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only to find means    to prevent itself from being trapped    by    the

surfacc    tcnsion, it    is also exposed to heat and cold,    to    the

dangerous actinic rays of the sun, to the buffeting of the wind, the force of the surface currents, and to the blows of hail and rain. Physically the environment is harsh and dangerous in the extreme. Biologically it is eąually dangerous. Those that float on the    surface of    the water are exposed to attack above    and

below:    every eye    can see them and there is no hiding    place.

Exposed to such dangers the Gyrinidae should be highly evolved and they are. Their perfection is wonderful. Their habit is also adapted to their protection. The study of their stru-cture and habits fills one with delight, every detail is marvel-lous in its perfection.

Both above and below the body is stream-lined and de-void of projections. The exoskeleton is hard, compact and rigid. Throughout the group the yariation in size is smali. Ali thus derive reasonable benefit from the surface tension: the smali being convex, the large dorsally flattened, so that each obtain the maximum advantage. Species swimming in open water and diving seldom, such as G. subsłriałus and G. marinus are roun-ded and have the front of the head above the water-line blunt, while those that swim in swift streams and dive often as does O. uillosus are elongated and have the head flatter and morę pointed in front. G. paykulli and other species which swim amongst reeds also have an elongated form. The reason for this is unknown. The advantage of an elongated form is offset by the difficulty which it offers to rapid turning. Placid water forms steer like an oarsman by varying the force of the leg strokes one sidc paddling morc vigorously than the other. It is believed that they may also utilize the resistance of the first pair of legs. Swift water species of elongated form offset the disadvantage by possessing, in their narrow last abdominal segment, suppleinented, I believe, by their setose parameres, an efficient rudder. That they steer in this way cannot be doubted for they can be obseryed to heel over when turning to such a degree that the pale under-surface is clearly to be seen. The means of propulsion is the middle and hind pairs of legs. The-se are short, broad and fiat; the segments so overlapping that, at the back stroke, they support each other and offer a single