Acaste is a medium-size trilobite with large, schizochroal (multi-lens) eyes, a round, forward-facing glabella (central head section), and a small, tapered pygidium (tail section).
Although its overall shape and body plan is very similar to that of Phacops (opposite), these two genera belong to different trilobite families, and were not directly related.
The physical likeness between the two was achieved through convergent evolution,
which means that both groups evolved similar features separately, probably due to living in a similar environment and following a similar lifestyle. In both types of creature, natural selection favoured certain adaptations, such as schizochroal eyes to see well, and so the two types end up looking very similar.
Indeed, it has been suggested that its visual acuity was more than the
creature actually required.
The two differ in some features, however, including ornamentation at the head end.