THE EVIDENCE OF THE SKELETON 



14; 



of its cells in a white fibrous tissue. What, then, is its topographical 

 position ? It is in all cases a median structure lying between the 

 cephalic stomach and the infra-< esophageal portion of the central 

 nervous system, and in all cases it possesses two anterior horns which 

 pass around the cesophagus and the nerve-masses which immediately 

 enclose the (esophagus (Fig. 61, A). These lateral horns, then, 

 which lie laterally and slightly ventral to the central nervous 

 system, and are called by Bay Lankester and Benham the sub- 

 neural portion of the entosternite, are very nearly in exactly the 

 position of the raccpuet- shaped head of the trabecuhe in Arnnioccetes. 

 It is easy to see that, with a more extensive growth of the nervous 

 material dorsally, such lateral 

 horns might be caused to take 

 up a still more ventral posi- 

 tion. Now, these two lateral 

 horns of the plastron of Li- 

 mulus are continued along 

 its whole length so as to form 

 two thickened lateral ridges, 

 which are conspicuous on the 

 flat surface of the rest of this 

 median plate. In other cases, 

 as in the Thelyphonida?, the 

 plastron consists mainly of 

 these two lateral ridges or 

 trabecuhe, as they might be 

 called, and Schimkewitsch, 

 who more than any one else has made a comparative study of the 

 entosternite, describes it as composed in these animals of two lateral 

 trabecular crossed by three transverse trabecule. I myself can con- 

 firm his description, and give in Fig. 61, B, the appearance of the 

 entosternite of Thelyphonus or of Hypoctonus. The supra-cesophageal 

 ganglia and part of the infra-cesophageal ganglia fill up the space Ph. ; 

 stretching over the rest of the infra-cesophageal mass is a transverse 

 trabecula, which is very thin ; then comes a space in which is seen 

 the rest of the infra-cesophageal mass, and then the posterior part of 

 the plastron, ventrally to which lies the commencement of the ventral 

 nerve-cord. 



In these forms, in which the central nervous system is more 



Fig. 61. — A, Entosternite of Limulus ; 

 B, Entosternite of Theta'phonus. 



Ph., position of pharynx. 



