, these elements must be regarded as placed temporarily in the groups of co-ordinating elements. It may be necessary to place them in a new group by themselves.

Two kinds of A elements may be distinguished :

:

(a.) Those which decussate with the corresponding element of the

opposite side.

(b.) Those which pass down on the same side of the cord as that on which the cell lies.

The elements A (a) inserted in the ganglion of Antenna II and Thorax II (fig. 1) may be taken as typical of the fibres which decussate. The cell lies in the lateral mass of ganglion cells, and the fibre gives off lateral branches to the neuropile before crossing to the other side. After the decussation has taken place the fibre turns backwards, and runs down the ganglionic chain, giving off collateral branches to the neuropile of each ganglion through which it passes. The fibre A (a) Thorax II, together with a corresponding fibre in Thorax I, have been traced as far as the last abdominal ganglion, but no definite ending has been made out. A (a), Antenna II, was only actually traced to Abdomen 5, but it showed no sign of ending in that ganglion, and probably continues to Abdomen 6. It will be observed that the decussation of A (a), Antenna II, takes place through the transverse bridge, behind the esophagus. Similar elements have stained in Thorax III and Thorax V.*

The element A(a)Br (fig. 1) may be best considered here. It consists of a large cell on the ventral surface of the brain, from which a moderately thick fibre runs at first forwards and upwards to the dorsal surface. After turning outwards, the fibre runs backwards to a point immediately in front of the oesophagus, where it passes across * In the present communication the thoracic ganglia are numbered consecutively I-XI. Of these I-VI form the anterior thoracic ganglion of the adult, whilst VII-XI form the five posterior ganglia.

to the other side, and then runs down the cord. On entering the thoracic ganglia, the fibre becomes very broad, and maintains a diameter many times that of any other fibre in the ganglionic cord. A single pair of elements of this kind exists, and it is easy to trace the fibres from the brain through the whole length of the ganglionic cord to the 6th abdominal ganglion. In the latter ganglion the fibre divides into several branches, but I have never obtained complete staining of these. The giant-fibres of the adult, with which these agree, are stated by Retzius* to divide in the last abdominal ganglion, and send branches through several of the nerve roots, which leave that ganglion.

In the brain this fibre gives off a few branches to the neuropile (fig. 1), but on its course down the cord, no collateral branches have ever stained, and I believe that none exist. The giant fibres differ in this respect from the other fibres of the class (A) which is now being considered.

Of the elements whose fibres do not decussate, A(b)Br. (fig. 1), may be taken as a type. The element starts with a cell of moderate size on the ventral surface of the brain, immediately anterior to the large cell of the giant fibre (A(a)Br). The fibre passes first forwards and upwards, giving off numerous branches to the anterior lobes of the brain on both sides, and then backwards through the brain, and down the ganglionic cord of the same side to the last abdominal ganglion. It gives off collateral branches to the neuropile of the ganglia through which it passes.

Similar elements have stained in Thorax III and Thorax IV.

B. Each element consists of a fibre starting from a cell in one of the thoracic ganglia, and running forwards to the brain. B Thorax V and B Thorax VI (fig. 1) are types of elements of this kind.

The cell of B Thorax V, lies in the lateral mass of ganglion cells. Soon after entering the neuropile the fibre gives off two branches, one running forwards and breaking up in the hinder portion of the ganglion immediately anterior, whilst the other runs backwards and breaks up in that immediately posterior. The main fibre turns inwards, crosses its fellow of the opposite side, and then bends forwards, running close to the median ganglionic cells, until it enters the brain. In the brain the fibre continues to run forwards, giving off many branches, and ends at about the level of the nauplius eye. The fibre has not been observed to give off collateral branches during its course through the thorax. Precisely similar elements occur in Thorax VIII, and somewhat similar ones in Thorax II.

In B., Thorax VI (fig. 1), the cell lies in the median mass of ganglion cells, and the fibre, on its course towards the brain, runs

6

* Retzius," Zur Kentniss des Nervensystems der Crustaceen," Biol. Untersuch." Neue Folge I, 1890.

along the outer border of the neuropile. It ends in the brain, at a point nearly as far forwards as the termination of B Thorax V, but somewhat lateral to it. In this fibre also no collaterals have been observed in ganglia other than that in which the element originates. A similar element occurs in Thorax I.

