Except when you upgrade an ordinary elastic with a delicate braid, or wrap chic black bands down the entire length of the tail. (harpersbazaar.com)
The core of a sperm's whip - like tail is the axoneme, a long tube consisting of nine pairs of microtubules arranged in a column around a central pair, all of which extend the entire length of the tail. (sciencedaily.com)
There are sizes available, while most fit all this allows you to choose the length of the tail which is helpful if you plan on using this to breastfeed (rookiemoms.com)