Figure 2

Thin distal and thick proximal parts of the genital operculum (GO) and first branchial appendage (B1) after the third embryonic molt (stage 20). Limulus polyphemus, prone. Semi-thin sections, LMs. Sections through these flap-like appendages (Figure 1B) show that the initial distal parts have a knobby appearance at this stage as a result of the outward protrusion of cells of the outer hypodermal layer (Figures 3, 4). A. A central lumen (L) is present in some places in the thin distal parts of the appendages shown here, including the second branchial appendage (B2). As shown in more detail below, the outer wall of the appendages consists of a layer of hypodermal cells and the cuticle they secrete. At intervals along the length of the appendages, some hypodermal cells extend processes into the lumen to start forming pillar-type trabeculae (T). Also at this stage, as further described below, a small cluster of cuticular vesicles (asterisks) is evident at the tips of the genital operculum and first branchial appendage. Scale, 115 μm. B. A narrow cleft (Cl) or space is evident between the thick proximal bases of the genital operculum and first branchial appendage as they lengthen from the opisthosomal ventrum (OV). The thin distal regions evident here have a central lumen (L) and developing trabeculae (T). B2, distal part of the second branchial appendage; Y, yolk. Scale, 120 μm.