The Naviculae and Frustuliae place themselves, two and two on the side of their pedal surface, and remain sometimes whole days in that position, and separate themselves again. I have seen a pair of the Frustulia agrestis (fig. 14. a.) united in this way, go slowly asunder, and remarked that, in spite of that apparent separation, both individuals were joined by two very narrow tubes (fig. 14. b. c.). And in magnifying them still more, I observed that these tubes passed through the cuirass and communicated with the brown content. I saw also upon the orifice of the tubes an areola forming itself by a thicker and darker content. They remained united about an hour, after which the separation took place as follows: The tube (c.) drew back in the animalcule (fig. 14. b.), and the tube (d.) towards (f.). Soon afterwards, having been able to observe the animalcule (f.) from downwards, or from the pedal surface, I saw (fig. 15.) in the middle line upon (d.) a larger opening, and upon (c.) a smaller one. In the last, the tube (fig. 14. b. d.) had drawn back; and the tube of the other animalcule entered into the first. In looking sidewise, I discovered, though rather indistinctly, both openings (d. c.).
Under the opening of the mouth, we find in the Pharyngoglossa, under the middle line and under the alimentary tube, a small round wart (fig. 16. d. and 16. b. d.), perforated in the middle, of a very doubtful function, and which, considering its position, I bring under this head.
