groove of the endostyle, and spread out in one plain. The central dorsal line of each sac is marked by the projecting dorsal lamina (D.L.), while the cut ventral edges are each indicated by one of the ridges of the endostyle (En.); consequently, as the interior of each sac is supposed to be facing downwards, the right-hand half of the sheet contains the left sides of the branchial sacs, and the left-hand half the right sides. The first fold on the right side, when well developed, is shown by a curved projection of the wall of the sac (I. br.f. in Fig. 4), while the rudimentary folds are indicated by the Roman numerals I., II., III., and IV. The short lines directed downwards from the wall of the branchial sac represent internal longitudinal bars, and divide the interior of the sac into meshes. The numbers (16, 2, 6, 8, &c.) placed above show the number of stigmata, and thus indicate the relative sizes of the meshes,

In figure 4, a typical branchial sac, such as has been described above, is represented. The dorsal lamina (D.L.) is followed by an area sixteen stigmata wide on each side, The first right fold (I. br.f.) is well developed, and has nine internal longitudinal bars, while the fold opposite to it, on the left side, is represented by four rows of narrow meshes, or, in other words, has five internal longitudinal bars. The other folds (II., III., IV. on each side) are in the usual rudimentary condition, being formed of two rows of narrow meshes each. They are separated by pairs of rows of wider meshes, containing each six, seven, or eight stigmata, and the IV. fold on each side is separated from the endostyle by a still wider area (ten or twelve stigmata). The other five sacs given in the chart (figures 5-9) are abnormal, and show some of the more common variations from the typical arrangement.

Figure 5 has the first right fold well developed (ten internal longitudinal bars), and separated from the dorsal

lamina by a considerable interval (eighteen stigmata). The second right fold (II.) is formed of two rows of narrow meshes (two and four stigmata), and is separated from the first right fold by a very wide interval (three rows of wide meshes). The third right fold (III.) is represented by a single row of narrow meshes, and the fourth is entirely absent. Its position (indicated by X in the diagram) is obviously after the two rows of wide meshes following III. It would then be separated from the endostyle by a space twelve or thirteen stigmata wide. The left side of this branchial sac has the three first folds well shown, while the fourth (IV.) is almost obsolete. In some parts of the sac it is represented by a single row of narrow meshes (two stigmata), but as the bars bounding these meshes fail here and there, the narrow meshes are thrown into the adjoining wider ones, and the fold disappears. In the figure the number of stigmata is enclosed in brackets, to show the partial nature of the row of meshes.

Figure 6 shows rather an irregular sac. The first right fold is well developed (eight bars), and separated from the dorsal lamina by a wide interval, while the first left fold (three rows of narrow meshes) has only an ordinary wide mesh (six stigmata) between it and the dorsal lamina. The second, third, and fourth folds on the right side form a series leading towards extinction; II. has two rows of narrow meshes, III. has a single row, while IV. is represented by a partial row only. The spaces alternating with these folds are rather narrow, being only five, four, and five stigmata wide, respectively. On the left side of the sac the single rows of narrow meshes representing the second and fourth folds are partial only, while the third fold (III.) is clearly shown by two rows of narrow meshes (two and three stigmata). The I. and II. and the II. and III. folds are separated by narrow intervals of five stigmata each, while the III. and IV. are separated by two rows of five stigmata, and the IV. is

divided from the endostyle by two rows of meshes, first a row of ordinary wide meshes (six stigmata), and then the usual much wider interval (sixteen stigmata).

Figure 7 shows a sac where the folds have almost entirely disappeared, with the exception of the first on the right side, which, as usual, is well developed. The second and third on the same side are represented by a single partial row of narrow meshes each; the fourth may have been present, but as the ventral edge of the sac was torn, the diagram could not be completed; the fourth left fold was also not seen. The first and third left are formed each of a single row of narrow meshes, while the second (II.) is only partially present. On both sides of this sac the spaces between the folds are well represented and comparatively wide, forming a contrast in this respect to the last sac examined (Fig. 6).

