SEM Studies on Arthropod Exoskeletons 2. Horseshoe crab Limulus polyphemus (L.) in comparison with extinct eurypterids and recent scorpions1

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Abstract
The exoskeleton of Limulus polyphemus (L.) (Xiphosura, Merostomata) is described on the basis of scanning electron microscopic (SEM) studies. Three principal layers are distinguished: “epicuticle”, exocuticle, and endocuticle. The exocuticle is composed of horizontal lamellae interconnected by vertical curved lamellae (“parabolic arcs”). Its structure is essentially as that in the exoskeletons of other arthropod groups. The endocuticle consists of vertical lamellae which seem to be homologous to the vertical curved lamellae in the exocuticle. Prior to the molting, most of the endocuticle is dissolved by the animal. Detailed descriptions are given on the structure of the horizontal and vertical lamellae, and on the course of the pore canals. A vertically lamellar, thick endocuticular layer also occurs in the extinct eurypterids. In recent scorpions, a typical development of the vertically lamellar structure is only found in the chela of the pedipalps and in the leg joints. The main part of the exoskeleton has a horizontally lamellar structure, but the horizontal lamellae are often widely spaced—thus the vertical lamellae dominate. The functional significance of the vertically lamellar structure is discussed.