The part of the cervix protruding into the vagina is of different lengths in different species. In the narwal, bottlenose whale and sperm whale it is quite short, however in the common porpoise it is comparatively long.
The inner part of the vagina has from four to twelve circular folds, which are very well delveloped and give the appearance of a chain of successive funnels. The function of the folds, found in no other mammal, is not understood. The inner part is largly glandular. Vaginal smears taken at various stages from 114 common dolphins showed a fairly standard pattern, with a few individual differences. Some individuals showed a substance consiting largly of sulphates, which prevents fertilization. This has been found by other scientists. It was found in roughly 4.5% of 221 animals caught in the Black Sea. This abnormality is not understood.
The vulva is a slitlike aperture with labia majora and minora (inner and outer folds, or "lips"). The front end of vulva shows a strand of tough, fibrous tissue, from which protrudes the clitoris. Many immature animals also show a band of fibrous tissue running from the front to the rear of the opening, placed so that it is likely to be ruptured during intercourse.
The mammary glands are two elongated, narrow and flat organs on either side of the vagina. The nipples are located in slits, and extend during suckling. In some embryos eight rudimentary nipples were found, suggesting ancestors may have had four pairs of nipples, and possibly produced multiple young.