13^ 



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 



B 



N. E / U LMS. 



Fig. 58. — Transverse Section through the Mesosoma op Limulus, to show 

 the Anterior (A) and the Posterior (B) Surfaces of a Mesosomatic or 

 Branchial Appendage. 



In each figure the branchial cartilaginous bar, Br.C, has been exposed by dissection 

 on one side. Ent., entapophysis ; Ent.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-vencral 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 



