Is this card really necessary? If there's a card for what the retina actually is, I don't think a cloze for the fact its a sensory membrane is explicitly required

The card itself is necessary since it outlines anatomy. Another card for the lens (1554693936539) relies on knowing what the retina is since they are cooperative structures. The 2nd cloze isn't absolutely necessary but more so helpful to establish that it's used for senses (e.g., black/white from rods and color from cones as hinted at in the Extra). If there's a specific resource that would show a lack of importance for this or if you could elaborate on rationale as to why you see it unnecessary, feel free to share it as a change note suggestion