The broad shell of a horseshoe crab makes its outline easy to recognise, but a photograph can hide much of its sensory anatomy. Two conspicuous eyes do not tell the entire story.
Start with a labelled account
The Smithsonian Ocean introduction describes the animal’s multiple eyes and points readers toward an educational account of its anatomy. Use that reference before trying to identify every small mark in an unlabelled photograph.
Separate the number described by the source from the number you can locate in one view. A structure may be present without being visible at the photograph’s angle or resolution.
Turn the picture into a question
Choose one labelled feature and ask where it would fall in the photograph you are examining. If you cannot find it confidently, record the limit rather than adding a guessed arrow.
This is a useful habit with marine animals more generally: an outline supplies recognition, while anatomy requires closer evidence. You can learn from a museum diagram or educational video without handling a wild animal. The result is a more careful description of what the source teaches and what your particular image actually shows.