270 THE ORIGIN OF VERTEBRATES 



To these may be added, owing to its attachment to the plastron, 



5. The posterior dorso-plastron muscle (65). This is the first of 

 the dorso-ventral muscles attached to the mesosomatic tergites, being 

 attached to the tergite of the first segment of the mesosoma. 



This muscle is of interest, in connection with the prosomatic 

 dorso-ventral muscles, because it is attached to the plastron, and runs 

 a course in close contact with the muscle (64), the two muscles being 

 attached dorsally close together, on each side of the middle line, the 

 one at the very posterior edge of the prosomatic carapace, and the 

 other at the very anterior edge of the mesosomatic carapace. 



Taking these muscles separately into consideration, it may be 

 remarked with respect to (61) that the cheliceral segment in its 

 paired dorso-ventral muscles, as in its tergo-coxal muscles, takes 

 up a separate position isolated from the rest of the prosomatic 

 segments. 



Next comes (62) the median dorso-preoral-entosclerite muscle, 

 which is strikingly different from all the other dorso-ventral muscles 

 in its large size and the extent of its attachment to the dorsal cara- 

 pace, according to Miss Beck's figures. The reason of its large size 

 is clearly seen upon dissection of the muscles in Buthus, for I find 

 that, strictly speaking, it is not a single muscle, but is composed of 

 a series of muscle-bundles, separated from each other by connective 

 tissue. There are certainly three separate muscles included in this 

 large muscle, which are attached in a distinct series along the pre- oral 

 entosclerite, and present the appearance given in Fig. 110, A, at their 

 attachment to the prosomatic carapace. Of this muscle-group the 

 most anterior and the most posterior bundle are distinctly separate 

 muscles ; I am not, however, clear whether the middle bundle 

 represents one or two muscles. 



This division of Miss Beck's muscle (62) into three or four 

 muscles brings the prosomatic region of the scorpion into line with 

 the mesosomatic, and enables us to feel sure that a single pair of 

 dorso-ventral somatic muscles belongs to each prosomatic segment 

 just as to each mesosomatic, and, conversely, that each such single 

 pair of muscles possesses segmental value in this region as much as 

 in the mesosomatic. 



It is very striking to see how in all the Scorpionida3, in which the 

 two median eyes are the principal eyes, this muscle group (62) on 

 the two sides closely surrounds these two eyes, so that with a fixed 



