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 paleostracan 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 
SAM add. 
hp cas 2. con. tub. 
Fic. 124.—D1aGRAM 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; 7. Lat., lateral line nerve; X., epibran- 
chial ganglia of vagus; R. br. prof. VII., ramus branchialis 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. 
