be easily recognised: 1st, the dorsal lamina, a narrow membrane, running down the centre of the dorsal wall, and projecting into the interior of the sac; 2nd, the fold placed on the right side, near the dorsal lamina, and separated from it by an area twice or thrice as broad as an ordinary mesh, and not traversed by any bars; 3rd, the remainder of the branchial sac, apparently destitute of folds, but divided by the internal longitudinal bars into rows of meshes which are not all equal, but are seen to be distinctly of two sizes, the larger of which is twice or thrice as wide as the smaller. The smaller meshes are placed two or three rows together, and are simply tracts along which the internal longitudinal bars are more closely placed than elsewhere. These tracts represent, I believe, the missing folds. In the great majority of the sacs examined they were separated by two or three rows of the larger meshes, and seven of them were present, three on the right side, where the single fold is placed, and four on the left side, where there is no fold, thus completing the eight characteristic of the genus Styela. Sometimes the most ventrally placed tracts of narrow meshes, those next the endostyle on each side, are either absent or reduced to a single row of meshes each. This was to be expected as, even in Ascidians with well-developed branchial folds, the pair next the endostyle are smaller than the others, and occasionally rudimentary.

The larger meshes contain usually six or seven stigmata,* (Pl. II., fig. 1), but as many as eight have been noticed, and sometimes there are only five. The smaller meshes have usually three or two (rarely one or four) stigmata each (Pl. II., fig. 1.) There are four rows of very much larger meshes in the sac, one on each side of the dorsal lamina, and one on each side of the endostyle. These contain usually sixteen to

As the stigmata are approximately equal, the width of the meshes may be measured by the number of stigmata they contain.

eighteen stigmata, but the number may descend as low as twelve or, in one case, ten; on the other hand, it may rise to twenty or, in one case, twenty-three. On the right side of the sac the dorsal row of wide meshes is followed by the branchial fold, which is traversed by usually eight or nine internal longitudinal bars (Pl. II., fig. 2); the number rarely exceeds this; ten was noticed only once or twice, and no greater number was met with; a smaller number of bars is, however, sometimes, present, and it may fall as low as four or five, in which case the fold is exactly like one of the longitudinal tracts of smaller meshes. (Pl. II., fig. 4.)

On the left side of the sac, immediately following the dorsal row of wide meshes, and consequently in the corresponding position to the fold on the right side, is a series of about four closely-placed internal longitudinal bars forming three rows of narrow meshes. (See Pl. I., fig. 4, I.) That these rows represent a fold there can be no doubt. They occupy exactly the position of one of the missing folds, and have the same structure as the fold on the right side in its more rudimentary condition; hence we may safely consider them as the dorsal or first rudimentary fold on the left side. Following this are usually two, sometimes three, rows of wide meshes, containing each six or seven stigmata, and representing the space of normal meshes between two folds. This space is bounded ventrally by the second rudimentary fold of the left side, which generally consists of two rows of narrow meshes, containing each two or three stigmata. Beyond these rows on the ventral side is the second space between folds represented by two rows of wide meshes, and followed by the third rudimentary fold of the left side, a series of two rows of narrow meshes. Then comes the last space between folds consisting of either two or three rows of wide meshes, and bounded ventrally by the fourth and last rudimentary fold of the left side, which is frequently represented by

a single row of narrow meshes, and may be entirely absent in some parts of the sac, on account of one of the internal longitudinal bars being broken in these places, and thus allowing the smaller meshes to communicate with the adjoining larger ones. This last fold is separated from the endostyle which marks the ventral edge of the sac by the ventral series of very wide meshes, which contain twelve to sixteen stigmata each.

The right side of the sac has, near the dorsal edge, the one well-marked fold which has been already described; and then the second, third, and fourth folds of the right side, which are all rudimentary, and have exactly the same structure as the corresponding series of meshes on the left side. The spaces between these folds are also usually exactly similar to those which have been described for the left side of the branchial sac.

Such is the normal condition of the branchial sac in this species, judging from the specimens I have examined, but it is liable to great variations, and scarcely two individuals are exactly alike, while in many there are great differences in structure; in the series of individuals examined I have found nearly every stage between the well-developed projecting first fold on the right side, with nine or ten rows of narrow meshes, and the very rudimentary fourth fold on the left side, consisting of a single row of narrow meshes, interrupted here and there, so as to leave not even the slightest indication of a fold.*

The accompanying chart (Plate I.) shows some of these variations diagrammatically. Each of the long lines (in Figs. 4-9) represents a horizontal section through a branchial sac which has been slit up along the bottom of the

* I should add that all the branchial sacs described in this paper are from adult individuals with fully developed genital organs, consequently, none of the irregularities can be due to immaturity.

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