. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . areabsent from the walls of all those diatoms whichpossess a raphe, because some species of Gyrosigma(= Pleurosigma), and also Epithemia Hyndmanni,possess both a raphe and fine pores, and it isreasonable to suppose that a similar structure ofthe wall occurs in other diatoms. In Eupodiscus Argus, which is a frequentmarine diatom, there are numerous cup-shapedchambers opening widely to the exterior, the wallsof these chambers forming an areolar
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . areabsent from the walls of all those diatoms whichpossess a raphe, because some species of Gyrosigma(= Pleurosigma), and also Epithemia Hyndmanni,possess both a raphe and fine pores, and it isreasonable to suppose that a similar structure ofthe wall occurs in other diatoms. In Eupodiscus Argus, which is a frequentmarine diatom, there are numerous cup-shapedchambers opening widely to the exterior, the wallsof these chambers forming an areolar network ofridges on the surface of the valve. The walls ofthe chambers are finely papillate, and severalminute, obliquely sloping canals perforate the baseof each chamber (fig. 62 D). The structure of Triceratium Favus is some-what similar, only the chambers opening to the exterior are polygonal andtheir walls are smooth. Minute poroids occur on the inner side of the innerwalls of these chambers, and pore-canals pass through the flange of the valveas minute tubes (fig. 61 B and D). In Isthmia nervosa there are larger primary and smaller secondary. B Fig. 63. Stephanopyxis Pal-meriana (Grev.) Grun. toshow the spines on the valvesby which the cells are unitedto form chains. B is anenlargement (x 1000) of threepairs of these spines toshow their tubular character.(After 0. Miiller.) Structure of cell-wall 91 B chambers. The inner walls of the secondary chambers possess a numberof poroids, and pore-canals are present here and there, passing ratherobliquely through the separating ridges (fig. 62 B and (7). In some diatoms, such as Stephanopyxis Palmeriana (fig. 63), Sceletonenmcostatum, and Lnuderia annulata, O. Mtiller (01)has shown that the spines by which the cells areunited to form chains are really tubes, throughwhich there is a protoplasmic continuity from cellto cell. The most numerous and best-developed poresare found in the centric diatoms. O. Miiller hassuggested tha
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