Kidney always so simple; in cartilaginous fish and some amphibians, there is also a shorter duct, similar to the amniote ureter, which drains the posterior (metanephric) parts of the kidney, and joins with the archinephric duct at the bladder or cloaca. Indeed, in many cartilaginous fish, the anterior portion of the kidney may degenerate or cease to function altogether in the adult. In the most primitive vertebrates, the hagfish and lampreys, the kidney is unusually simple: it consists of a row of nephrons, each emptying directly into the archinephric duct. Invertebrates may possess excretory organs that are sometimes referred to as
Urinary bladder a series of ridges, thick mucosal folds known as rugae that allow for the expansion of the bladder. The Latin phrase for "urinary bladder" is "vesica urinaria", and the term "vesical" or prefix "vesico -" appear in connection with associated structures such as vesical veins. The modern Latin word for "bladder" - "cystis" - appears in associated terms such as cystitis (inflammation of the bladder). The outside of the bladder is protected by a serous membrane. The bladder wall itself is smooth muscle. The inner side of the bladder is lined with a mucosal membrane consisting of a surface glycocalyx