112 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



muscles {2^) arise near the midline just behind the dorsal eyes; the 

 muscles of the pedipalps {25a) take their origins farthest forward; 

 and the muscles of the other appendages (2^b-2jc) are distributed 

 on the following areas of the "fixed cheeks." Diagrammatically, there- 

 fore, we may represent the segmentation of the prosomatic carapace 

 as given at E of figure 47. The horseshoe-shaped acron (Acr) bear- 

 ing the eyes encloses the region of the prosomatic somites (I-VIII), 

 and sends posteriorly, between the lobes of the anteriorly curved 

 cheliceral and pedipalp somites, a median tongue bearing the dorsal 

 ocelli (dO). The structural conformity with the trilobite head (fig. 

 46 J) is exact, except for the greater number of somites included in 

 the xiphosurid prosoma. 



Students of the embryology of Limulus (Kishinouye, 1893, Iwanofif, 

 1933) have indicated the segmental divisions of the prosoma as sub- 

 tending the lateral areas of the carapace bearing the compound eyes. 

 Branches of the segmental nerves, the "haemal nerves" of Patten and 

 Redenbaugh (1900), extend into these parts, but, as in the case 

 of the trilobites, the location of the compound eyes on the lateral plates 

 of the prosoma is sufficient proof that these plates belong to the eye 

 segment, or acron. Hence, they cannot be lateral extensions of the 

 median somites. 



The gills of the trilobite legs, borne on coxal epipodites (fig. 48 D, 

 Eppd), have not been retained on the prosomatic appendages of 

 Xiphosurida, though an epipodite is present on the fourth leg (E, 

 Eppd), but gill-bearing epipodites are highly developed on the opistho- 

 somatic appendages, which are otherwise much reduced. 



The prosomatic appendages of Limulus, except the chelicerae, as 

 shown by Benham (1885), have the typical arthropod coxal muscu- 

 lature, consisting of dorsal promotor and remotor muscles (fig. 48 F, 

 I, J), and ventral muscles {K, L). Of the latter, two (jj, j^) are 

 promotors and remotors, but two others {32111, 32n) are united 

 proximally and evidently function as adductors. The dorsal muscles 

 arise on the tergal carapace (C). The ventral muscles, however, are 

 attached on an internal plate, or "entochondrite" (k), suspended in 

 the ventral part of the body by dorsal muscles (t-s). The same 

 structure (B) is characteristic of most of the Chelicerata, and a 

 similar structure occurs in the gnathal segments of many of the 

 Mandibulata (figs. 50 E, H, 51 B, k). Since the ventral muscles of 

 the appendages should primarily arise on the ventral body wall, the 

 "entochondrite" might be supposed to be a sternal derivative, but 

 Schimkewitsch (1895, 1906) claims that in the Arachnida it is pro- 

 duced from transformed muscle tissue. In various mandibulate 



