PHYLUM CHORDATA 225 



through the mouth and used for respiration, but are present 

 in terrestrial forms only during development. Between 

 the slits run the aortic arches carrying blood, and these 

 great arteries are often supported by cartilaginous or bony 

 structures. The nervous system of chordates is formed 

 during development by the turning in of the ectoderm on 

 the dorsal surface (Fig. 92). At first there is simply a 

 longitudinal depression (A), and the deeper part of this is 

 later pinched off (B, C) to form a longitudinal tube inside 

 the body wall (C). This tube usually becomes thickened 

 and expanded somewhat at the anterior end to form a brain, 

 and nerves grow out from it to all parts of the body, but 

 even in ah adult man it is still tubular in its fundamental 

 structure. 



In some of the most primitive living chordates, such as 

 the lamprey eels and the lancelets, the chorda is functional 

 and constitutes practically the sole skeletal structure within 

 the body, but in the great majority of the vertebrates it is 

 replaced to a greater or lesser extent^ by a series of verte- 

 brae which together make up the spinal column, or " back- 

 bone." There is also a transition in the aortic arches in 

 passing from primitive to more specialized chordates. 

 The aquatic representatives have functional gill slits and 

 well-developed aortic and skeletal arches. The frogs and 

 toads begin active life with a fish-like form and functional 

 gills, but later lose the gills and have the blood diverted 

 to the lungs and skin for aeration. Reptiles, birds, and 

 mammals never breathe through gills, but nevertheless have 

 gill slits and gill arches during early developmental stages. 

 The primitive arterial arches degenerate in the adults or 

 are diverted to supply new parts with blood (Fig. 105). 

 The skeletal parts of the arches which serve to support gills 

 in primitive chordates are for the most part transformed 

 into jaws, throat cartilages, and the bones of the face in 

 more specialized types. 



Thus, though the same fundamental plan of structure is 

 present in all chordates, parts may be so modified as to 



