APPENDIX I

ON comparing the East Spitzbergen species found by Professor Kükenthal with the West Spitzbergen species found by Professor Nathorst, we concluded that they are identical, but that L. Spitzbergensis differs considerably from L. glacialis in size and in the shape of the caudal plate. We were at first disposed to consider it a new species, especially on account of its possessing second antennæ which were said to be wanting in Lepidurus glacialis. Closer examination, however, showed it to be a small variety of L. glacialis, most probably derived from the latter by being obliged to ripen at an earlier stage of development, in adaptation to the shortness of the more northerly summer.

That this view is correct seems probable from the following considerations:

(1) The possession of second antennæ does not distinguish it from L. glacialis, for we have succeeded in finding these appendages on the latter.

(2) The position of the sperm-forming centre (see § on reproduction) is identical in the two.

(3) The genital tube is very much simpler, the diverticula showing hardly any traces of branching, therein exhibiting a more larval condition.

(4) The same may be said of the smaller size of the caudal plate, which develops gradually, as Brauer has shown in his paper on the development of L. productus.

(5) The small size of the whole animal also agrees with the supposition.

It is interesting to find that Packard's measurements for L. glacialis (from Cape Krustenstern ?) make it even smaller than the Spitzbergen variety. From this, however, it is difficult to draw any certain conclusions, as his drawings give a fully-developed tail-plate (see Monograph of the North American Phyllopoda). It thus appears that L. glacialis may be much stunted by unfavourable surroundings.

That the specimens from Spitzbergen were not young specimens follows from the facts that they (several hundred) were nearly all the same size, and that they were caught in the end of August, a week or so before the close of the short summer, while the freshwater pools were still unfrozen. Professor Kükenthal informs me that this season in the latitude in which they were found lasts about ten weeks.

Packard's measurements for a fully developed L. glacialis make it doubtful whether we are to look upon this variety as permanent. It is possible that in favourable summers they may further develop (without any great increase of size) into stunted L. glacialis. This question, however, can only be certainly answered by cultivating specimens further south, in an aquarium, to see whether they develop into L. glacialis. In the meantime it will be useful to call the animal L. glacialis var. Spitzbergensis, or, for shortness, L. Spitzbergensis.

APPENDIX II

THE EYE-PIGMENT OF APUS

It was very difficult to decide whether the cells marked p in the diagram (Fig. 43) of the eye of Apus were really cells, as there drawn, or only collections of very minute pigment cells. [Grenacher, in his drawings of the single eyes of Apus, leaves the matter rather indefinite. He indicates rather than draws the pigment cells with nuclei. His drawing leaves the impression that he took it for granted that they were large pigment cells, without actually ascertaining the facts.] We were at first inclined to take the latter view, having found that under a very high power, 1 the granules themselves were not easy to distinguish from cells. Each one consists of a stainable nucleus surrounded by a pigment crust, the whole being enclosed in a layer of some hyaline substance. These "cells " were of all sizes (from 1-2 μ), and were found in all stages of fission (see Fig. 65). There are thus two ways of regarding these pigment masses in the eye of Apus. Either the whole is a kind of loose syncytium of minute pigment cells, as we at first thought, or these pigment granules are formed inside a large cell around stainable protoplasmic granules, as starch is formed round the leucoplasts. This we now think to be the case.

1 Zeiss apochromatic 2 mm. homogeneous immersion, 1.40 n.a., eye-piece No. 12, giving 1500 diam.

Although we cannot be certain that we have seen the nuclei of the large pigment cells as shown in the diagram (Fig. 23, p. 139), we concluded that there must be such nuclei, and that the pigment masses were real cells and not syncytia. We were chiefly led to this conclusion by noticing the long regular lines of granules running down the nerves towards the optic ganglion, as shown in the diagram. It seemed to us that these rows of single granules would not be so straight and even, unless enclosed within a long pseudopodium-like process of the pigment cells. Were the granules semi-independent cells, their arrangement could hardly be so straight and regular. We

FIG. 65. Pigment granules (? cells) from the eye of Apus, X ca. 3000, showing a stainable nucleus, surrounded by a thin crust of brown pigment, the whole enclosed within a hyaline substance.

were further induced to take this view from finding that, in some specimens, the pigment in the unpaired "eye" was composed of similar eye-pigment granules, also arranged in long pseudopodium-like strands. In most of the specimens. examined, the pigment in the unpaired "eye" was similar to that in the pigment cells of the rest of the body, i.e. it was in the form of very minute olive green granules. The occasional finding of eye-pigment in the unpaired "eye" was especially interesting in reference to the origin we attributed to that organ out of an anterior pair of Annelidan eyes.

Around the paired eyes, the green pigment reaches up to their very rim, and indeed stretches over the outer edges of

the eye itself, but there it changes into the black brown granules above described.

These "eye-pigment" granules certainly appear to be very primitive formations. The utilisation of excretory

matter as pigment is at once suggested, the incrustation of brown stuff round the nucleus reminding one forcibly of the incrustation of excretory matter round the blood corpuscles under the dorsal organ (see Appendix IV.). In the pigment granules, however, it was quite regular, whereas it was irregularly massed around the blood corpuscles. These corpuscles, again, are very much larger than the pigment-forming granules, and moreover fairly uniform in size, whereas the latter are of all sizes.