372 STRUCTURE OF VERTEBRATES. 



branchs, and perhaps other Fishes, as also in Amphibians 

 and Lacertilians, but in higher Vertebrates they arise 

 independently." Most of the visceral muscles consist of 

 unstriped fibres, but those of the outer parts and of the 

 heart show the usual striped structure. 



Skeletal System.— K^2ctX. from the exoskeleton of skin 

 teeth, scutes, shields, etc., the skeleton consists of the 

 following parts — 



!The skull and its associated "arches." 

 The backbone and associated ribs. 

 (The notochord is transitory except in 

 the simplest Vertebrates.) 



(b) Appendicular f Fore-limbs, and pectoral girdle. 

 Skeleton. \ Hind hmbs, and pelvic girdle. 



Skull. — In very early stages the cavity in which the brain 

 lies is surrounded by a membranous sheath comparable to 

 that which ensheathes the notochord. The elements of the 

 skull are as follows ; — 



(a) The foundation is formed from two pairs of plates — 

 posterior parachordals and anterior trabeculse — which lie 

 beside and in front of the notochord. These are extended 

 upwards by a further formation of cartilage, the result of 

 which is a cartilaginous brain-box or chondrocranium. The 

 end of the notochord may also help a little. 



(p) The gristly brain-box is added to laterally by a pair of 

 cartilaginous nasal capsules in front, and a similar pair of 

 auditory capsules behind. 



(c) About the mouth there are some lip or labial cartilages, 

 but much more important is a series of arches (never more 

 than eight) which loop round the pharynx on each side, 

 running between the primitive gill-clefts. The two which lie 

 furthest forward — the mandibular and the hyoid arches — are 

 of great moment in the development of the skull ; the others 

 form supports for the region of the pharynx, which in Fishes- 

 and at least all young Amphibians bears open gill-slits. In 

 the Elasmobranch fishes, the mandibular and hyoid arches 

 do not form any direct part of the cartilaginous brain-case, 

 but in the Teleosteans and thence onwards they, or the bones- 



