VERTEBRAL COLUMN. 



161 



3 



Although the scales are dermal structures, they are said to be 

 frequently exposed on the surface in the adult. This, if true, 

 must be due to the fact that the overlying epidermis and dermis 

 have been rubbed off. This may frequently happen in the hand- 

 ling of animals during their preservation. 



Vertebral column. In the Chondrostei the notochord is 

 persistent and its sheath is stout but unsegmented and unossified. 

 The neural and haemal arches on the other hand are segmented ; 

 they are attached to the sheath but do not extend round it. In 

 other living Ganoids (for the vertebral column of extinct genera 

 the reader is referred to the special accounts), vertebral bodies 

 are formed by the extension of the arch tissue round the sheath, 

 its chondrification, segmentation and ossification. In Poly- 

 pterus and Amia the 

 vertebral bodies are 

 amphicoelous as in 

 Teleosteans ; in Lepi- 

 dosteus they are con- 

 cave behind and con- 

 vex in front (opistho- 

 coelous). 



In some extinct Ganoids 

 (Fig. 90a) bony plates 

 are found, corresponding 

 to each arch, on the 

 ventral side of the noto- 

 chord. They are called hypocentra, and carry the haemal arches. 

 A corresponding plate, which may be composed of two pieces, is 

 found on the dorsal side, and called pleurocentrum. Such incipient 

 vertebrae are called half-vertebrae. They may, each of them, extend 

 completely round the notochord (Fig. 90b), as in the tail of the 

 living Amia, in which case one of them only carries the neural and 

 haemal arches. Ring-vertebra is the term used when the pleurocent- 

 rum and hypocentrum are joined to form a ring round the noto- 

 chord, as in the amphicoelous vertebrae of Polypterus, Teleostei, etc., 

 in which the bony ring has thickened so as to constrict the notochord 

 in the middle of each vertebra. In Lepidosteus the arches are continuous 

 with the bony centrum. In all other Ganoids with bony centra the 

 arches are separated from the centra by persistent cartilage. 



A B 



Fig. 90. — A. Vtitt'bra of Calurus fuicaius. B. Caudal 

 vertebrae of Eunjcormus rpeciosus (after Zittel). 1 

 neiiral arch ; 2 the bifurcated neural spine ; 3 hypo- 

 centrum ; 4 pleurocentrum ; 5 rib. 



The caudal fin is diphycercal in Polypterus. In other living 

 genera it is heterocercal. In Amia it is hemiheterocercal (ex- 

 ternally homocercal, internally heterocercal). and the dorsal 

 lobe of the caddal fin is reduced to the covering of fulcra. 



z.-n. 



M 



