Figure 3

Narrow cleft (Cl) like that in Figure 2B between the emerging thick proximal bases of the genital operculum (left) and first branchial appendage (right) after the third molt (stage 20). Limulus polyphemus, prone. TEM. Both appendages have an outer hypodermal layer (H) with only a very thin cuticle (Cu) at this stage. The central part of each appendage is filled with cells (C) and cell debris (CD), the latter apparently from cells that deteriorate in forming a central lumen for passage of hemolymph. Small hypodermal cell protrusions into the cleft give these appendages a knobby appearance that is typical of this stage (Figures 2A, B and 4). The external, ventral surface of the embryo is toward the bottom of the photo. Scale, 5 μm.