90 THE ANATOMY OF VEETEBEATED ANIMALS. 



snake (Bachiodon), vfhich has a series of what must be termed 

 teeth, formed by the projection of the inferior spinous pro- 

 cesses of numerous anterior vertebrae into the oesophagus. 

 And, in the highest Vertehrata, teeth are confined to the 

 premaxUlge, maxillae, and mandible. 



The Circulatm-y Organs. — The heart of the vertebrate em- 

 bryo is at first a simple tube, the anterior end of which 

 passes into a cardiac aortic trunk, while the posterior end 

 is continuous with the great veins which bring back blood 

 from the umbilical vesicle — the ornphalonieseraic veins. 



The cardiac aorta immediately divides into two branches, 

 each of which ascends, in the first visceral arch, in the 

 form of a forwardly convex aortic arch, to the under side 

 of the rudimentary spinal column, and then runs, parallel 

 with its fellow, to the hinder part of the body, as a pri- 

 mitive subvertebral aorta. The two primitive aortae very 

 soon coalesce throughout the greater part of their length 

 into one trunk, the definitive subvertebral aorta; but the 

 aortic arches, separated by the alimentary tract, remain 

 distinct. Additional arterial trunks, to the number of four 

 in the higher Vertebrata, and more in the lower, are succes- 

 sively developed, behind the first, in the other visceral arches, 

 and further connect the cardiac and subvei-tebral aortae. 



In the permanently branchiate Vertebrata, the majority of 

 these aortic arches persist, giving off vessels to the branchial 

 tufts, and becoming converted into afferent and efferent 

 trunks, which carry the blood to and take it from these 

 tufts. (Fig. 25, A, B, C, D, E.) 



In the higher Amphibia, which, though branchiate in the 

 young state, become entirely air-breathers in the adult con- 

 dition, such as the Batrachia (Fig. 25, F) and Ccecilia, the 

 permeable aortic arches are reduced to two (the middle pair 

 of the three which supply the external gills, and the fourth 

 pair of embryonic aortic arches) by the obliteration of the 

 cavities of the dorsal ends of the others. Of the posterior 

 arches, the remains of the fifth and sixth become the trunks 

 which give off the prdmonary arteries, and, in the Batr-achia, 



