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AN INTRODUCTION TO ZOOLOGY 



joined by the fusion of their dorsal extremities. The epibranchials 

 are short stout rods lying almost vertically in the side walls of the 

 pharynx and articulating dorsally with the pharyngeo- and ventrally 

 with the cerato-branchials. These latter are more elongated rods 

 passing forwards and inwards in the ventral pharyngeal wall. The 

 hypobranchials are not so regular as the other elements. The first 

 pair are short slender rods running forward and joining the first 

 cerato-branchials to the basihyal. Hypobranchials 2, 3 and 4 are 

 also small rods, but running backwards ; the first pair join in the 

 middle line and also connect with the succeeding pair, while the 

 third and fourth pairs put their respective arches in connection with 

 the basibranchial plate. The fifth arch lacks a hypo branchial, its 

 ceratohyal, which directly abuts on to the basibranchial, is much 

 expanded and has a conspicuous notch at its postero-lateral corner 

 through which an important blood-vessel, the ductus Cuvieri, passes. 

 The basibranchial plate is a median flattened sheet of cartilage 

 helping to form the roof of the pericardial cavity. The posterior 

 borders of all epi- and cerato-branchials, except the fifth, bear a 

 series of small unbranched branchial rays. In this manner is con- 

 stituted a fairly complex skeletal framework, the branchial basket. 

 Other series of curved rod-like elements, the extra branchiate, are 

 developed just below the skin in relation to the second, third and 

 fourth arches, but as they are not firmly attached to the other 

 branchials they are often missing in prepared skeletons. The 

 relation of the various parts of a typical visceral skeleton may be 

 represented as follows : 



Development. The mandibular arch is first seen as two curved 

 rods of cartilage, one on each side, lying in the hinder margin of the 

 mouth and meeting in the mid- ventral line ; each later splits into 

 upper and lower jaws. The hyoid arch arises in a similar manner 

 in the septum between the spiracle and the first gill cleft, but the 

 rods do not meet in the middle line. Its upper end soon acquires 

 an articulation with the auditory capsule, and later it splits into two. 

 The branchial arches also arise as two half hoops of cartilage in the 



