PMC and SEM of prism larval stage of C. subdepressus.

A PMC forming a ring near the posterior end 14 hpf; cells are connected by cytoplasmic bridges (arrow). B Triradiate calcareous spicule secreted by PMC initiating the formation of the larval skeleton 15 hpf. C The apical tuft (at) and future mouth (m) are present at the anterior region. On the posterior end the postoral arms (po) begin to extend and the blastopore is visible (bp). Scale bars =  (A, C); (B)