The life history as observed in the laboratory of two porcellanid crabs belonging to the genus Polyonyx has been described viz., Polyonyx hendersoni Southwell - a species generally found inside Zoanthus colonies and sponge cavities and P. loimicola Sankolli-a commensal with a polychaet worm Loimia medusa (Savigny). In both the species the larval development consists of two zoeal and a megalopa stage. Detailed account of these stages is given along with illustrations. Necessary comparisons of the larval characters of these two species are made with those of the only two known species viz., P.quadriungulatus Glassei and P. gibbesi Haig - both of which happen to be commensal forms, revealing certain morphological features common to commensal species. Generic features based on the knowledge of the larvae of these four species and a key to the first zoeal stage are also given.