62 CLASS PISCES. 



strengthened by incomplete cartilaginous rings, analogous to the 

 cartilaginous rings found in the trachea of the higher Vertebrata. 

 Like the tracheal rings, they serve to keep open the tube in this 

 case the mouth and pharynx through which the respiratory 

 medium passes to the respiratory organ, but they differ from 

 them in being divided up into segments which are movable upon 

 one another by means of muscles. The Visceral Arches, as 

 these structures are called, are placed in the splanchnic meso- 

 derm, as shown by embryology, and therefore have nothing to 

 do with ribs, with which they have sometimes erroneously been 

 compared. They may be described as consisting of a series of 

 cartilaginous rods on each side, joining one another ventrally, 

 but usually (except in the case of the first two) ending freely 

 dorsally without connection with other skeletal structures. 



The first arch is called the Mandibular ; its skeleton lies near 

 the lips and constitutes the jaws. The second is called the 

 Hyoid Arch : it lies in the pharynx wall between the spiracle 

 and the first branchial cleft. The remainder ^ of which there are 

 usually five,- are the Branchial Arches : they lie in the pharynx 

 wall between the branchial clefts, the last always occurring 

 behind the last cleft. In Heptanchus there are seven branchial 

 arches and seven clefts ; in Hexanchus and Chlamydoselachus 

 there are six. 



The mandibular arch becomes closely associated with the 

 cranium. It always becomes divided into two pieces : of these 

 the dorsal piece forms the skeleton of the upper jaw and is called 

 the palato- quadrate bar, while the ventral piece constitutes the 

 cartilage of Meckel. The dorsal piece is longitudinally directed 

 beneath the skull from the auditory to the ethmoidal region ; it 

 gives articulation at its posterior (quadrate) end to Meckel's 

 cartilage. This upper segment of the mandibular arch presents 

 two principal arrangements in fishes. In the one of these, that 

 which is generally called the hyostylic, its hind end is not attached 

 to the skull directly but is held up by the stout dorsal segment 

 of the hyoid arch, which is for this reason called the hyo-mandib- 

 ular. This is the arrangement found in most fishes. In 

 Chimaera and Dipnoi, however, a different arrangement is found. 

 In these and in some extinct fishes the palato- quadrate bar is 

 fused with the skull not only posteriorly in the auditory region, 

 but anteriorly in the ethmoid region and in the intermediate 



