Annual development of testis and spermatogenesis in Urechis unicinctus

We documented the histological structure and annual development of the testis and the process of sper-matogenesis in Urechis unicinctus using light microscopy and transmission electron microscopy.The testis was localized at the end of the body and formed a band.Each end of the testis was connected to the body wall and res-piratory gut wall by muscle tissue.The testis was composed of interior connective tissue and spermatogenic cells,including spermatogonia clusters at the periphery of the connective tissue and interior spermatocytes surrounded by connective tissue.Spermatocytes that were clustered at different stages were transported into the coelomic fluid and developed into sperm,which left the cell clusters and entered the nephridia.Based on the histological charac-teristics of the testis,we divided the annual development into 5 stages: proliferative,growing,mature,release,and resting stages.In addition,we observed spermatogenesis in the testis and coelom using both optical and electron microscopy and documented ultrastructural changes in the nucleolus,acrosome,and sperm tail.