CLASs I PISCES 7 



The root of the teeth of fishes is distinguished from the crown by the 

 absence of an enamel layer. It consists either entirely of vasodentine, or also 

 exhibits bone cells with radiating primitive tubules (osteodentine). 



The internal skeleton of fishes shows that in the different Orders of this 

 class there persist until adult life almost all the stages of developmenL which 

 are only temporary among the higher Vertebrates. The whole process of the 

 gradual segmentation, chondrification, and ossification of the vertebral column 

 and of the rest of the skeletqp is quite clear in the different groups of fishes, 

 and the fossil forms from the Palaeozoic and Mesozoic deposits furnish the 

 most important Information in this respect. 



Vertebral Column. — The first trace of the internal skeleton in all Vertebrata 

 is confined to an elastic rod-like structure (chorda dorsalis) extending longi- 

 tudinally on the dorsal side of the body, consisting of oil-bearing cells, and 

 enveloped in a laminated sheath. Upon the chorda dorsalis lies the spinal 

 chord ; below it, the ventral cavity of the body. 



The vertebral column persists throughout life in this primitive stage only 

 in Ämphioxus. The notochord, indeed, still remains unsegmented in the 

 Cyclostomes, but the spinal chord is already protected by arch-shaped bars of 

 cartilage developed in the sheath of the notochord (neural arches) ; ajid at the 

 front end there is developed a cartilaginous capsule for the reception of the 

 brain. 



In all typical fishes the notochord is divided into ring-shaped segments 

 (vertebrae), and is partly or completely cartilaginous or bony. The chondri- 

 fication or ossificatioit always arises in the outer skeletogenous layer of the 

 notochordal sheath, and begins with the upper and lower arches. 



A complete vertebra consists of a vertebral body (centrum) surrounding 

 the notochord, two upper half-arches surrounding the neural canal (neuro- 

 pophyses), and two lower half-arches (hemapophyses). The upper arches are 

 united with a dorsal spinous process (processus spinosus, spina dorsalis, or 

 neural spine), while the lower arches either unite in 

 the caudal region into a ventral spinous process (spina ! 



ventralis), or remain as lower transverse processes «.« 

 (parapophyses). 



In many fishes provided with a cartilaginous 

 vertebral column (Selachians and Acipenseroids), there 

 are interposed between the upper and lower arches 

 cartilaginous intercalary pieces (intercalaria) which ha 

 sometimes exceed the arches in size (Fig. 15, i). ^^^ ^^ 



The vertebral centra or vertebral bodies in cartila- rj.^^^ camiai vertebrae of 

 ginous fishes usually Surround a persistent remnant Centrophoms ux side viev/. m, 



ö 111-1 • 1 11 TT- Haemal arches; i, Intercalary 



of the notOChoi'd, which pierces the whole OI the cartilages ; na, Neural arches ; 



vertebral column and partly fills the intervertebral Hasse)!^^^ ^ ^^"^'^ 

 Spaces. The cartilaginous, calcified, or bony anterior 



and posterior faces of the vertebral bodies are deeply hollowed like double 

 cones (amphicoelous), and thus give the centrum an hour-glass shape. 

 In the Selachii there is usually a partial calcification, in the Ganoidei 

 and Teleostei an ossification of the primitive cartilaginous centrum. 

 While, however, the bony fishes generally exhibit complete ossification 

 of the whole vertebra with all its processes, the ganoids of the Palaeozoic 

 and Mesozoic deposits display every possible stage in the process of 



