v. 5 DEVELOPMENT OF THE JAWS 143 



that is so characteristic of many early vertebrates. Modern research 

 has shown that the armour has become progressively reduced along 

 various lines of iish evolution. Older ideas of comparative anatomy 

 regarded the 'cartilage fishes' as showing a primitive stage, preceding 

 the appearance of bone. We now realize that this is the opposite of the 

 truth and that the dogfish and its relatives represent a higher type, 



rosC 



Fig. 96. Skull and branchial arches of the dogfish (Scyliorliimts). 

 au.c. auditory capsule; b.b. basibranchial; b.li. basihyal; c. centrum; cer.b. ceratobranchials; 

 cer.h. ceratohyal; d.r. foramen for dorsal root; e.b. extrabranchials; e.c.f. external carotid 

 foramen; e.l. ethmoid ligament; ep.b. epibranchials; gr. groove for anterior cardinal sinus; 

 g.r. gill rays; hy.a. foramen for hyoid artery; hymd. hyomandibula; i.d. interdorsal; io.c. 

 interorbital canal; I.e. labial cartilages; M.c. Meckel's cartilage; na. neural arch; nas.c. nasal 

 capsule; o.n.f. orbito-nasal foramen; op. foramen for ophthalmic nerve; op.g. groove for 

 op.V; op.V, op. VII ophthalmic branches of V and VII; orb. orbit; ph.b. pharyngobranchials ; 

 p.sp.l. prespiracular ligament; r. rib; rost. rostral cartilages; spd. supradorsals ; tr. transverse 

 process; vr. foramen for ventral root; II-IX, foramina for cranial nerves. (After Borradaile.) 



able to defend themselves by mobility, by biting, and by efficient 

 sensory and nervous organization. Heavy defensive armour is a 

 primitive form of protection for animals, as for man. 



Besides its use in feeding and defence, the mouth can also be used as 

 a means of 'handling' the environment, for instance in the nest-build- 

 ing activities of many fishes. Indeed, it is difficult for us to realize the 

 utility of the jaws for an animal not provided with any other means of 

 seizing hold of objects. 



The development of the mouth to a point at which it could be used 

 in these varied ways was, therefore, a very important stage in evolution. 

 Recognition of the Gnathostomata as a separate group of animals is 

 far more than a matter of classificatory convenience, it marks the 

 achievement of the possibility of life in a greatly increased range of 

 environments. 



