The sixth appendage is much larger and stronger than the others, and like the fifth, is without long spines. The coxa is large and quadrate; the second and third joints are short, like those of the fifth appendage; the fourth, fifth, and sixth joints are longer and more or less bell-shaped; the seventh and eighth joints are much larger than the others and are flattened. The metastoma (Fig. 162, _m_) is an oval plate immediately behind the mouth; it covers the inner parts of the coxae of the sixth pair of appendages, and represents the chilaria of _Limulus_. But, unlike the latter, it is not a paired structure; nevertheless the presence of a longitudinal groove on its anterior part renders probable the view that it is derived from a paired organ.[233] The front margin of the metastoma is indented and toothed. On its inner side in front is a transverse plate, the endostoma, which is not seen from the exterior, since the front margin of the metastoma extends a little beyond it. Behind the prosoma are twelve free segments, of which the first six form the =mesosoma= (Fig. 161, 7–12). The tergum on the dorsal surface of each segment is broad and short, the middle part being slightly convex and the lateral parts slightly concave; the external margin is bent under, thus forming a narrow rim on the ventral surface. The tergum of each segment overlaps the one next behind. The segments increase in breadth slightly up to the fourth segment, posterior to which they gradually become narrower.