THE ENDOCRANIUM. 21 



and lateral plates. (Figs. 16, 142.) The most characteristic organs found in 

 these metameres, such as the cartilaginous branchial bars and the segments of 

 the heart, arise from the mesoderm. 



The neuromeres usually remain separate, but there is a tendency for the 

 anterior ones to move forward and join the vagus group. In vertebrates, the 

 entire group has joined the vagus neuromeres, forming the most posterior part of 

 the medulla. (Fig. 58.) 



Nerves. In the arachnids, the great complex of vagus and branchial nerves 

 has already made notable progress in that separation of components from the 

 primary segmental nerves, and in their regrouping into compound nerves whose 

 constituent parts have a common function, that is so characteristic of vertebrates. 



We may recognize, for example, the beginning of the lateral line nerve in 

 the combined sensory components of the first three vagal appendages of the 

 scorpion. In Limulus, the primitive condition of the vertebrate cardiac nerves 

 is seen in the eight pairs of segmental cardiac nerves that arise from the vagal and 

 branchial neuromeres. (Figs. 59, 78, c 7 I4 .) The visceral arch nerves are repre- 

 sented by the branchial nerves, and the hypoglossal, by the combined group of 

 motor components that supply the great, branchio-thoracic muscles. (Fig. 77.) 

 The intestinal nerves are also indicated; i l ~ 



It is only necessary to unite the vagal and branchial neuromeres into a 

 more compact mass, and to complete the union of the sensory, branchial, hypo- 

 branchial, cardiac, and intestinal components into compound nerves, to realize 

 the characteristic condition so familiar in vertebrates. (Compare Figs. 5 7 and 58.) 



It will be observed that the similarity exists, not only in the union of the 

 originally separate components into the same physiological groups, but that the 

 number of neuromeres and components is approximately the same; that their 

 topographical position is the same; and that the general course and distribution 

 of the resulting nerves is the same. 



The Endocranium. 



All the higher arachnids are provided with a cartilaginous endocranium that 

 is the forerunner of the primordial cranium of vertebrates. It may be traced 

 back to such primitive arthropods as Branchipus, Apus, and other phyllopods. In 

 Branchipus, it is a small plate of cartilage, lying on the haemal side of the mesen- 

 cephalon, and serving for the attachment of the mandibular muscles. 



In the higher arachnids, it is more voluminous, serving mainly for the at- 

 tachment of the leg and jaw muscles, and for the great longitudinal muscles that 

 move the cephalothorax on the branchial section of the body. Its structure is 

 similar to that of the primordial cranium of vertebrates, and it has the same topo- 

 graphical relation to the brain and to the alimentary canal. The rudiments of 

 the following parts may be recognized: occipital ring, trabeculas, pituitary fora- 

 men, and palato-pterygoid arch. (Figs. 209-220.) 



