Roles of hesC and gcm in echinoid larval mesenchyme cell development. To understand the roles of hesC and gcm during larval mesenchyme specification and differentiation in echinoids, we performed perturbation experiments for these genes in two distantly related euechinoids, Hemicentrotus pulcherrimus and Scaphechinus mirabilis. The number of larval mesenchyme cells increased when the translation of hesC was inhibited, thereby suggesting that hesC has a general role in larval mesenchyme development. We confirmed previous results by demonstrating that gcm is involved in pigment cell differentiation. Simultaneous inhibition of the translation of hesC and gcm induced a significant increase in the number of skeletogenic cells, which suggests that gcm functions in skeletogenic fate repression. Based on these observations, we suggest that: (i) hesC participates in some general aspects of mesenchymal cell development; and (ii) gcm is involved in the mechanism responsible for the binary specification of skeletogenic and pigment cell fates.