5/2 THE BRANCHIAL BARS. 



Branchial skeleton. 



The most primitive type of branchial skeleton in any existing 

 form would appear to be that of the Petromyzonidse, which is 

 developed in a superficial subdermal tissue, and consists of a 

 series of bars united by transverse pieces, so as to form a basket- 

 work. It is known as an extra-branchial system, and an early 

 stage of its development in the Lamprey is shewn in fig. 47. In 

 the higher forms this system is replaced by a series of bars, 

 known as the branchial bars, so situated as to afford support to 

 the successive branchial pouches. Outside these bars there may 

 be present in some primitive forms (Elasmobranchii) cartilaginous 

 elements, which are supposed to be remnants of the extra- 

 branchial system (fig. 327, Ex.Br] ; while a series of membrane 

 bones is also usually added to them, which will be dealt with in a 

 separate section. The branchial bars are developed as simple 

 cartilaginous rods in the deeper parts of the mesoblast which 

 constitutes the primitive branchial arches. 



The position of the branchial bars in relation to the somatopleure and 

 splanchnopleure can be determined from their relation to the so-called head 

 cavities. These cavities atrophy before the formation of the cartilaginous 

 branchial bars, but it will be observed (fig. 328), that the artery of each 

 arch (aa) is placed on the inner side of the head cavity (pp}. The cartila- 

 ginous bar arises at a later period on the inner side of the artery, and 

 therefore on the inner side of the section of the body cavity primitively 

 present in the arches. 



An anterior arch, known as the mandibular arch, placed in 

 front of the hyo-mandibular cleft, and a second arch, known as 

 the hyoid arch, placed in front of the hyo-branchial cleft, are 

 developed in all types. The succeeding arches are known as 

 the true branchial arches, and are only fully developed in the 

 Ichthyopsida. 



In some Sharks (Notidani) seven branchial arches may be 

 present (not including the hyoid and mandibular). In other 

 Ichthyopsida five are usually present, in the embryo at any rate, 

 while in the Amniota there are usually two or three post-hyoid 

 membranous arches, in the interior of which a cartilaginous bar 

 is usually formed. The general form of these bars at an early 



