312 



THE ORIGIN OF VERTEBRATES 



venous sinus, while the opercular and chilarial segments are respec- 

 tively the foremost mesosomatic and the last prosomatic segments : 

 they signify that the palreostracan ancestor must have possessed a 

 separate set of segmental dorso-ventral muscles confined to the bran- 

 chial, opercular and chilarial or metastomal segments, which, on the 



\L£p.pit 

 -■M.adi- 



-Af con- St*. 



Fig. 124. — Diagram constructed from a series of Transverse Sections through 

 a Branchial Segment,' showing the arrangement and relative positions 

 of the Cartilage, Muscles, Nerves, and Blood-Vessels. 



Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured 

 blue are the internal sensory nerves to the diaphragms and the external sensory 

 nerves to the sense-organs of the lateral line system. Br. cart., branchial 

 cartilage; M. con. sir., striated constrictor muscles; M. con. tub., tubular 

 constrictor muscles ; M. add., adductor muscle ; D.A., dorsal aorta ; V.A., ventral 

 aorta; S., sense-organs on diaphragm ; n. hat., lateral line nerve; X., epibran- 

 chial ganglia of vagus ; E. br. prof. VII. , ramus brandiialis profundus of facial ; 

 J.v., jugular vein ; Ep. pit., epithelial pit. 



one hand, were respiratory in function, and on the other were attached 

 to the longitudinal venous sinus. Further, these muscles must all 

 have received a nerve-supply from the neuromeres belonging to the 

 chilarial and opercular segments, an unsymmetrical arrangement of 

 nerves, on the face of it, very unlikely to occur in an arthropod. 



