The blue pattern seen when the skin of Diadema antillarum is viewed by reflected light is due to iridophores. The quality of their blueness varies with the background provided by neighbouring chromatophores.
Microdissection reveals that they consist of gelatinous plates contained in a fibrous and cellular capsule. The study of fixed preparations shows that the plates are sheathed and that their structure is greatly altered by fixation which distorts, disrupts, or dissolves them.
The structure of the iridophores bears little resemblance to the description of the so-called ‘eyes’ of D. setosum given by Sarasin and Sarasin, with which they appear to correspond.
Examination of their optical properties indicates that Rayleigh scattering by the colloidal contents is the most likely cause of their colour.