The cocoon of the leech Theromyzon trizonare consists of fibrils packed into an arrangement that produces both C- and S-like patterns of bow-shaped lines in sections oblique through the membrane. Sections normal to the cocoon membrane show layers containing cross-sections of fibrils (approximately 16 nm dia.) that are separated by a center-to-center distance of approximately 23 nm. In cross-section, each fibril presents a central hole approximately 5 nm in diameter. A structureless layer covers most of the exterior surface of the cocoon membrane, and short protuberances are apparent in some zones. © 2004 Elsevier Ltd. All rights reserved.
Dimitriu, C., & Shain, D. H. (2004). Ultrastructural properties of the Theromyzon (Annelida: Hirudinae) cocoon membrane. Micron, 35(4), 281–285. https://doi.org/10.1016/j.micron.2003.10.049