On the question of whether merfolk are mammals or fish one can look towards the physical nature of their form and compare it with the standard zoological criteria for classification of vertebrates. The popular definition of the merfolk as half-human half-fish, is partially inaccurate due to the majority of the vital organs taking humanoid rather than piscine form; all except that is the lungs and genitalia. As merfolk can breathe underwater, but have no visible gills, it would be incorrect to assume either mammalian or aquatic lung development, but more possibly a unique combination of the two. Likewise, while most merfolk depicted in art and literature have no visible human reproductive organs, there are many anecdotes of the sexual desires of mermaids being towards men, but no tales of merpeople exhibiting any sexual interest towards fish or indeed any other seaborne creatures. One can deduce from this that the reproductive instincts of merfolk are humanoid. Mermaids are also famously depicted frequently with naked breasts, which take on obvious human form, and are synonymous with breast-feeding, and the feeding of young, which is common to mammals. Whilst fish are known to care for their young in some exceptions, by offering shelter or protection during development, and other fish are even known to give birth to live young, no fish has ever been recorded as having mammaries. The scales and tail-fins on the mermaid are another point in question, as most mammals, save for the armadillo, have smooth skin or fur. Even mammals that have fully adapted to living in the ocean, such as dolphins and porpoises, have smooth skin rather than scales and their tails also take a different form, and thrash up and down to produce movement rather than, as most fish tails do, from side to side. Indeed it is only the tail, and the ability to breathe underwater that distinguishes the merfolk from humans at all (it is, however, not known whether merfolk are warm or cold blooded, and speculations on the form of their circulatory system would be rather fruitless). Other notable human characteristics, such as a pair of arms, working flexible fingers, a navel, hair, and a distinct lack of fins on the upper body, far outweigh the piscine classification traits and sway the likelihood of merfolk being mammals as being the most likely.