144 THE ORIGIN OF VERTEBRATES 



concentrated towards the cephalic end than in Liniulus, 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 

 same growth of the brain-region of the central nervous system would 

 cause the entosternite to be separated into two lateral trabecular, 

 which would then take up the ventro-lateral position of the two 

 trabecular of Ammoccetes. 



On the other hand, it might be that two lateral trabecular, 

 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, etc., 

 was formed. From an extensive consideration of the entosternite in 

 different animals, Schimkewitsch has come to the conclusion that this 

 latter explanation is the true one. He points out that the lateral 

 trabecules 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 trabecular are more 

 cartilaginous, while the transverse are more fibrous. Schimkewitsch, 

 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 segmen tally 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 Schimkewitsch, that the 

 essential part of the entosternite consists of two lateral trabecular, 

 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 trabecular function as supports 

 for the attachment of muscles, which are specially attached at 

 certain places. At these places transverse fibres belonging to some 



