158 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



mandibulars, is said to be hyostylic. The upper jaw is also 

 'Connected with the skull by two strong fibrous bands (ligaments), 

 one in front (ethmopalatine ligament) and one behind (pre- 

 spiracular ligament) running in front of the spiracle and contain- 

 ing a small pre-spiracular cartilage. 



Each of the five branchial rods on each side becomes jointed 

 into a series of segments, named, from above downwards, pJwryngo-, 

 epi-, cerato-, and hypo-lranchials. A median ventral basi-branchial 

 cartilage lies between and connects together the arches of opposite 

 sides. 



Slender cartilaginous gill-rays for the support of the gill-folds 

 radiate backwards from the posterior margins of the cerato-hyals 

 and first four pairs of cerato-branchials. 



The visceral skeleton includes a few other cartilages besides the above. 

 They are (1) a pair of rod-like labials at each corner of the mouth, (2) three 

 flattened rods, the extra-branchials, on each side, external to the three 

 middle branchial arches. 



(2) Vertebral Column and Ribs. The vertebral column con- 

 sists of a series of joints or vertebrae, which are united together 

 to form a flexible rod. The vertebrae are of two kinds, trunk 

 vertebrae and tail vertebras ; the latter are the more typical, and 

 each of them consists of a deeply biconcave (amphiccelous) body or 

 centrum lined by bone, a neural arch under which the spinal cord 

 runs, and a hcemal arch protecting the blood-vessels of the tail, 

 and produced downwards into a hcemal spine. The trunk vertebrae 

 chiefly differ in the absence of complete haemal arches; those in 

 front possess horizontal projecting transverse processes which bear 

 short ribs; those behind have downwardly projecting haemal 

 processes ( = transverse processes of those in front, and sides of 

 haemal arches of tail vertebrae). The neural arches of both kinds 

 of vertebra are similar. Each consists of four pieces, a lateral 

 neural plate projecting from the centrum on each side, and two 

 small rounded dorsal cartilages (neural spines) placed one behind 

 the other. The spaces between the neural plates are filled up by 

 hexagonal intercalary cartilages. 



The notocliord persists throughout the entire extent of the verte- 

 bral column, but by the development of the vertebral centra is 

 deeply constricted at regular intervals, though not quite divided 

 dnto a series of separate sections. 



II. Appendicular Skeleton. Each unpaired fin is in typical cases 



