138 THE ORIGIN OF VERTEBRATES 
At the base of each of these appendages, where it is attached to 
the body of the animal, the external chitinous surface is characterized 
Fic. 58.—TRaNSVERSE SECTION THROUGH THE Mrsosoma OF LimuLus, TO sHOW 
THE ANTERIOR (A) AND THE PosTERIOR (B) SuRFAcES OF 4 MEsOsoMATIC OR 
BRANCHIAL APPENDAGE, 
In each figure the branchial cartilaginotis bar, Br.C., has been exposed by dissection 
on one side. Hnt., entapophysis; Hnt.l., entapophysial ligament cut across; 
Br.C., branchial cartilaginous bar, which springs from the entapophysis; H., 
heart; P., pericardium; Al., alimentary canal; N., nerve cord; L.V.S., longi- 
tudinal venous sinus; Dv., dorso-ventral somatic muscle; Vp., veno-pericardial 
muscle. 
by a peculiar stumpy, rod-like marking, and upon removing the 
chitinous covering, this surface-appearance is seen to correspond to a 
well-marked rod of cartilage (Br.C.), which extends from the body 
