These fall to two major groups, informally called the cyclostomes and noncyclostomes. The holonephros is the kidney of the larvae of cyclostomes and the Gymnophiona. "Romundina" shares many characteristics of both cyclostomes and Gnathostomes to varying degrees. Biologists disagree about whether cyclostomes are a clade. Northern brook lampreys are jawless fishes, also known as cyclostomes. A common feature of both cyclostomes is the absence of myelin in neurons. The opisthonephros is the functional adult kidney in lampreys (cyclostomes), most fishes, and amphibians. They are Cyclostomes (Circle mouths), a primitive fish-like creature, and are not related to eels. Cyclostomes were regarded as either degenerate cartilaginous fishes or primitive vertebrates.