The branchial apparatus is restricted to the external and 

 internal divisions of the first branchial arch and the ventral 

 stub of the outer division of the second. Neither of these 

 arches is closely associated with a branchial sac. The sac of 

 the first arch may not even develop, whereas that of the sec- 

 ond appears but is moved backward during development and 

 then disappears. There is an irregular circlet of cartilaginous 

 rod around the common posterior branchial opening of 

 Myxtne; that of the left side encircles the pharyngocutaneous 

 duct as well. 



The development of this type is only partly known. The 

 general pattern is one of direct development to the adult 

 condition. The primordia of the nasal capsule arise as up- 

 ward processes from the anterior ends of the trabeculae. 

 There are two cross connections between the trabeculae in 

 front of the hypophyseal fenestra. The posterior one gives 

 attachment to the hypophyseal cartilage and the anterior 

 one disappears. Most interesting is the development of the 

 internal and external arch elements. 



Holmgren described blastemas for paired palatoquadrates, 



ACERASPIS 



BIRKENIA 



PTERASPIS 

 Figure 5-29. Lateral views of reconstructions of fossil ognaths: A, osteostracan, B, anospid, and 

 C, heterostrocan. (Partly after Stensio, Jorvik, and Romer) 



134 • THE HEAD SKELETON OF FISHES 



