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, i.e. those supplied by 

 the Vlth nerve, which were preserved owing to their having taken on 

 a 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 (cf. 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 (03) 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. 



