If I should ask you whether the nest of a bird constituted a part of the organism or a part of its environment, I presume that everyone present would resent the question as an insult to his intelligence. [...] The situation becomes somewhat less clear, perhaps, when we consider the calcareous tube of a marine annelid. Here is something which is definitely secreted by the epidermal cells of the organism, and which forms a sort of permanent integument. It does not, however, in this case retain any organic connection with the body of the worm. But when we pass to the shell of the mollusc we find that there is such an organic connection with the body, so that the animal cannot be dislodged without extensive injury to its living tissues. [...] Does such a shell belong to the organism or its environment?