Context:
"The nervous system was so complex and highly developed as to leave Lake aghast. Though excessively primitive and archaic in some respects, the thing had a set of ganglial centres and connectives arguing the very extremes of specialised development. Its five-lobed brain was surprisingly advanced; and there were signs of a sensory equipment, served in part through the wiry cilia of the head, involving factors alien to any other terrestrial organism."
(Source)
I think that 'to be' verb should be involved:
Factors ARE alien to any other terrestrial organism
Questions:
Why didn't the author use 'to be' verb in the sentence?
If it's optional- could you please explain these options to me?
