Most vertebrates have some form of duct to transfer the sperm from the
testes to the
urethra. In
cartilaginous fish and
amphibians, sperm are carried through the
archinephric duct, which also partially helps to transport urine from the kidneys. In
teleosts, there is a distinct sperm duct, separate from the
ureters, and often called the vas deferens, although probably not truly
homologous with that in humans. The vas deferens loops over the ureter in
placental mammals, but not in
marsupial mammals. In cartilaginous fishes, the part of the archinephric duct closest to the testis is coiled up to form an
epididymis. Below this are a number of small glands secreting components of the seminal fluid. The final portion of the duct also receives ducts from the kidneys in most species. In
amniotes (
mammals,
birds, and
reptiles), the
archinephric duct has become a true vas deferens, and is used only for conducting sperm, never urine. As in cartilaginous fish, the upper part of the duct forms the epididymis. In many species, the vas deferens ends in a small sac for storing sperm. The only vertebrates to lack any structure resembling a vas deferens are the primitive
jawless fishes, which release sperm directly into the body cavity, and then into the surrounding water through a simple opening in the body wall. ==Additional images==