The bat creatures' arms are connected to their wings. However in some shots (usually the ones require them to pick up or push someone) their arms and wings are suddenly separate appendages.
McKenzie states that the underwater city lies at "over 11,000 fathoms", however, the deepest point in the Earth's oceans (The Challenger Deep) is only 5,960 fathoms (of course in the film's fictional reality, there may be depths unknown to us 'surface dwellers')
Obvious wires visible when the bat creature swoops down to attack the heroes during the climactic battle in the laboratory.
Wires can be seen during the sub chase scene.