C. Elements of this kind are inserted in ganglia Thorax VII-X in fig. 1. Each consists of a small cell in the anterior portion of the lateral mass of ganglion cells. The fibre, after taking a Z-shaped course through the neuropile, to which it gives off numerous arborescent branches, turns backwards, and after running between a fibre of Series D and the central mass of ganglion cells, ends in a tuft of fine branches at the posterior end of the ganglion next behind that in which the cell is situated.

D. These elements appear to be intimately associated with those of Group C, and both groups generally stain in the same preparations.

The cells of elements D lie in the posterior portion of the lateral ganglionic mass. The fibre of each element passes forwards and inwards through the neuropile, giving off numerous arborescent branches to the latter. On entering the next ganglion in front, the fibre has reached the outer border of the median ganglionic mass, and after giving off a little tuft of branches in front of the tuft in which one of the C elements ends, it pursues a direct anterior course, ending in a tuft of branches in the ganglion next but one to that in which it started. This terminal tuft lies opposite the terminal tuft of one of the C elements, and behind the lateral tuft of the D element of the next ganglion. The three tufts lie at exactly the same level in the cord, being all in the focus of the microscope at the same time.

II. Elements consisting of a cell in the ganglionic cord, giving off a fibre, which, after sending arborescent branches to the neuropile, passes out from the cord by one of the nerve roots. Many of these elements, not improbably all, are motor, and in some cases the fibre has been traced through its whole course from the cell, until it breaks up on the muscle. (Fig 1, E. Thorax XI.)

Typical elements of this class are inserted in fig. 1 (Thorax VIIXI, E-J). Each of these ganglia contains one or more elements of the various kinds. E and F have the cell situated in the lateral mass of ganglion cells, and the fibre passes out through the anterior root of the ganglion and goes to one of the limbs. In G and H the cell lies in the central mass, but the fibre passes through the same anterior root as E and F to a limb. The element J, on the other hand, passes through the posterior root and goes to the muscles of the body wall. It arises in a very small cell in the median mass, and the fibre runs for some distance outwards. It then turns and takes a circular course through the neuropile, forming a complete loop, after which it passes outwards and enters the posterior nerve root. From the anterior part of the

inner side of the loop a straight arm passes inwards and meets a similar arm from the opposite side. A direct fusion of the two arms has, however, never been observed.

III. Sensory elements, in which the cell lies outside the ganglionic cord.

These have been demonstrated by me up to the present only in the abdomen of lobster embryos. The cells are similar to those described by Lenhossék and Retziust in the earthworm, and by Retzius‡, in polychaetes and molluscs. They are spindle-shaped and lie in the ectoderm (or immediately beneath it) of the dorsal surface of the abdomen. The distal end of the spindle either runs out as a fine fibre which ends freely, or the end of the fibre broadens out, forming a T-shaped figure on the end of the spindle. The fibre arising from the proximal end of the cell passes forwards and downwards to the nerve cord, where, after entering one of the ganglia, it bifurcates, giving rise to a Y-shaped figure. One of the branches runs forwards, the other backwards along the ganglionic cord. They have been seen to pass through two ganglia, but as to where they end I am not at present in a position to make a definite statement.

Theoretical.

I shall now endeavour to draw attention to some points of interest connected with the observations described above. With regard to the elements C and D, it might be maintained that they represent a purely embryonic arrangement, which has not yet reached the active state. This, however, appears to me improbable from the fact that the two systems remain in practically the same state from very early embryos, in which the eye-pigment has just begun to deposit, to the oldest larvæ (about one week) which I have been able to examine; and also from the fact that they take up methylene blue in a way which, according to present experience, only active nerve tissue does. Assuming then that the system is in the active state, it is important to notice the position of the three tufts of fibres, which stand opposite to each other, where elements C and D end. Although my observations agree entirely with those of Retzius, Kölliker, and the majority of recent investigators, in the fact that direct anastomosis of the portion of the element which stains with methylene blue has never been observed, it seems impossible to understand the meaning of this arrangement on any other supposi

* Lenhossék, "Ursprung, Verlauf, und Endigung der sensibeln Nervenfasern bei Lumbricus," ,"Arch. Mikr. Anat.,' vol. 39, 1892.

† Retzius, "Das Nervensystem der Lumbricinen," Biol. Untersuch.' Neue Folge III, 1892.

Ditto, Neue Folge IV, 1892.