The Collaborative International Dictionary of English v.0.48:

Retinula \Re*tin"u*la\, n.; pl. Retinulae. [NL., dim. of NL. &
   E. retina.] (Zool.)
   One of the group of pigmented cells which surround the
   retinophorae of invertebrates. See Illust. under
   Ommatidium.
   [1913 Webster]