Development of a holothurian,Cucumaria japonica, is examined. Holoblastic radial equal cleavage results in formation of a coeloblastula. Gastrulation begins on the second day. It is represented by an invagination and accompanied by immigration of the mesenchyme cells and marks the start of development of the vitellaria larva. The archenteron immediately divides into rudiments of the hydrocoel, the somatocoel and the rudiment of the gut. No unified enterocoel is formed. Ends of the hydrocoel fuse to form a ring canal on which five primary tentacles, radial canals, a polian vesicle, and a hydroporic canal are formed. The somatocoel divides into right and left somatocoels, which then fuse to form a united coelomic sac, encircling the gut. The ventral mesentery then disappears. Thus, by day 6 the pentactula is completely formed. At day 10 the tentacles move from the ring canal to the radial canals, and the somatocoel gives rise to two derivatives: the preoral and anal coelomes. By the end of the second week of development, three parts can be distinguished in the alimentary canal, and a longitudinal muscle band begins developing in the mid-ventral ambulacrum. At the stage of two months, a rudiment of the respiratory system appears.