THE ENDOCRANIUM OF THE SCORPION. 



317 



the posterior sides of the bases of the chilaria. A small transverse muscle joins 

 the distal ends of the two bars. Two other small muscles run to the chilaria 

 from the thin portions of the endocranium, near the bases of the bars. 



It is especially noteworthy that the body of the endocranium is composed of 

 fibroid cartilage, \vhile the bars just described are of capsuliginous cartilage, 

 exactly like that found in the abdominal appendages. The development of the 

 bars shows that they represent a pair of gill bars belonging to the chilaria, that 

 have been secondarily united with the cranidium. 



Foramina. There are two pairs of foramina for the passage of nerves. One 

 pair lies just outside the marginal wall, appearing on the haemal side of the endo- 

 cranium, a little posterior to the haemal processes, f 1 . The intestinal branches 

 of the haemal nerves belonging to the sixth thoracic neuromere pass through this 

 pair. (Fig. 214, //.;/".) The intestinal branch of the chilarial haemal nerve passes 

 through the second foramen,/ 2 . 



The occipital foramen is the large canal enclosed by the basioccipital and the 

 supraoccipital plates. Through it passes the spinal cord, the chelarial and 

 opercular nerves. 



Endocranium of the Scorpion (Figs. 43, 216, 217, 218). The endo- 

 cranium of a small American scorpion, probably Buthus carolinianus, was recon- 

 structed, by plotting serial sections of the whole thorax. 

 The endocranium of the large African scorpion was dis- 

 sected out and the drawings and measurements were 

 made under the simple microscope. 



In the American scorpion, Fig. 216, the endocranium 

 consists of two nearly parallel plates of fibre-cartilage, or 

 trabeculae; they are united at their posterior ends by a 

 broad basilar plate, which extends laterally into w r ing- 

 like, posterior-lateral processes, I. p., and backward in 

 two long posterior processes, p.p. The thickened median 

 portion forms the basioccipital. The trabeculae arc- 

 united in front of the basioccipital by a thick membrane, 

 on the neural surface of \vhich lies the enlarged anterior 

 end of the bothroidal cord, m.c. Opposite the ends of the 

 membrane, are two plate-like haemal processes, directed 

 haemally and laterally. The middle portion of the 

 trabeculae are thin, nearly horizontal plates. The anterior ends are greatly 

 thickened and end in two pointed processes. 



There is a distinct occipital ring enclosing the posterior part of the brain. 



In the large African scorpion (Figs. 217, 218), the endocranium is heavy and 

 well developed and in its general form resembles the one just described. The 

 neural surface of the diverging trabeculas is flat and their united posterior ends 

 form a thick basilar plate from which spring, right and left, the broad vertical 

 plates representing the posterior lateral processes, 1. p. The supraoccipital, 



FIG. 216. Endocranium 



of an American scorpion 



(Buthus) seen from the hasmal 



surface. Reconstructed from 



sections and dissections. 



