The fine structure of the cartilage in the annelidSabella penicillum

Summary The fine structure of the tentacular cartilage cells of the annelidSabella penicillum has been studied by scanning (SEM) and transmission electron microscopy (TEM). The cells contain a large, transparent vacuole centrally, and a thin layer (>0.1 m) of cytoplasm around. Some granular endoplasmic reticulum, mitochondria and smaller clear vesicles are present in the cytoplasm. The nuclei are dense in the TEM preparations. The surrounding matrix is most dense close to the cells, and form a chondroid matrix, 0.2 to 0.5 m thick, consisting of a flocculate material. The boundary to the surrounding matrix is condensed in the central cartilaginous cells, but not in the pinnular. The rigid structure of the close chondroid matrix is demonstrated in the SEM. The structure is compared to other invertebrate cartilages so far described, and its functions are discussed.