General Features 



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What Is a Vertebrate? "Vertebrates are backboned animals" — a 

 definition often given, but inexact. In laboratory courses on vertebrate 

 comparative anatomy, the animal usually studied as an example of a 

 vertebrate of the fish type is a small shark known as the '"dogfish." 

 This fish has no backbone. It has no bone whatever in its internal skele- 

 ton. But it does possess a dorsal axial series of structures, movably 

 articulated, each of them in its form and relations to other parts closely 

 resembling a bony vertebra. Their material, however, is cartilage, 

 much softer than bone and very different from bone in internal consti- 

 tution and mode of development. The shark has a vertebral column, 

 but it is cartilaginous. 



To define vertebrates as animals having the vertebral column would 

 seem necessarily to be correct. The fact is, however, that most zoologic 

 classifications include within the group called "vertebrates" certain 

 animals which, in a strict sense, do not have a vertebral column; that 

 is, if by "column" is meant a longitudinal series of definitely formed 

 vertebrae articulated to one another. In the "round-mouthed" eels 

 (cyclostomes), represented by the lamprey (Fig. 317) and a smaller eel, 

 the hagfish, the axial skeleton is a cylindric rod or cord, the notochord, 

 consisting of a somewhat gelatinous internal substance enclosed by a 

 sheath of tough fibrous tissue (Fig. 101). The notochord itself is quite 

 unsegmented. Its fibrous sheath extends upward on either side to en- 

 close the dorsal nerve cord (spinal cord). In the neural arch thus 

 formed are embedded minute rods or plates of cartilage lying at either 

 side of the spinal cord (Fig. 318). These little cartilages evidently cor- 

 respond to merely the dorsal parts of vertebrae but they are, at most, 

 very small rudiments of vertebrae and do not articulate together to 

 form a "column." Yet cyclostomes are called vertebrates. Some of the 

 older classifications include under vertebrates the little externally fish- 

 like Amphioxus (Fig. 312), which has a well-developed notochord but 

 not the slightest rudiment or trace of a vertebral column. 



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