Comparative xenobiology
Comparative xenobiology was a component within the study of xenobiology that compared various elements of different species. 

When  had an electromagnetic lifeform that stowed-away aboard in 2377, the ship's children speculated as Neelix told the ghost story about the creature, attempting to guess what it might be. Icheb said that "It was obviously a space-dwelling lifeform"; while Mezoti inquired ""was it non-corporeal?"; Rebi asked if it was "hostile"; Neelix' acknowledgement of those comments led Azan to guess Species 5973, a multispectrum particle lifeform, which was incorrect. Mezoti then guessed if it was an interphasic species, but before she received her answer, the frustrated Neelix belted out, "We can either debate comparative xenobiology or I can continue with the story. Now it's up to you." ()

A module baring this title was used in Keiko O'Brien's classroom aboard Deep Space 9 in 2369, intended to educate her students in the field of xenobiology. Within the module were references to various non-humanoid lifeforms and animals encountered by the  a century earlier. 

Data contents of the first row (from left to right): 
 Ornithoid life form discovered on Pyris VIII (cf. )
 Horta – Silicon cycle life form (cf. )
 Carbon cycle lifeform from Excalbia (cf. )

Data contents of the second row (from left to right) were:
 Tribble – Polygeminus grex from Iota Geminorum IV (cf. )
 Regulan bloodworm from Regulus II (cf. )
 Flying parasite – Blastoneuron from Large Magellanic Cloud? (cf. )
 Denebian slime devil – Denebia carnivora from Deneb IV (cf. ) 

Along with the names and origins of these species, the diagram contained a comparative anatomical cross section of each creatures. (, et al.)

Background information 
This okudagram was created with data and images taken from the pages of the (non-canon) Star Fleet Medical Reference Manual, which was co-written by Doug Drexler. A fully legible version of the graphic was later (re)published in its new form in the reference book The Making of Star Trek: Deep Space Nine, p. 23.

The single most notable error in the text of this module was the fact that the Ornithoid life forms were discovered on Pyris VII not "Pyris VIII".
Biology