THE EVIDENCE OF THE SKELETON 143 
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-cesophageal 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 cesophagus (Fig. 61, A). These lateral horns, then, 
which lie laterally and slightly ventral to the central nervous 
system, and are called by Ray Lankester and Benham the sub- 
neural portion of the entosternite, are very nearly in exactly the 
position of the racquet-shaped head of the trabecule in Ammoccetes. 
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 Thelyphonide, the 
plastron, consists mainly of 
these two lateral ridges or 
trabecule, as they might be 
called, and Schimkéwitsch, 
who more than any one else has made a comparative study of the 
entosternite, describes it as composed in these animals of two lateral 
trabeculz 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 
Fic. 61.— A, ENTOsTeRNITE oF LIMULUS; 
B, ENTOSTERNITE OF THELYPHONUS. 
Ph., position of pharynx. 
