144 THE ORIGIN OF VERTEBRATES 
concentrated towards the cephalic end than in Limulus, the whole of 
the concentrated brain-mass is separated from the gut only by this thin 
transverse band of tissue. Judging, then, from the entosternite of 
Thelyphonus, it is not difficult to suppose that a continuation of the 
sane growth of the brain-region of the central nervous system would 
cause the entosternite to be separated into two lateral trabecule, 
which would then take up the ventro-lateral position of the two 
trabecule of Ammoccetes. 
On the other hand, it might be that two lateral trabecule, 
similar to those of Thelyphonus and situated on each side of the 
central nervous system, were the original form from which, by the 
addition of transverse fibres running between the gut and nervous 
system, the entosternite of Thelyphonus and of the scorpions, ete., 
was formed. From an extensive consideration of the entosternite in 
different animals, Schimkéwitsch has come to the conclusion that this 
latter explanation is the true one. He points out that the lateral 
trabecule can be distinguished from the transverse by their structure, 
being much more cellular and less fibrous, and the cell-cavities more 
rounded, or, as I should express it, the two lateral trabecule are more 
cartilaginous, while the transverse are more fibrous. Schimkéwitsch, 
from observations of structure and from embryological investi- 
gations, comes to the conclusion that the entosternite was originally 
composed of two parts— 
1. A transverse muscle corresponding to the adductor muscle of 
the shell of certain crustaceans, such as Nebalia. 
2, A pair of longitudinal mesodermic tendons, which may have 
been formed originally out of a number of segmentally arranged 
mesodermic tendons, and are crossed by the fibrils of the transverse 
muscular bundles. 
These paired tendons of the entosternite he considers to corre- 
spond to the intermuscular tendons, situated lengthways, which are 
found in the ventral longitudinal muscles of most arthropods. 
It is clear from these observations of Schimkéwitsch, that the 
essential part of the entosternite consists of two lateral trabecule, 
which were originally tendinous in nature and have become of the 
nature of cartilaginous tissue by the increase of cellular elements in 
the matrix of the tissue: these two trabecule function as supports 
for the attachment of muscles, which are specially attached at 
certain places. At these places transverse fibres belonging to some 
