Figure 12

Cartilage-like endoskeleton (En) in the thick, proximal base of opisthosomal appendages. Limulus polyphemus, prone. Semi-thin sections, LM. A: In this example after the third embryonic molt (stage 20), the endoskeleton at the base of the genital operculum (GO) has a rod-like structure with cells surrounded by a thin, dense matrix. In the proximal end of the endoskeleton, some matrix chambers are swollen and without cell contents (*). The base of the first branchial appendage (B1) has large vacuoles, probably from deteriorated cells. Cl, cleft between the appendage bases; GL, gill lamella; M, muscle cells. Scale, 50 μm. B: Some endoskeleton is present at the thick base of the first branchial appendage (B1) in this first instar. The gill lamellae (GL) have variable spacing and decreasing development and length toward the proximal end (left) of the appendage. OV, opisthosomal ventrum; T, developing pillar-type trabeculae. Scale, 80 μm.