In outward appearance many of the mammals have changed little since they evolved from the non-mammalian synapsids in Late Triassic times. Throughout the Jurassic, Cretaceous, Paleogene and Neogene periods they have remained small, compact creatures, not adapting into any of the wide ranges of lifestyles occupied by the great reptiles and dinosaurs. However, some of them have a number of interesting specializations. The surviving groups in the world are monotremes, marsupials and placentals (multituberculates died out back in the Paleogene).
The zwim is a semiaquatic, insectivorous placental mammal. It inhabits the streams and rivers of the Palaearctic ecozone and is particularly common in regions of deciduous forest. It has a length of about 30 centimeters (1 foot), most of which is taken up by a long flattened tail. The tail, and the long webbed hind feet, allow the animal complete freedom in the water. Its long sensitive snout is used for probing under stones and in dead vegetation for the insects and other invertebrates on which it feeds, both at the bottom of the stream and on land. It lives in burrows on the river banks and can defend itself against predatory reptiles and fish by biting with its sharp teeth. The saliva is venomous and any bite is quickly effective. The zwim is a social animal and as many as a dozen burrows can be found within a short distance from one another on heavily wooded river banks. Large numbers may congregate at the wallowing-pools of the bricket in order to feast on the parasites that are shed there.