278 



THE ORIGIN OF VERTEBRATES 



This muscle-pair is, as it should be, the pair of dorso-veutral 

 muscles belonging to the segment immediately following on the 

 group of segments represented by the recti muscles, i.e. according 

 to previous argument, the segment belonging to the sixth pair of 

 locomotor appendages or ectognaths; a muscle, therefore, which 

 would arise in the vertebrate from the mandibular, and not from 

 the premandibular cavity. A similar muscle probably existed in 



© 





m nhl sup 



.IV" 1 nerve 



CZ> 



cn> 



m out sup 



Fig. 112. — A, Diagram of Position of Oblique Muscle in Scorpion; B, Diagram 

 of Transition Stage ; C, Diagram of Superior Oblique Muscle in Verte- 

 brate. 



I.e., lateral eyes; c.e., central eyes; CIV., central nervous system; Al., alimentary 



canal; c, aqueductus Sylvii. 



Eurypterus (M.obl. in Fig. 106, B), and, as in the case of the for- 

 mation of the oculomotor group, derived from the recti group of the 

 scorpion, would form the commencement of the superior oblique 

 muscle in Thyestes and Tremataspis. 



It is instructive to notice that the original position of attachment of 

 this muscle is naturally posterior to that of the oculomotor group of 

 muscles, and that Furbringer, in his description of the eye-muscles 

 of Petromyzon, asserts that this muscle in this primitive vertebrate 



