Formation of the mesodermal organs during the metamorphosis and definitive organogenesis in the holothurian Apostichopus japonicus

The fine structure of the mesodermal organs (water-vascular system, somatocoel derivatives, connective tissue, and mesenchymal cells) in late doliolaria, pentactula, and 1-month-old juveniles of the holothurian Apostichopus japonicus was studied. It has been shown that all the structures of the water-vascular system, the longitudinal muscle band anlagen, extracellular matrix, and different types of mesenchymal cells are present at the late doliolaria stage. However, even in 1-month-old juveniles, many organs do not reach their full development as in adults. During the developmental period under study, several stages of myogenesis can be distinguished, ranging from the appearance of single myoepithelial cells to the beginning of muscle bundle formation. These data are consistent with our hypothesis of the origin, development, and evolution of the muscular system in echinoderms. One-month-old juveniles already have all the main types of coelomocytes characteristic of adults. This means that the immune system of A. japonicus is already formed.