The eggs are stalked and provided with chitinoid often operculate shell.
Such a calyx is operculate or calyptrate.
The shell is thick, and operculate in some forms; thin, and provided with filaments, in others; in the latter cases it may contain only a few yolk-granules suspended in an albumen-like substance.
Therein an order Thoracica comprehends the pedunculate Lepadidae, together with the operculate and sessile Balanidae and Verrucidae; a single species without cirrhi constitutes the order Apoda, and a single species with only three pairs of cirrhi the order Abdominalia.
Here the two elements, ovum and yolk-cells, are surrounded by a shell of operculate or of spindle-capped types.