STRUCTURE OF THE VERTEBRATES 



209 



These internal gills ma}- be covered entirely by endoderm, or 

 by both endoderm and ectoderm. 



In Amphioxus, it will be recalled, the afferent branchials break 

 into capillaries in the gills, and as the blood stream slows down 

 the exchange of gases takes place. The capillaries then re-collect 

 into efferent branchials which pass into the dorsal aortae. This 

 is essentially the same condition which prevails throughout the 

 gill-breathing fish and amphibia. In the amniotes the relation- 

 ships of the branchial arteries are the same, although they do not 

 break into capillaries. 



In the dogfish the gill is a paired structure, developing along 

 the inner surface of the gill bar. The two halves, or hemi-branchs, 

 of each gill lie in adjacent gill slits. Therefore the gills which lie 

 in a gill opening belong to different branchial cartilages: one 

 half belongs to the anterior bar, the other half to the next 

 posterior gill support. 



This relationship of the hemi-branchs is carried over to the 

 higher groups. The greatest change which occurs in the more 

 specialized fish is the modification of the septa separating the 

 gills from each other. The septa of the dogfish extend from the 

 pharynx to the outside of the animal, and the septum is cov- 

 ered externally by the body skin. Each gill slit, in other words. 



A. Dogfish 



Gilt- 

 support 



Septum- 

 Afferent 

 branchial 

 artery 



Efferent 

 artery 



Skeletal arch 



B. Sturgeon 

 (Chondrostei) 



C. Gar pike 

 (Holostei) 



D Typical Teleost 



Fig. 123. Cross Sections of Gills, showing reduction of the septum. 



