I myself can confirm his description, and give in Fig. 61, B, the appearance of the entosternite of Thelyphonus or of Hypoctonus. The supra-oesophageal ganglia and part of the infra-oesophageal ganglia fill up the space _Ph._; stretching over the rest of the infra-oesophageal mass is a transverse trabecula, which is very thin; then comes a space in which is seen the rest of the infra-oesophageal mass, and then the posterior part of the plastron, ventrally to which lies the commencement of the ventral nerve-cord. [Illustration: FIG. 61.--A, ENTOSTERNITE OF LIMULUS; B, ENTOSTERNITE OF THELYPHONUS. _Ph._, position of pharynx.] {144}In these forms, in which the central nervous system is more 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 same growth of the brain-region of the central nervous system would cause the entosternite to be separated into two lateral trabeculæ, which would then take up the ventro-lateral position of the two trabeculæ of Ammocoetes. On the other hand, it might be that two lateral trabeculæ, 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, Schimkéwitsch has come to the conclusion that this latter explanation is the true one. He points out that the lateral trabeculæ 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 trabeculæ are more cartilaginous, while the transverse are more fibrous. Schimkéwitsch, from observations of structure and from embryological investigations, 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 correspond to the intermuscular tendons, situated lengthways, which are found in the ventral longitudinal muscles of most arthropods.