110 



VERTEBRATA. 



by Aristotle, who called them " animals with blood " (vol. I., p. 132) ; 

 he also put forward the possession of a bony or cartilaginous 

 skeletal axis as a common characteristic. But it was Lamark who 

 first recognised the presence of a vertebral column as the most 

 important character, and introduced before Cuvier the name of 

 Vertebrata into the science. This designation, however, in its strict 

 significance, is only an expression for a definite grade of develop- 

 ment of the skeleton, which may persist in its first unsegmented 

 condition as the notochord (Am})hioxus, Myxine). The most im- 

 portant characteristics therefore of the Vertebrata do not depend 

 upon the presence of internal vertebrae and of a vertebral column, 

 but upon a combination of characters which 

 have to do with the general relations of posi- 



:tion, the mutual arrangement of the organs 

 and the mode of embryonic development. We 

 may accordingly define the Vertebrata as 

 laterally symmetrical organisms with an 

 axial skeleton, on the dorsal side of which 

 is placed the central nervous system (brain 

 and spinal cord), while on its ventral side 

 lie the alimentary canal with its two open- 

 ings (oral and anal) and the rest of the 

 viscera and the heart ; the latter being placed 

 on the ventral side of the alimentary canal. 

 The Skeleton. The presence of an in- 



FIG. 569. Transverse section 



through the chorda dorsaiis ternal skeleton is a character of great im- 



^S&*S P rtanoe ; * in the ^vertebrates the 

 chordai sheath ; 8k, skeietoge- firm supporting structures are almost always 

 produced by the hardening and segmenta- 

 tion of the external skin, in the Vertebrates the relation of the 

 hard to the soft parts of the body is reversed. The hard parts are 

 placed in the axis of the body, and send out processes towards the 

 dorsal and ventral surfaces, which constitute respectively a dorsal 

 canal for the reception of the central nervous system (brain and 

 spinal cord) and a ventral arch over the vascular trunks and the 

 viscera. In the simplest and lowest Vertebrates the axial skeleton 

 remains as an elastic cord the notochord (chorda dorsaiis), which in 

 the higher Vertebrates is present in embryonic life and constitutes the 

 first rudiment of the vertebral column (fig. 569). When the internal 

 skeleton acquires a firmer consistency it, like the external skeleton 

 of the Invertebrates, becomes segmented. This modification is intro- 



as 



