In this connexion must be mentioned the interesting experiments of W. F. R. Weldon[155] upon the respiratory functions of _Carcinus maenas_ at Plymouth, since these were the first noteworthy observations directed towards the exact measurement of the action of natural selection upon any animal, a field of observation in which Weldon will always be looked upon as a pioneer. An extended series of measurements by Weldon and Thompson on male specimens of _Carcinus maenas_ of various sizes between the years 1893 and 1898 showed a steady decrease in the ratio of carapace breadth to length; the Crabs appeared to be becoming steadily narrower across the frontal margin, and the same thing, though not to the same extent, was happening in female Crabs. Weldon supposed that this change might be correlated with the silting up of Plymouth Sound and the consequent fouling of the water. To test this hypothesis he kept a very large number of male Crabs in water to which fine porcelain clay was added and kept in continual motion. In the course of the experiments the survivors and the dead were measured, and it was found that the mean carapace breadth of the survivors was less than that of those that succumbed. The experiment was repeated with the fine sand that is deposited and left at low water upon the stones on Plymouth beach, and the same result was observed. It was also noticed that the individuals which died had their gills clogged with the sand, while those that survived had not. As a further confirmation, a great many young male Crabs were isolated and kept in pure filtered water, and they were measured before and after moulting; these measurements, when compared with measurements of the frontal breadth in Crabs of the same size taken at random upon the beach, were found to show a greater breadth than the wild Crabs, thus indicating that a selection of narrow Crabs was taking place in Nature which did not take place when the Crabs were protected from the effects of fine sand in the water. The whole chain of evidence goes to show that the carapace breadth in _Carcinus maenas_ in Plymouth Sound is being influenced by the rapid change of conditions occurring in the locality. Various objections have been urged against this conclusion, but, though they merit further investigation, they do not appear very weighty. The fresh-water Crab, _Thelphusa fluviatilis_, common in the South of Europe and on the North coast of Africa, belongs to the Cyclometopa, and is interesting from its direct mode of development without metamorphosis. =Fam. 1. Corystidae.=—The orbits are formed, but, unlike all the other families of the Cyclometopa, are incomplete. The body is elongate and oval, and the rostrum and front edge of the mouth rather as in the Oxyrhyncha, in which Tribe they are sometimes included. _Corystes_, with a few species in European seas. _C. cassivelaunus_ at Plymouth.