VERTEBRATE SKELETON' 



slight and the whole sheath very thin. The chorda is still smaller 

 in Amniotes and its sheath is a single layer, apparently the elastica 

 externa. In existing Tetrapoda the vertebrae are usually better 

 ossified than in most fishes. Neural arches are well developed, and 

 in the tails of the lower forms, the haemals as well, but in the higher 

 Amniotes the haemal arches (chevron bones) are small or are entirely 

 absent. A few Tetrapods have distinct intercentra in the trunk. 

 There are always zygapophyses in the trunk region 

 and there may be (reptiles, p. 43) additional means 

 of strengthening the articulation of the vertebrae. 

 All Tetrapoda, except those lacking hind limbs or 

 with them rudimentary, have a sacrum. Haemal ribs 

 never occur, but all have pleural ribs on some or all of 

 the trunk vertebrae. 



AMPHIBIA have, at most, four vertebral regions 

 — cervical (one vertebra), dorsal, sacral and caudal. 

 Gymnophiona and Aistopodous Stegocephals, which 

 lack limbs, lack a sacrum and the Caecilians have no 

 caudal vertebras. The caudals of Anura (fig. 40) are 

 fused to a single bone, the urostyle or cocc3rx. Most 

 Anura have eight presacral vertebra?, one sacral and 

 the urostyle, the number of vertebrae entering the 

 latter being unknown, this structure developing during 

 metamorphosis. Gymnophiona may have 275 verte- 

 brae. The vertebras are relatively short in Anura, 

 longer in the other orders. Each bears a diapophysis 



Fig. 40. — ® _ ^ _ 1. X ^ 



Vertebral column which supports the rib which is usually very 



(Wieders'he''fm! short, somctimcs a mere particle on the end of the 



•86). a, atlas; diapophysis. 



r, ribs; 5, sacrum; . . . . c, 



u. urostyle. The Vertebrae are most primitive in Stegocephala, 



some of which resemble the Chondrostei, some are 

 more hke Holostei, others are peculiar in vertebral structure. The 

 most primitive had a persistent notochord with elements on the sides 

 of the sheath, apparently homologous with those of sturgeons (p. 

 32). One group of ossicles, apparently belonging to the caudal half 

 sclerotome, consisted of a pair of neurals, bearing zygapophyses (fig. 

 42, B) and forming a neural arch. In the transverse plane with these 

 and on the ventral side of the chorda are a pair of caudiha^mals, usually 

 united below to a single plate, called the hypocentnim arcale. The 



