Exostome structure

The exostome is formed from the common periclinal cell wall pair between the inner wall of the OPL and the outer wall of the PPL. The endostome is formed from the common periclinal cell wall pair between the inner wall of the PPL and the outer wall of the IPL (Figure 12.1). Autolysis of the anticlinal walls of the cells between the exostome and the endostome separates the two rings of the diplolepidous peristome, while autolysis of the periclinal walls between adjacent teeth separates the structures within each ring of the peristome. In reduced peristomes this autolysis may be incomplete, leaving adjacent structures partially attached. 

Preperistome plates adhere to the outer face of the exostome and are formed by deposition in the cell layer external to the OPT. These plates were previously used to delimit Mesonodon (Buck, 1980 Gradstein et al., 2001). However, these structures have been found to occur sporadically in a range of species in both Entodon and Erythrodontium and to be lacking in many specimens of Mesonodon (Figure 12.2D). They appear to be an occasional by-product of peristome reduction and of no taxonomic significance in this family. 

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