28 



VERTEBRATE SKELETON 



in the lower Elasmobranchs, Chondrostei (lig. 28) and Dipnoi which 

 resemble Cyclostomes in having only arches and no centra. In 

 other fishes centra are formed by the bases of haemals and neurals 

 around the notochord. As a rule only the caudineurals develop 

 neurapophyses, and in the tail the caudihaemals form haemapophyses, 

 while the cranial half sclerotomes usually do not form such out- 

 growths. As a result many fishes have two or more rings, more or 

 less complete, to each myotomic somite. The ring with arches is 



Fig. 28. — Vertebrae of Acipenser (R. Hertwig, '91). cd, caudihaemals and -neurals; 

 cr, cranihaemals and -neurals; {i, intercalaria) ; n, notochord; wo, neural arch; ns, neural 

 spine; r, ribs; s, notochordal sheath; sc, spinal canal. 



called the centrum, the one without is an intercentnun or intercalare 

 (fig. 20), but, as will appear later, the two together are the equivalent 

 of the centrum of Tetrapoda. 



This double condition of centra is called diplospondyly. Sometimes a whole 

 myotome and the corresponding nerves have apparently been lost, while the 

 vertebral parts have been retained, polyspondyly resulting. In the higher 

 Ganoids and in Teleosts centrum and intercentrum unite to form a single (true) 

 centrum, a condition found in most Amniotes. Except in a very few cases the 

 caudal half of one primitive sclerotomic segment fuses with the cranial half 

 of the next posterior, the result being an alternation of centra and vertebras with 

 muscular segments (p. 22). 



In the trunk there are no complete haemal arches, the haema- 

 pophyses being, as explained above, haemal ribs, but in some fishes 

 parts of the haemapophyses form a reduced haemal arch around the 

 dorsal aorta, as in Chondrostei where the haemal ribs are greatly 

 reduced or absent. Usually the basal stumps He higher on the sides 



