Structural and ultrastructural features of the inlet and outlet regions of the urinary bladders of the leech, Hirudo medicinalis L.

Each of the 34 nephridia in the leech, Hirudo medicinalis, has its own separate bladder. Urine flows from the last portion of the nephridium, the final canal, into the bladder through a special inlet which prevents backflow of urine. This inlet consists of a vestibule formed by two serially arranged septa, each with a small pore. As no muscles or cilia are associated with either the nephridia or the bladder inlet, urine flow into the bladder is passive. Urine leaves the bladder through an outlet that consists of a urethra with sphincters at both ends and an opening, the nephridiopore, in the ventral skin. The sphincter muscles are distinct from the body wall muscles and receive double innervation: urine retention and release is therefore active.