Roundworms have taken this last body plan and adapted to thousands of environments with it. From microscopic to maybe a meter long, roundworms are just what they sound like-round worms. At one end, they have a mouth, at the other, an anus, with a tube digestive system in between.
Availability of two-factor authentication for Apple ID Two-factor authentication is an extra layer of security for your Apple ID designed to ensure that you're the only person who can access your account, even if someone knows your password.
These days, there's pretty much three forms of authentication in general use on the web: Single-factor authentication, e.g.: PIN or password. Two-factor authentication, e.g.: Single-factor plus a software- or hardware-generated token code, or a smart card. Two-step authentication, e.g.: Single-factor plus a code sent to the user out-of-band.