These organs must have been independently developed, as they could not have been inherited from a common progenitor; and in the several groups they are formed by the modification of the fore legs, of the hind legs, of the maxillae or lips, and of appendages on the under side of the hind part of the body.
Organo hauek zein bere aldetik garatuak izan behar dute, arbaso komun batengandik heredatuak ezin izan direnez gero; eta talde ezberdinetan aurreko hanken, atzeko hanken, maxila edo ezpainen eta gorputzaren atzeko sabelaldeko apendizeen eraldaketaz formatuak dira.