We describe the design and implementation of Jack-in-the-Net (Ja-Net) platform software in a wireless LAN environment. Ja-Net is a platform for adaptive services in a large scale, open network environment. Ja-Net achieves built-in capabilities to create/emerge services adaptability according to dynamically changing network conditions and user preferences. A service is implemented on the Ja-Net platform by a collection of autonomous system components called cyber-entities. Cyber-entities are autonomous with simple behaviors and interact with each other using Ja-Net ACL (Agent Communication Language) to jointly provide a service. For instance, a cyber-entity migrates from node to node and duplicates itself. Also, a cyber-entity may establish a relationship with an interaction partner to form a group to provide a service. The relationship strength between cyber-entities is a measure of the usefulness of the relationship and is adjusted based on the level of satisfaction indicated by a user who received the service. We design and implement the Ja-Net platform on notebook PCs connected by a wireless LAN. We implement the sample application AddAppli on our Ja-Net platform.