During the spring of 1955, while attending classes at the Pacific Marine Station, under the direction of Dr. Alden E. Nobel, Chairman of the Department of Zoology, College of the Pacific, Urechis caupo was introduced as a representative of the fauna of the mud flats of Tomales Bay. The unusual manner in which this animal lives would stimulate the curiosity of any student interested in biology, and especially those associated with marine life. The fact that the animal burrows its own home in a U-shaped tunnel and dwells there with three permanent commensals (Hesperonoë adventor, Skogsberg, a plynoid annelid; and 2 pinnotherid crabs, Scleroplax granulate, Rathburn, and Pinnixa franciscapa, Rathburn) is a unique characteristic. The goby fish, Clevelandia ios (Jordan and GIlbert), may also be considered a commensal, but it uses the burrow as a retreat rather than a permanent residence (Fisher, 1946). Studies on the early embryology of Urechis stimulated the writer to further study of the reproductive cycle. The puzzling descriptions and information pertaining to the immature gametes that are found in the coelomic fluid, coupled with the fact that no permanent gonad has been observed or indeed, that it may be wholly lacking, supplied a challenge that he could not ignore. The primary purpose of this investigation is to attempt to evaluate the reproductive cycle and to attempt to locate some type of germinal epithelium.