THE endocranium; summary. 



319 



The supraoccipital forms an arching roof over the posterior thoracic and vagus 

 neuromeres. The thoracic nerves pass out of the endocranium through the wide 

 open sides, while the nerves of the four vagus neuromeres, together with the nerve 

 cords, pass backward and out of the cranium through the foramen magnum. 



Telyphonus. — The endocranium of Telyphonus (Fig. 219), can be readily 

 reduced to the type of those described above. 



Fig. 219. — Endocranium of Telyphonus, A, Neural surface; S, haemal; C, side. Xsi/3- 



V. Summary and Comparison. 



I. Endocranium. 



1. A cartilaginous endocranium is eminently characteristic of the phyllopod- 

 arachnid-vertebrate stock. It appears to be absent in the insects, myriapods and 

 Crustacea. Its simplest adult condition is seen in Apus and Branchipus where 

 it is an unpaired, unsegmented basilar plate of iibrocartilage, with thickened 

 margins and projecting cornua. This is also its early embryonic condition in 

 Limulus. 



2. The evolution of the form, location, and mode of growth of the endocra- 

 nium was determined primarily by the size and functional activity of the mus- 

 cles and appendages with which it was associated. 



3. The basilar plate served primarily for the attachment of muscles arising 

 from the bases of the mandibles, or of the anterior thoracic appendages. It is first 

 seen in the anterior midbrain region as a thickened horizontal membrane, lying 

 between the nerve cord and the alimentary canal and extending across the median 

 line from the base of one appendage to its mate. 



In Adus and Branchipus it is confined to the mandibular metamere; in 



