39 



THE VERTEBRATES. 



the first time a well-developed series of neural and haemal arches, the forerunners 

 of a true vertebral column. 



The cephalic appendages, a, that were so characteristic of the ostracoderms, 

 are here rudimentary and probably functionless. With the relative decrease in 

 the size of the head and the increase in the size of the trunk, the whole body is 

 better balanced and more suitable for an active, free swimming existence. Loco- 

 motion was probably effected largely by the flexible trunk and tail, although im- 

 mediately behind the branchial region there are traces of supports for small 

 pectoral fins, the first appearance in this phylum of paired appendages of the 

 vertebrate type. Pelvic fins were apparently absent. 



The arthrodires clearly represent a higher type of animals than the ostraco- 

 derms. They have successfully emerged from the precarious period of profound 

 metamorphosis in which the ostracoderms were engaged. While it lasted, an 

 active life was inhibited by the changes going on in the old organs, and by the 

 imperfect adjustment of the new. 



With the arthrodires that period is past. They have increased notably in 

 size; the eyes are fully adjusted to their new location within the neural tube; the 

 mouth has become capacious, and the jaws large and powerful, with formidable 

 cutting, or toothed margins, well suited for capturing and devouring animal food; 

 the respiratory region is set apart from the digestive and urogenital regions, and 

 the body is better balanced and better adapted for an active, free swimming life. 

 The arthrodires, therefore, quickly developed from the sluggish, plant-eating 

 stage of the ostracoderms, into the most active, rapacious, and formidable animals 

 of their time. But as a class they were short-lived, for the structural conditions 

 within had as yet attained only a temporary equilibrium, and the new mode of 

 life was rapidly producing new creative forces. Out of these conditions arose 

 the first true vertebrates of this phylum, the dipnoi and the teleostomes. 



The anatomical changes involved in the creation of the new types of animals 

 were comparatively insignificant. The head became relatively smaller and more 

 compact, the trunk larger, and the whole body assumed a more fish-like appear- 

 ance. (Figs. 261-264.) The lateral eyes grew still larger and took up a position 

 on the sides of the head, well behind the olfactory pits, which have also greatly 

 increased in size. The premaxillae fused, forming the fronto-nasal process, and 

 together with the maxillae became permanently fixed to the floor of the cranium. 

 The distal ends of the mandibles united in the median line, and their proximal 

 ends articulated with a cranial cartilage, to which the hyoid arch was also at- 

 tached; they then lose their rotary and transverse movements, and swing forward 

 and backward against the maxillary arch in typical vertebrate fashion. 



With the fusion in the median line of the three pairs of oral arches, their 

 dermal armor becomes the three pairs of fixed dental plates characteristic of the 

 dipnoi (Fig. 261, p.mx,moc.d.p.), and from which the isolated socketed teeth of 

 the higher vertebrates arose. 



The plates in the dorso-lateral walls of the primitive branchial shield now 



