A Benzite male in 2364

Respiration devices were apparatuses used to supplement the air an individual breathed.

Some Benzites were unable to breathe a standard oxygen/nitrogen atmosphere, and therefore had to wear respiration devices in those environments during the early 2360s. By the early 2370s, however, they had fallen into disuse. (TNG: "Coming of Age"; DS9: "The Ship")

It has never been clearly stated in dialogue what the respiration devices worn by the Benzites were truly used for or what happened to them. Mike Okuda explained the absence of the apparatus in "The Ship" by pointing out "there's been some advances in Benzite technology!" (Star Trek: Deep Space Nine Companion)
Mordock's respiration device was originally conceived as being "a special suit using circulating gas tubes." Mendon's device was later described as being "a special device at his chest which converts the oxygen level to his use."

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