320 ENDOCRANIUM, BRANCHIAL AND NEURAL CARTILAGES. 



Limulus and other arachnids, it arises from all the thoracic and first vagus 

 metameres, and in the adult it may cover a much wider territory than the entire 

 brain. 



4. The extension backward of the basilar plate in the arachnids, and the 

 increased volume of its posterior portion, was due to the diminished size of the 

 anterior thoracic appendages and the increase in size of the posterior ones. Also 

 to the fact that as the thoiacic metameres united more intimately with each other 

 and with the forehead, the posterior portion of the basilar plate served as a more 

 and more important point of attachment for those voluminous longitudinal trunk 

 muscles that helped to move the whole cephalothorax on its hinge-like joint. 

 Another factor that aided in the development of the occipital region was the crowd- 

 ing forward of the neural arches of the vagus segments to form the supraoccipital 

 plate, and the union of the latter with the underlying basilar plate to form the 

 occipital ring. The occipital region was also reinforced by the addition of the 

 branchial bars of the rudimentary vagus appendages to the posterior part of the 

 basilar plate. 



5. The basilar plate is anchored to the haemal wall of the cranium by the 

 powerful plastro-tergal muscles that arise from the keel-like haemal plates. The 

 size of these plates, their form, and the direction of their fibrous constituents, 

 is determined by the amount and direction of the strain on them, and hence is 

 determined indirectly by the same factors that control the form and development 

 of the basilar plate and the occipital ring. 



6. Thus the evolution of the arachnid endocranium becomes intelligible. 

 Its four main axes, or planes, of growth that have given rise to the transverse 

 basilar plate, the longitudinal trabeculae, the vertical haemal plates, and the heavy 

 occipital ring, are due more remotely to those causes controlling the concrescence 

 of the cephalic metameres, and more directly to the nature of the three axes of 

 muscular strain, transverse, longitudinal, and vertical, that have acted upon it. 

 It will be observed that owing to the relation of the endocranium to the mouth, 

 brain, stomodaeum, and intestine, the framework of the endocranium could not 

 be constructed along any other lines than those indicated. (Fig. 220.) 



7. There is no reason to doubt that the conditions prevailing in living arach- 

 nids, also obtained in the trilobites and merostomata. In the gigantic eurypter- 

 ids, with powerful, oar-like appendages and movable head, the endocranium 

 probably reached a higher grade of development than in any of their living repre- 

 sentatives. 



8. The fully developed arachnid endocranium is in every essential respect a 

 duplicate of the primordial cranium of a primitive vertebrate embryo. They 

 agree : a. In their position relative to the brain; b. in their general form; c. in their 

 mesodermic origin and histological structure; d. in their absence of segmenta- 

 tion, although spreading over several metameres; e. in their great size compared 

 with the brain; and /. which is the most important agreement of all, they agree 

 in their four axes, or planes of growth; namely, the transverse growth of the basilar 



