574 



THE BRANCHIAL BARS. 



a stage in the evolution of the Chordata long prior to the 

 appearance of cartilaginous or osseous branchial bars, and 

 reaches its climax in the Amniota. 



In a fully developed branchial bar the primitively simple rod 

 of cartilage becomes divided into a series of segments, usually 

 four, articulated so as to be more or less mobile : and either 

 remaining cartilaginous or becoming partially or wholly ossified. 

 Each bar (fig. 327) forms a somewhat curved structure, embracing 

 the pharynx. The dorsal and somewhat horizontally placed 

 segment is known as the pharyngobranchial (Ph.Br), the next 

 two as the epibranchial (E.Br) and ceratobranchial (C.Br), and 

 the ventral segment as the hypobranchial (H.Br). There is 

 also typically present a basal unpaired segment, uniting the bars 

 of the two sides, known as the basibranchial (B.Br). The arches 

 often bear cartilaginous rays which support the gill lamellae. 



In Teleostei dental plates are usually developed as an 

 exoskeletal covering on parts of the branchial arches. 



In the Amphibia four or three branchial arches are present in 

 the embryo. These parts are more or less completely retained 

 in the Perennibranchiata and Caducibranchiata, but in the 

 Myctodera and Anura they become largely reduced, and 

 entirely connected with the hyoid. 



In the Anura they never reach any considerable development, 

 and are soon reduced to a 

 plate (fig. 330) the coalesced 

 basihyal and basibranchial 

 plate the posterior processes 

 of which represent the rem- 

 nants of the branchial arches. 



According to Parker the pos- 

 terior process of this plate in the 

 adult is a remnant of the fourth 

 branchial bar ; the next one is 

 the third branchial bar, while the 

 anterior lamina behind the hyoid 

 is stated by him (though this is 

 somewhat doubtful) to be a rem- 

 nant of the first two bars. 



In the Amniota, the bran- 

 chial arches become still more 



Pmx 



FIG. 330. YOUNG FROG, WITH TAIL 

 JUST ABSORBED ; SIDE VIEW OF SKULL. 

 (From Parker.) 



An. auditory capsule; in front of it is 

 the cranial side wall ; A.N. external nostril ; 

 St. stapes; Mck. Meckelian cartilage; B.Hy. 

 basihyobranchial plate; St.Hy. stylohyal 

 or ceratohyal; Br.i. first branchial arch. 



Bones: E-0. exoccipital; Pr.O. pro- 

 otic ; Pa. parietal ; Fr. frontal ; Na. nasal ; 

 Pmx. premaxillary ; MX. maxillary; Pt. 

 pterygoid; Sq. squamosal; Qn-J't. quadra - 

 tojugal; Art. articular; D. dentary. 



