PHYLUM CHORD AT A 



307 



Medullary 

 tube 



Coeloni 



blood vessels making it possible for respiration to take place. In the 

 higher vertebrates, however, these slits, which start to develop during 

 embryonic life, either become closed or never are open, and the respiratory 

 function is assumed by lungs. The latter are developed as ventral 

 outpocketings of the pharynx somewhat posterior to the pharyngeal 

 slits. Because of their formation from evaginations of the wall of the 

 pharynx which unite with invaginations of the body wall, the pharyngeal 

 slits are lined with entoderm internally and ectoderm externally. 



The dorsal tubular nervous system develops from a strip of ectodermal 

 cells lying in the median line on the dorsal side of the body. This strip 

 of cells sinks inward, forms a groove, and then, by the meeting of the 

 side walls above, becomes a tube, 

 called the medullary tube (Fig. 201). 

 This development of a central nerv- 

 ous system by invagination is pecul- 

 iar to the chordates. Typically 

 this tube, when first formed in the splanchnic 

 embryo, opens at the anterior end mesoderm 

 to the outside and at the posterior 

 end turns ventrally around the end 

 of the notochord to become con- 

 tinuous with the posterior end of 

 the archenteron. Later the open- 

 ing to the outside is closed and 

 the connection with the archenteron 

 is severed, thereby producing a 

 closed neural tube the cavity of 

 which is known as the neurocoel. In the vertebrates the anterior end 

 of this neural tube becomes dilated and forms the brain, while the 

 remaining part becomes inclosed in the vertebral, or spinal, column and 

 forms the spinal cord. 



Another characteristic of the chordates is the gradual obscuring 

 of the metameric condition in the body wall by the fusion of the meta- 

 meres and the shifting of superficial muscles. Throughout the phylum, 

 however, the deeper structures remain metamerically arranged. 



335. Advances Shown by the Chordates. — In the characteristics just 

 enumerated the chordates show advances over all lower phyla, and these 

 advances are the foundation upon which their supremacy in the animal 

 kingdom rests. An internal skeleton does not give so great a leverage 

 to muscles as does the exoskeleton of the arthropods, but this mechanical 

 disadvantage is more than balanced by the freedom of movement which 

 is allowed. An organism incased in an exoskeleton can never be so 

 mobile, and on the whole can never become so efficient, as can one the 

 body and appendages of which possess an internal skeleton. The 



Mesoderm 

 from which 

 will develop 

 the body 

 muscles 



'omafic 

 mesoderm 



Alimentary 

 canal 



Fig. 201. — Somewhat diagrammatic rep- 

 resentation of a section through the body of 

 an amphioxus larva at a stage later than any 

 shown in Fig. 48, with which it may be 

 compared. The ectoderm is white, the 

 entoderm lined, and the mesoderm stippled. 



