Developmental morphology of a holothurian, Cucumaria japonica (Dendrochirota, Holothuroidea), a species with accelerated metamorphosis

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 accompa nied by immigration of the mesenchyme cells and marks the start of developm ent of the vitellaria larva. The archenteron immediately divides into rudim ents of the hydrocoel, the somatocoel and the rudiment of the gut. No unifi ed 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 hydro poric canal are formed. The somatocoel divides into right and left somatoco els, which then fuse to form a united coelomic sac, encircling the gut. The ventral mesentery then disappears. Thus, by day 6 the pentactula is comple tely 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 ca n 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.