268 THE ORIGIN OF VERTEBRATES 
branchial appendages, and not after the fashion of the air-breathing 
scorpion, by means of the dorso-ventral muscles of the mesosoma. 
The only mesosomatic dorso-ventral muscles which were retained 
were those of the foremost mesosomatic segments, 1.2. those supplied by 
the VIth nerve, which were preserved owing to their having taken on 
@ prosomatic position and become utilized to assist in the movements 
of the lateral eyes. 
Let us turn now to the consideration of the corresponding muscu- 
lature in Limulus and in the scorpion group. These muscles con- 
stitute the markedly segmental muscles to which I have given the 
name ‘dorso-ventral somatic muscles.’ They are most markedly 
segmental in the mesosomatic region, both in Limulus and in Scorpio, 
each mesosomatic segment possessing a single pair of these vertical 
mesosomatic muscles, as Benham calls them (ef. Fig. 58 (Dv.)). In 
the prosomatic region the corresponding muscles are not so clearly 
defined in Limulus; they are apparently attached to the plastron 
forming the group of plastro-tergal muscles. From Benham’s descrip- 
tion it is sufficiently evident that they formed originally a single pair 
to each prosomatic segment. 
In Scorpio, according to Miss Beck, the dorso-ventral prosomatic 
muscles are situated near the middle line on each side and form the 
following well-marked series of pairs of muscles, shown in Fig. 110, A, 
taken from her paper, and thus described by her :— 
1. The dorso-cheliceral-sternal muscle (61) is the most anterior 
of the dorso-ventral muscles. It is very small, and is attached to the 
carapace near the median line anteriorly to the central eyes. 
2. The median dorso-preoral-entosclerite muscle (62) is a large 
muscle, between which and its fellow of the opposite side the eyes are 
situated. It is attached dorsally to the carapace and ventrally to the 
pre-oral entosclerite. 
3. The anterior dorso-plastron muscle (63) is attached dorsally 
to the carapace in the middle line, being joined to its fellow of the 
opposite side. .They separate, and are attached ventrally to the 
plastron. Through the arch thus formed the alimentary canal and 
the dorsal vessel pass. 
4, The median dorso-plastron muscle (64) is attached dorsally to 
the posterior part of the carapace. It runs forward on the anterior 
surface of the posterior flap of the plastron to the body of the plastron, 
to which it is attached. 
