SKELETAL SYSTEM. 431 



the trunk, head, and limbs, as well as of the heart, show the usual 

 striped structure. 



Skeletal system. — Apart from the exoskeleton of skin- 

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

 following parts : — 



( The skull and its associated " arches." 

 (a) Axial ) The backbone and associated ribs. 



Skeleton. *j (The notochord is transitory except in 

 ( the simplest Vertebrates.) 



(/>) Appendicular ( Fore limbs, and pectoral girdle. 

 Skeleton. \ Hind limbs, and pelvic girdle. 



Skull. — The notochord grows forward anteriorly as far 

 as that region of the brain known as the optic thalami. 

 Around notochord and brain the mesenchyme forms a 

 continuous sheath, which is the foundation of the skull. 



As in the case of the notochordal sheath of the trunk 

 region, so also here cartilage is formed in the primitive 

 membranous cranium. The first cartilages to appear are 

 the two parachordals, which lie on the lower surface of the 

 head at the sides of the notochord, and the two trabeculae 

 lying in front. The parachordals grow round and above 

 the notochord, producing the basilar plate, while the 

 trabeculae unite in front to form the ethmoid plate. The 

 continuance of the process of cartilage formation, together 

 with the addition of cartilaginous nasal capsules in front 

 and auditory capsules behind, completes the formation of 

 the primitive cartilaginous brain-box or chondrocranium of 

 the lower Vertebrates. 



Also connected with the head region, and of great import- 

 ance, are the visceral or gill arches which loop around the 

 pharynx on either side, and separate the primitive gill-clefts. 

 At the time when cartilage begins to be formed in the 

 membranous cranium, the arches also become chondrified, 

 and at the same time divided into segments. 



Of these arches there are never more than eight. The 

 most anterior is the mandibular arch which bounds the 

 mouth, the second the hyoid; these two are of great 

 importance in the development of the skull. The others, 

 in Fishes and at least young Amphibians, bound open gill- 

 slits and support the pharynx ; above Amphibians, they 

 are less completely developed. 



