FLORA AND FAUNA OF MILL RACES
37
in the head-race contained about 12,000 rotifers per cc. Another sample close by contained an estimated 20,000 per cc., and in a third, the numbers feil as low as about 500 per cc. Numbers were sometimes very different, even at either end of a particular moss shoot, and no adequate reasons have been found for this. In some instances, crowding was largely confined to the mossbases, and in others, to the apicesof the mossshoots. Sometimes no such preferences were apparent. It is possible that, at the bases, the crowded stems and rhizoids afforded protection from the current and, at the apices, the closely set leaves did the same. Rotifers were nearly always found in the axils of the leaves, and seldom on the bare stem. The leaves of the Willow Moss (Fontinalis) are large, about 4 mm. long and boat-shaped, and where they occurred, they were often packed with rotifers on the concave side in the lee of the flow. Nematodes and Suctorians also occurred in high local concentrations, with comparatively small numbers elsewhere. No corresponding differences in environment were apparent. The Oligochaetes Nais and Elaeosoma, though their smaller numbers made it less dramatic, showed the same patchiness. These irregularities, while very interesting, greatly increased the difficulty of estimating numbers.
T H E DISTRIBUTION OF
Cordylophora lacustris
Cordylophora deserves a little more attention because of its intrinsic interest and because, according to the textbooks, it is in entirely the wrong habitat. In June, all the Cordylophora was greyish, but in August, the colonies or mats in the faster water were a bright salmon-pink, those in slower water remaining grey-white. The pink colonies were fertile at least during the period August lOth to 29th, when they were studied. They produced both male and female gonophores on different upright stems, the gonophores replacing tentacled heads at the end of the short lateral branches. The male gonophore is an oblong whitish sac about 1.5 mm. long and contains tadpole-shaped sperms. The female gonophore is a sac at first opaque, then transparent, containing 5 to 15 spherical eggs. When fertilised, an egg elongates to resemble a small lozenge, develops cilia, and escapes as a planula larva, which soon settles down to produce a new colony. It was soon evident that Cordylophora was growing more vigorously in the faster water ; the upright branches were longer, and the tentacled heads bigger. But Pennak in " Fresh-water Biology of the United States " says that it grows in static or slowly flowing water. Allman in his monograph on Cordylophora