Ring- or saddle-shaped, glandular, thickened external body wall epithelium of some (usually genital) segments, in the sexually mature oligochaetes and leeches. Helping to stick together the copulating partners; secreting the cocoon shell.

Chaetogaster diaphanus.
C. diaphanus 1 , a chain of 5 zooids, first of them mature, with clitellum and an egg.