Wall of digestive tract is not digested because of the following reasons :
1. There is a layer of glycocalyx inner to enteric epithelium which checks the action of enzymes.
2. The protein digesting enzymes are always in inactive stage. They become active on getting mixed with other types of enzymes.
3. There is lining of mucus inner to enteric epithelium which dilutes the action of enzymes on wall of alimentary canal.
Diphyodont is the condition of having two set of teeth during the life of an organism. For example : humans are diphyodont as they have two set of teeth namely temporary milk or deciduous teeth which are replaced by permanent or adult teeth.