Tyranids have redundant organs

• Devastation of Baal:

"To human eyes, a tyranid organism was a single thing, a beast like any other. This was not so. Each monster in the limitless swarms was a carefully designed colony of symbiotic creatures. Once incorporated into the hive fleet’s genetic knowledge, the baseline genome of those organisms chosen for a primary host was pared back to the bare essentials, and gifted with the characteristics common to all tyranid creatures – thick, chitinous armour, a hexapedal anatomy, multiple redundant organs – characteristics that, above all else, made them incredibly difficult to kill. Only then were the true adaptations added.

Though the finished creature may have looked like a complete, single being, it was made up of a multiplicity of individual creatures, many of them semi-sentient in their own right. This was most obvious to the casual observer in the weapons borne by the larger constructs, whose repurposed anatomies still retained recognisable biological shapes. There were other, less obvious examples of forced parasitism. Thinking blood. Organs that could live separately from the creature they served. Subsidiary brains that awaited the death of the main nerve stem or the presentation of some unusual circumstance that required specialist knowledge not present in the basic mentality of the creature; both events that might never come to pass. Organs could be installed, fully aware, and live for centuries, never realising their potential. The hive fleet was so huge it could afford to be profligate with flesh."