Peridiniales. Dinophysis baltica Glenodinium bipes Lohmanniella oviformis stellaris n. sp. Mesodinium rubrum Tintinnopsis beroidea Gonyaulax catenata Peridinium finlandicum Jachromaticum (?) Difflugia lobostoma sp.balticum (?) Ciliata. Didinium Gargantua Laboea compressa conica delicatissima vestita Ebria tripartita Systematically Unknown Forms. Radiosperma, corbiferum Xanthidium multispinosum Some solitary nauplii were found, but their species could not be determined for want of sufficient material. This was also the case with a very few cells of Peridiniales and various more or less spherical cells, perhaps ova, Infusoria or only the contents of broken walled individuals. A few cells of pollen of foliferous trees and firs had been driven out into the sea. After this summary we shall proceed to the discussion of the separate species. DIATOMACEAE. A. OCEANIC SPECIES. Achnanthes taeniata GRUN. (Fig. 2.) 1880 Cleve and Grunow, p. 22, pl. 1, f. 5. 1894-95 Cleve II, p. 189. 1896 a Cleve, p. 5. 1896 b Cleve, p. 4, 13. 1897 b Gran, p. 9, pl. 1, f. 10. 1897 Östrup, p. 329, 353, pl. 2, f. 15. 1905 Jörgensen, p. 105, pl. 8, f. 27. 1908 Gran, p. 122. 1910 Meunier, p. 326, pl. 33, f. 41-44. When the low salinity of the Finnish waters was taken into consideration it was probable, that a rather poor plankton would be found. On that account this treatise was begun with some scepticism, but the results were encouraging. Especially Achnanthes taeniata was very interesting, occurring very abundantly, particularly in the inmost part of the Gulf of Finland, where the River Neva carries much nutritive water into the sea. The maximum number of cells pr. litre was here four millions. The cells are very small compared with many other plankton-organisms; but when the long chains occur in such large quantities as here, they can be observed with the naked eye. Achnanthes taeniata of May gave very interesting material for studying the development of the resting-spores. Every phase of development was to be found from the delicately-walled cells to the thick-walled finished resting-spores. The slender-walled cells were often very beautifully preserved with quite distinct H-formed chromatophores. During the formation of the resting-spores the contents of the cells will be coloured formed by cell-division are apparently placed two in each cell. At the same time the chains will twist into spirals. black by the osmic acid in the preserving fluid. The spores being The breadth of the chains examined was 13-30 μ. Both the thin-walled chains and those with resting-spores occurred at every station, the resting-spores for the most part dominating. This species occurred most abundantly in the Gulf of Finland, especially at the inmost station (F 41). The maximal number of cells pr. litre amounting to 2600000 cells without resting-spores and 1305000 resting-spores was found at the depth of 5 metres. At this station the spore-free cells still dominate. Towards the bottom the number of the resting-spores increase in proportion to the thin-walled cells, this fact confirming the rule, that the heavy resting-spores will sink more rapidly than the lighter thin-walled cells. But any considerable sinking could not be noticed yet. The richly laden upper layers of the water indicate that this species is still in energetic development. At the other two stations in the Gulf of Finland, viz: F 50 and F 61, great numbers of Achnanthes taeniata are to be found, decreasing from east to west. At these stations the resting-spores prevail, and the development has proceeded further that at station F 41. While the maximum has not sunt anything worth mentioning at station. F 50, we find it at the depth of 40 metres at station F 61. The diatom is here decreasing, having begun to sink towards the bottom, the greater number of the cells forming resting-spores. At the two stations F 23 and F 30 in the Gulf of Bothnia the behaviour of this species is rather different. At the most northerly station (F 23) we find it pretty numerous with the maximum in the upper layers of the water, but at the same time with resting-spores dominating. The last mentioned fact indicates the retrogression of the species. At station F 30 probably this diatom, occurring in very few cells has almost finished its development and will soon disappear from the plankton. At station F 74 in the northern part of the Baltic it is found somewhat more abundantly than at station F 30, the resting-spores being numerous and the maximum number of cells occurring at the depth of 30 metres. Chaetoceras debile CLEVE. 1894 Cleve, p. 13, pl. 1, f. 2. (Fig. 3.) 1895 Östrup, p. 456, pl. 7, f. 89. 1895 Ch. vermiculus Schütt, p. 39, f. 7 a-c. 1908 Gran, p. 92. 1910 Meunier, p. 242, pl. 27, f. 19-23. 1913 Meunier, p. 43, pl. 7, f. 1-11. A few solitary specimens of some resting spores were to be found. These had two spines on one of the valves and were very probably belonging to Chaetoceras debile. The breadth of the chains being very little (8-12) the two little wort-like structures of the typical Ch. debile were not distinctly developed; but Meunier (1913) has resting-spores (pl. 7, f. 11), which are quite similar to those of the Finnish material. Fig. 3. Chaetoceras debile. te ven a drawing of this species with This diatom is not specified in the tables, being only distinguishable from the other Chaetoceras-spores on closer examination and higher magnifying powers, than used when counting. But it can be said with great certainty, that this species occurred in minimal numbers compared with Chaetoceras Wighami, which will be dealt with beneath. The specimens drawn were found at station F 23 at the depths of 0 and 10 metres. It is not noted from the other stations, but the possibility that is has been overlooked is not improbable. 1896 Ch. bottnicum Cleve, at Aurivillius, p. 14, pl. 1. 1897 a Ch. Wighami Gran, p. 27. 1901 Levander, p. 6, 13. 1908 Gran, p. 88. 1910 Meunier, p. 244, pl. 27, f. 26. 1913 Meunier, p. 42, pl. 6, f. 32-34. Chaetoceras Wighami was found at every station occurring both as very thin-walled cells and as resting-spores. The cells were either observed singly or in short chains consisting of very few cells. The resting-spores lying one in each cell were decorated on both valves with fine spines, which were easily seen on the larger cells, but were very indistinct on the smaller ones. The breadth of the chains was 6-17, the greater number of the resting-spores however not exceeding 10 u. At station F 41 Ch. Wighami occurred rather numerously being here without resting-spores, while only a few straggling cells or spores were found at the stations F 50, F 61, F 74 and F 30. At station F 23 the resting-spores were dominating with maximal number at the depth of 30 metres. At this station it is still fairly numerous, but no doubt in regression. Coscinodiscus lacustris GRUN. (Fig. 5.) 1884 Grunow, p. 85, pl. 4, f. 30-33. 1899 Van Heurck, p. 525. 1910 Meunier, p. 274, pl. 30, f. 33-34. The undulated valve of Coscinodiscus lacustris having one excentric concavity and one excentric convexity can be dissolved into narrow sectors consisting of 3 (2-4) rows of pores. The sectors are separated by thickened lists just inside the margin, while the very border of the valve is very delicately radially striped. The size was highly varied, the diameter ranging between 20 and 70 u. In spite of Coscinodiscus lacustris being a very characteristic species, when it was deprived of its contents by intense heating, it was rather difficult to determine this species, when it was counted, as the contents were of a dense black colour. When looking at the valve it was particularly difficult to distinguish the small cells of this species from those of Thalassiosira baltica. On that account the numbers put down in the tables concerning these two species are not quite exact. Under Coscinodiscus lacustris are noted every cell, which has been determined with certainty as this diatom, while those, of which the identification is uncertain, are counted as Thalassiosira baltica, this species on closer examination appearing to be the dominating one. |