Mucronalia, foot reduced, but still operculate, eyes present, animal fixed by its very long proboscis which is deeply buried in the tissues of an Echinoderm, no pseudopallium.
An operculum is present only in Amphibola; a contrast being thus afforded with the operculate pulmonate Streptoneura (Cyclostoma, &c.), which differ in other essential features of structure from the Pulmonata.
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.
Here the two elements, ovum and yolk-cells, are surrounded by a shell of operculate or of spindle-capped types.
The eggs are stalked and provided with chitinoid often operculate shell.