Fig. 24 is another stalked diatom called Gomphonéma acuminátum, found commonly in ponds and ditches. There are nearly forty species belonging to this genus. A pair of frustules are also shown which exhibit the beautiful flinty outline without the coloured contents (technically called endochrome). Fig. 27 is a side view of a beautiful diatom, called Eunótia diadéma from its diadem-like form. There are many species of this genus. When seen upon the upper surface, it looks at first sight like a mere row of cells with a band running along them; but by careful arrangement of the light its true form may easily be made out. Fig. 28 represents a very common fresh-water diatom, named Melosíra várians. The plants of this genus look like a cylindrical rod composed of a variable number of segments, mostly cylindrical, but sometimes disc-shaped or rounded. An end view of one of the frustules is seen at the left hand, still coloured with its dots of “endochrome,” and showing the cylindrical shape. Immediately above is a figure of another frustule seen under both aspects with the endochrome removed. A rather curious species of diatom, called Cocconeïs pedículus, is seen at Fig. 29 as it appears on the surface of common water-cress. Sometimes the frustules, which in all cases are single, are crowded very closely upon each other and almost wholly hide the substance on which they repose. Fig. 30 is another diatom of a flag-like shape, named Achnanthes, having a long slender filament attached to one end of the lower frustule, representing the flag-staff. There are many wonderful species of such diatoms, some running almost end to end like a bundle of sticks, and therefore called Bacillária; others spreading out like a number of fans, such as the genus Licmophora; while some assume a beautiful wheel-like aspect, of which the genus Meridion affords an excellent example. A very remarkable, and not uncommon, fresh-water diatom is the Bacillária paradóxa. It looks, when at rest, like a broad brown ribbon of varying length. The diatoms lie across the ribbon, on edge, and slide upon each other exactly like the ladders of a fire-escape, so that the broad ribbon is converted into a fine long thread, which speedily closes up again into the original ribbon, and so _da capo_. The reason for this movement, and how it is effected, is absolutely unknown; indeed, nothing certain is known as to the way in which diatoms move, nor has ever a probable guess yet been made.