The stenotele capsule has been observed to be composed of an external membrane, a thick chitinous or keratin layer, and an inner membrane. A sac-like extension of the capsular wall into the capsule bears spines and stylets on its inner surface and evagination of this structure occurs on discharge. Profiles of tubular or membranous structures often are seen within the capsules of resting stenoteles. These structures are presumably related to the external filament. The spines often reveal a flattened aspect which suggests that at least some of them might more accurately be called "vanes." A cnidocil has been found to accompany each stenotele.

This study revealed several aspects of the developmental stages of stenoteles: A vacuole is formed which is nearly surrounded by the nematocyte nucleus. The vacuole content changes in density and a capsular wall is formed at the periphery of the vacuole. Tubules differentiate from the capsular matrix, and spines and stylets develop somewhat later. An operculum is formed from the nematocyte cytoplasm.