The material described below came from washings of holothurians in the West Indies. It was collected in part by the author and Dr. R. U. GOODING during the summer of 1959 in the Bahamas, Barbados, Puerto Rico, and Jamaica. This field work was aided by a grant (G-8628) from the National Science Foundation of the U.S. The rest of the material was collected by Dr. J. H. STOCK in 1958 at Curaçao and Bonaire, with the support of the Netherlands Foundation for the Advancement of Research in Surinam and the Netherlands Antilles (WOSUNA). The study of the specimens has been aided by grants (GB-1809 and GB-5838) from the National Science Foundation. I am indebted to Dr. ELISABETH DEICHMANN of the Museum of Comparative Zoology, Harvard University, for the identification of the holothurian hosts collected in 1958 and 1959.