THE ORIGIN OF VERTEBRATES 
i) 
“I 
e) 
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 Scorpionide, 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 
