When kept under identical conditions, Uca pugnax regenerates limbs and molts more rapidly than U. pugilator from the same location. 2. The tropical species U. rapax and U. speciosa also regenerate faster than U. pugilator from the same location. U. thayeri is the slowest to replace missing limbs, the slowest to harden the carapace after ecdysis, and also requires the greatest injury before autotomy will take place. 3. When kept in groups, U. thayeri stops regeneration after basal growth and will not molt; U. pugilator is only slightly retarded when kept in groups. U. rapax is also affected by grouping, showing a lengthened proecdysial phase and terminal plateau, thus also delaying molting. 4. Newly regenerated limbs of U. rapax, U. thayeri, and U. speciosa, aside from being lighter than old limbs, have a conspicuous banding pattern. This pattern becomes less obvious during the week or two after ecdysis as melanophores move into the interband regions and the limb as a whole darkens due to increased dispersion of pigment in the melanophores. In U. pugilator the newly regenerated limbs are very pale in color and remain that way for several months under laboratory conditions, even when animals are maintained on a dark background. 5. When treated with 0.5 mg/liter methylmercury, growth was inhibited entirely in U. thayeri. Partial inhibition was seen in U. pugilator, and the least inhibition in U. rapax. A few individuals of U. rapax were able to complete regeneration and molt, but there was no melanin in the regenerated limbs. 6. Inhibition of melanogenesis in regenerated limbs was also seen in U. thayeri and to a smaller extent in U. pugilator at 0.1 mg/liter methylmercury. The lack of black pigment may be due to an inhibition of cell migration but more likely of melanin synthesis. Some of these crabs developed melanin when kept in clean water after ecdysis. 7. Seasonal differences were noted in all species, but especially in U. Thayeri. In this species, regenerated occurred much more rapidly in March-April than in January.
