Meme-Free Zone

I'm thinking that the exoskeleton doesn't need to have permanent openings for many functions. For excreting, for example, your internal blob of material hits the inside of the exoskeleton, at which point the exoskeleton begins to develop on the inside edge of the blob (the outer edge is already against the exoskeleton). Once the inner membrane is complete, the exoskeleton splits, the blob is expelled, and then the exoskeleton fuses again. Once it is fused the membrane dissolves. Could be relatively quick.