448 AMPHIBIA. 



by its skin. The tadpole has sensory cells arranged in 

 distinct lateral lines, but of these the adult retains no definite 

 trace, though there are many nerve-endings and "touch- 

 spots " in the skin. 



Skeleton. — The vertebral column consists of nine vertebrae, 

 and an unsegmented portion called the urostyle. 



The first vertebra bears two condylar facets for the two condyles of the 

 skull and an odontoid process which lies between the condyles. Its arch 

 is incompletely ossified. Each of the next six has an anteriorly concave 

 or procoelous centrum, a neural arch surrounding the spinal cord, a, 

 transverse process from each side of the base of the arch, and an anterior 

 and a posterior pair of articular processes. The eighth vertebra has a 

 biconcave or amphicoelous centrum. The ninth is convex in front, with 

 two convex tubercles behind, and bears large transverse processes with 

 which the hip-girdle articulates. The urostyle has anteriorly a dorsal 

 arch enclosing a prolongation of the spinal cord, but both arch and nei-ve 

 cord disappear posteriorly. Remnants of the notochord, around which 

 the vertebral column was constructed, persist only in the centres of the 

 vertebrae. 



The skull consists ia) of the persistent parts of the original 

 cartilaginous brain-box or chondrocranium ; (V) of ossifi- 

 cations within parts of the chondrocranium ; (c) of membrane 

 or investing bones, and (d) of associated visceral arches. 



Part of the chondrocranium persists as an encasement of 

 the brain. Two exoccipitals bounding the foramen magnum 

 and forming the condyles, two pro-otics or ossifications of 

 the original auditory capsule, and an unpaired spheneth- 

 moid forming the front of the brain-case are cartilage- 

 bones. Two parieto-frontals and two nasals above, a paired 

 vomer and an unpaired dagger-shaped parasphenoid beneath, 

 and two lateral hammer-shaped squamosals are membrane- 

 bones. 



To these are added the small pre-maxillse in the very 

 front of the skull, the long maxillae on each side, the 

 quadrato-jugals which continue the latter to the minute 

 nodule which represents the quadrate bone. 



On the roof of the mouth, extending from the quadrate 

 forwards to near the vomers, are the triradiate pterygoids, 

 while at right angles to the anterior end of the parasphenoid 

 and behind the vomers are the palatines. 



Each half of the lower jaw, based on Meckel's cartilage, 

 consists of three pieces, — the largest an articular angulo- 



