The Platypus has heavily adapted over the course of its evolution. Many people have heard the phrase "God has a sense of humor, just look at the Platypus." This is said because the Platypus has traits of both reptiles, birds and mammals.
Some of the many many adaptations of the Platypus include:
Tail that stores fats. This long flattened tail is also used for swimming.
Venom is used to fight off attackers and used for competition among each other. There is a spur located on each foot that can sting. This venom is a DLP which stands for defensin like protein. The venom is used to kill or wound prey or predators. Interestingly enough, only the males have a functional spur and use it in the mating season for dominance.
Electroreception. This Platypus can use receptors on its bill to sense the contracting fibers of muscle in animals nearby. This is used to aid the animals in finding prey and also in sensing predators. However, water must be used as a medium.
Ten sex chromosomes. Unlike normal mammals who have just two sex chromosomes (XY in male and XX in female) the Platypus has ten! Also, a Platypus lacks a gene, SRY, which is used to determine sex in mammals. Therefore, it is genetically impossible to determine the sex of a Platypus. Of course, there are physical attributes to differentiate between the two.
Picture From: http://www.ryanphotographic.com/platypus.htm