

THE ENDOSKELETON 



155 



purpose that they have had no rivals for the office; thus, al- 

 though the changes in form and arrangement have been num- 

 berless, the teeth of even the highest vertebrates, composed of 

 dentine overlaid by enamel, attest their origin from placoid 

 scales. Aside from the testimony of comparative anatomy, 

 the embryonic history of the teeth, even in the highest 

 form, is a direct corroborative testimony to this mode of 

 origin. 



Associated with the first pair of visceral arches, the primi- 

 tive jaws, are the cartilages of the second pair, the hyoid, 

 also emancipated from the function of bearing gills and modi- 

 fied in part to assist in the action of the jaws. Like the first, 

 these arches also consist of two pieces, a dorsal and a ventral 

 one. The first, called the hyomandibular, is more or less de- 

 tached from the other and forms an intermediate piece, tech- 

 nically called a suspensorium, between the cranium and the 

 true jaws. To this is also attached the ventral piece, or hyoid 

 proper. 



The remaining five arches, the genuine branchial arches, 

 are all much alike, and are gill-bearing, associated with gill- 

 slits. Each one consists of four pieces, two dorsal and two 

 ventral, the two sets bent at an angle with each other. Along 

 the mid-ventral lines the pairs are united and held to one 

 another by median pieces, the basi-branchials, of which there 

 is typically one for each pair, but in living selachians the full 

 number is seldom represented. The additional gill arches in 

 the two primitive forms have been referred to above and are 

 represented in the diagram, Fig. 41, A, by dotted lines. 



Aside from these definite and well-developed visceral arches, 

 as they may be called collectively, there are the rudiments 

 mentioned above, functionally of little importance, and held 

 by some to be the reduced remnants of still other arches. 

 Such are the labial cartilages, lying at the angle of the mouth 

 and used to strengthen the integumental folds of the lips; 

 another of these is the spiracular cartilage, crescentic in shape 

 and surrounding the spiracidum or blow-hole, perhaps a modi- 

 fied gill-slit, anterior to the others. These, if admitted tcf the 



