Retinal structure in Synbranchus marmoratus with observations on two new cone myoid inclusions.

The retina of a South American swamp eel, Synbranchus marmoratus (Synbranchidae), was studied by Golgi impregnation, light and electron microscopy. Its principal features include (1) the presence of a dense matrix, possibly a new type of tapetum lucidum, in the pigment epithelium, (2) a well developed photoreceptor layer containing large rods, single, double and triple cones, and (3) well developped inner nuclear and plexiform layers, with the exception of horizontal cells which are few and relatively small. These and other observations are discussed in relation to the photic environment and habits of this fish. The presence of microfilament bundles and two unusual features, microtubuleladen dense bodies and paracrystalline inclusions, in cone myoids are discussed in terms of their possible involvement in retinomotor responses.