Ultrastructure of nematocytes and one-celled tentacles of the freshwater coelenterate, Calpasoma dactyloptera.

Calpasoma dactyloptera, a tentacled form of minute, freshwater coelenterate, has been investigated by light and electron microscopy and time-lapse cinematography. Each tentacle consists of a protrusion from a single ectodermal epithelial cell termed a tentaculocyte. Thus, unlike most coelenterate tentacles, neither mesoglea nor endoderm is present in the tentacle. Large numbers of nematocytes are present, however. When the nematocytes are poised, they sit within tentaculocyte vesicles which represent invaginations of the plasma membrane. A cnidocil protrudes into the external medium. The bottom of each nematocyte is elongated as a stalk which extends to the tentacle base, coursing through tubular membrane lined channels within the tentaculocyte. A network of fibers and microtubules, originating in the cnidocil, extends to the base of the nematocyte stalk.