THE RESPIRATORY SYSTEM 



249 



BRAIN , 



MYOTOME 



brane thus formed the pouches are converted into gill-shts. The branchial 

 arches develop from the regions between the gill-slits. Each arch has an 

 endodermal pharyngeal lining and an external ectodermal covering. The 

 core of each arch is mesodermal. 



The levator and depressor muscles of the gills are developed from the 

 hypomeric mest)derm enclosed in each branchial arch. The connective 

 tissue, the cartilage or bone, and 

 the blood-vessels of each arch are 

 derived from the mesenchyma. 



History of the Gills. Pharyn- 

 geal gills are peculiar to chordates 

 and are one of the most constant 

 characteristics of the group. This 

 should not be understood to imply 

 that invertebrates are without 

 structures from which gills might 

 have evolved. The origin of gills 

 from endodermic diverticula sug- 

 gests the possibility that their be- 

 ginnings may be seen in the 

 intestinal diverticula of fiat worms. 

 Were these diverticula to meet the 

 skin and become perforate, aper- 

 tures similar to gill-slits would be 

 formed. 



Gill-slits first appear in the 

 hemichordates. Rhabdopleura has 

 none, but Cephalodiscus has a single 

 pair. In most hemichordates the 

 number is considerable and in- 

 creases throughout life. Early in 

 their development, their number is 

 doubled by the growth of "tongue- 

 bars" which extend from the 

 dorsal side of the gill aperture to 

 the ventral side. Later the gill 

 bars thus formed become intercon- 

 nected by cross rods or sjmapticulae such as occur also in urochordates 

 and cephalochordates. 



In urochordates, the number of gill-slits varies from a single pair in 

 Appendicularia to the many characteristic of most genera. The gill-slits 

 of this group open into an atrial cavity developed by an ectodermal 

 ingrowth along the dorsal side of the body. 



COELOM"-- 

 ESOPHAGUS 



Fig. 231. — The pharyngeal regio 1 of a 

 young Squalus embryo, showing the 



visceral arches and clefts. 