Figure 8 shows a very remarkable case where at first sight the folds seem to be entirely absent. The first right (I.) does not project, and is indicated merely by five rows of very narrow meshes (containing each one or two stigmata). Extending from this to the endostyle is a series of wide meshes, and one naturally supposes at first that the remaining folds on the right side are obsolete. When, however, the stigmata are counted, and the sizes of the meshes compared, it becomes evident that six of them form two series, a larger and a smaller, placed alternately, while the seventh is the largest of all, and separates the most ventral of the smaller meshes from the endostyle. The series of smaller meshes has six, six, five stigmata, while the three larger ones have each eight; but, although there is only this slight difference in size, I think there can be no doubt that the three narrower rows represent the three missing folds (II., III., and IV.), while the three wider rows are the spaces between them.

The first left fold has two rows of narrow meshes (three

Ꮓ

and five stigmata), and is separated from the dorsal lamina by twelve stigmata. The rest of this half of the sac resembles the corresponding region on the right side. The second, third, and fourth folds are represented by single rows of meshes, five, six, and five stigmata wide respectively, while they are separated by rows of wider meshes (nine or ten, eight, and seven stigmata); the fourth is separated from the endostyle by a space of twelve stigmata.

Figure 9 gives only one-half of the sac (the right side), and is brought in to show a condition of the first fold even more rudimentary than in figure 8. As in that sac, it does not project internally, and is composed of five rows of narrow meshes, but these meshes are wider than those of the sac shown in figure 8; they contain each two or three stigmata, and are precisely like the rows of narrow meshes in the rudimentary folds.

The remaining folds on this side of the sac (II., III., and IV.) are represented each by a single row of narrow meshes (three stigmata), and they alternate with single rows of wider meshes (seven, six, five stigmata). The fourth is separated from the endostyle by a space of ten to twelve stigmata in width.

In conclusion, there are two points which I think this investigation has brought out clearly.

1. The immense variability which may exist within the limits of a single species in the details of structure of an important organ like the branchial sac.

2. The continuous series connecting a well-developed branchial fold (such as I. on the right side, in figures 4 and 5) with the most rudimentary indication of where the fold ought to be. Figures 4-9 show examples of all stages between these extremes. Commencing with I., on the right in figure 5, we pass through the corresponding folds in figures 4, 7, and 6 to figure 8, where there is no projection, but the bars are still close; then to figure 9, where the bars are more

open

and the meshes of the same size as in I. on the left in figure 4. Then we pass to I. on the left in figures 5 and 6, where there are only three rows of narrow meshes; then through two rows and a single row, both of which are well represented in the diagrams, to a single row extending only part of the way, as in IV. on the left, figure 5, and IV. on the right, figure 6. Then the step is short to the last stage, shown at X. in figure 5, where fold IV. on the right is entirely absent.

EXPLANATION OF THE PLATES.

PLATE I.-Chart of Diagrams (page .)

Fig. 1. Diagram of transverse section of branchial sac of Styela

grandis.

Fig. 2. Diagram of transverse section of branchial sac of Styela

oblonga.

Fig. 3. Diagram of transverse section of branchial sac of Styéla

grossularia.

Fig. 4. Diagram of a typical branchial sac of Styela grossularia. Figs. 5-9. Diagrams of branchial sacs of Styela grossularia, showing some of the more important variations.

D. L.-Dorsal lamina.

En.-Endostyle.

I., II., III., IV.-The branchial folds.

PLATE II.—All the figures represent parts of the branchial sac of Styela grossularia, van Beneden, seen from the interior. Fig. 1. A part showing the alternating series of wide and narrow

meshes.

Fig. 2. The right dorsal region, showing a well-developed first

right fold.

Fig. 3. A part showing an almost rudimentary fold where the internal longitudinal bars stop, and the rows of narrow meshes are only partial.

Fig. 4. A feebly-developed first right fold, where there is no projection, and only four rows of narrow meshes.

br.f.-branchial fold.

i.l.—internal longitudinal bar.