Among cytheracean Ostracoda the normal pores, through which sensory receptors reach the exterior lateral surface of the valve, have been classified as simple or sieve-type. Although the function of the sensory receptors is only partly understood, the morphology of the normal pores through which they pass has been used as a taxonomic criterion at the generic and family levels. It is recognized that both sieve-type and simple normal pores may occur in the same family, but genera have been regarded as having one type or the other.

Scanning electron microscopy of normal pores of members of many cytheracean ostracod families has revealed that: (1) there is widespread polymorphism in both simple and sieve-type normal pores, related to differences in function of the sensory receptors passing through them, (2) both simple and sieve-type normal pores occur together on valves of species belonging to several families, and (3) although number and arrangement of normal pores seem to be diagnostic at the specific and generic levels, there are instances of extreme variations in both.

The initial temporary effect of these new data may be to decrease the usability of normal pores as a diagnostic character. However, preliminary results suggest that, as scanning electron microscope data become available for more ostracod genera and families, an even greater diversity of normal pore structure will be found. This diversity should make normal pore structure a much more useful criterion for systematic, phylogenetic, or paleoecologic inferences.

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