Cellular structure and ultrastructure of the black coral Antipathes aperta: 2. The gastrodermis and its collar cells

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Abstract
The gastrodermis of the black coral Antipathes aperta is associated with eight distinct types of cells, including two types of microbasic b‐mastigophores (nematocysts), spumous and vesicular mucus cells, and ganglion cells that are essentially the same as in the epidermis. Three additional types of cells are unique to the gastrodermis, and are readily distinguished from those of the epidermis by their electron‐opaque inclusions. These include lipoidal cells, zymogen digestive cells, and an unusual type of epitheliomuscular collar cell. The pharyngeal region is characterized by the presence of electron‐opaque nematocysts, a scattering of zymogen cells, and a large number of collar cells. The latter are distinguished in part by the presence of dense microfibrillar processes that surround the microvilli and extend into adjacent collars. This interconnection results in the formation of an extensive pharyngeal meshwork. These collar cells are additionally distinguished by the placement of the collar and flagellum adjacent to a flared cup of cytoplasm. This portion of the cell is capable of endocytosis of relatively large unicellular prey, and apparently is capable of forming digestive vesicles as well. The pharyngeal gastrodermis grades into simple lobate septal filaments toward the base of the coelenteron, where large, granular nematocysts all but replace the smaller electron‐opaque types Collar cells are found here as well, but in fewer numbers compared to the zymogen cells. Ultrastructural results are compared with those of other coelenterates and discussed in terms of food and modes of nutrition.