472 



THE BRANCHIAL BARS. 



■jPP 



Fig. 328. Horizontal sec- 

 tion THROUGH THE PENULTI- 

 MATE VISCERAL ARCH OF AN EM- 

 BRYO OF Peistiurus. 



ej). epiblast ; re. pouch of 

 hypoblast which will form the 

 walls of a visceral cleft ; pp. 

 segment of body-cavity in vis- 

 ceral arc-h ; aa. aortic arch. 



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 Ichthyop- 

 sida 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 carti- 

 laginous bar is usually formed. The 

 general form of these bars at an eai'ly 

 stage of development is shewn in the 

 dog-fish (Scyllium) in fig. 321). 



The simple condition of these bars in 

 the embryo renders it highly probable that 

 forms existed at one time with a simple 

 branchial skeleton of this kind : at the 

 present day however such forms no longer 

 exist. The first arch has in all cases 

 changed its function and has become con- 

 verted into a supporting skeleton for the 

 mouth ; the hyoid arch, though retaining 

 in some forms its branchial function, has 

 in most acquired additional functions and 

 has undergone in consequence various peculiar modifications. The 

 true branchial arches retain their branchial functions in Pisces and 



some Amphibia, but are second- 

 arily modified and largely ab- 

 orted in the abranchiate forms. 

 Since the changes undergone 

 by the true branchial bars are 

 far less complicated than those 

 of the hyoid and mandibular 

 bars it will be convenient to 

 treat of them in the first in- 

 stance. 



These bars are, as already 

 mentioned, most numerous in 

 certain very primitive forms 

 (seven in Notidanus), while as 

 we ascend the series there is a 

 gradual tendency for the pos- 

 terior of them to disappear. 

 This tendency is the result of 

 a gradual atrophy of the pos- 

 terior branchial pouches, which commenced at a stage in the 

 evolution of the Chordata long prior to the appearance of carti- 

 laginous or osseous branchial bars, and reaches its climax in the 

 Amniota. 



In a fully developed branchial bar tlie primitively simple rod of 



Fig. 329. Head of embryo Dogfish, 

 11 lines long. (From Parker. ) 



Tr. trabecula ; Pl.Pt. pterygo quadrate ; 

 37. Pt. nietapterygoid region ; Mn. mandi- 

 bular cartilage ; Hy. hyoid arch; Br.l. first 

 branchial arch ; Sp. mandibulo-hyoid cleft ; 

 Cl^. hyo-branehial cleft ; Lch. groove below 

 the eye; Na. olfactory rudiment; E. eyeball; 

 An. auditory mass; CI, 2, 3. cerebral vesi- 

 cles; Hm. hemispheres; f.n.p. nasofrontal 

 process. 



