146 ORGANIZATION OF THE HEAD v. 6 



parts of the pharynx wall and the anterior arches could be called into 

 play to help in the collection of food. In this way the mouth came to 

 be used for prehension, and the grasping jaws of the gnathostomes 

 appeared as the more anterior arches became modified to allow more 

 efficient seizing, and the skin over them was modified to form the 

 teeth. The mouth probably shifted backwards during this process and 

 its lateral edges joined the first gill-slit. The rods supporting the 

 posterior wall of that slit thus became bent over into the characteristic 

 position of the vertebrate jaws. 



There is some uncertainty as to the means of support of the jaws 

 in the earlier stages of their evolution. The front end of the palato- 

 pterygo-quadrate bar is attached to the cranium in the dogfish by the 

 ethmo-palatine iigament'. In most elasmobranchs the hind end of the 

 upper jaw is not fixed to the cranium but is slung from the latter by 

 the hyomandibula and by a prespiracular ligament. This means of 

 support, known as hyostylic, was for long supposed to have been the 

 original one. But the earliest gnathostomes (the acanthodians) do not 

 have this arrangement (p. 187), indeed, their hyoid arch is an almost 

 typical branchial arch, not modified to support the jaw. In the primi- 

 tive condition one would not expect the hyoid arch to have any con- 

 nexion with the mandibular. In the acanthodians the jaw is supported 

 by direct attachments to the cranium at its hind as well as front end, 

 a condition known as autodiastylic. 



The early elasmobranchs themselves do not have a hyostylic jaw 

 support, but an arrangement in which the upper jaw is both attached 

 to the cranium and also supported by the hyomandibula. This 

 amphistylic condition persists to-day in the primitive shark Hexan- 

 chus. Apparently the jaws, which at first swung from the skull, later 

 became fixed at the hind end to the hyoid, and this finally became the 

 only means of support posteriorly. The advantage of this last arrange- 

 ment is presumably that it allows a wide gape for swallowing, the prey 

 whole. As the sharks sought to eat larger and larger fishes, those in 

 which the hind end of the upper jaw was less firmly fixed to the skull 

 were the more successful and so the hyostylic condition was achieved. 



If this theory of the origin of the jaws is correct we may expect to 

 find some trace of a cartilaginous support for the side wall of the 

 pharynx in front of the original first gill-slit, a premandibular visceral 

 arch. Many sharks have two pairs of labial cartilages in this position, 

 which have been held to represent arches. However, there are strong 

 grounds for believing that this is represented by the trabeculae cranii, 

 the rods lying on each side in front of the parachordals and contribut- 



