CHAPTER IV. 



SUBPHYLUM D. VERTEBRATA. 



THE animals included in this great group all possess an 

 internal axial skeleton forming the vertebral column or back- 

 bone ; and a dorsal spinal cord. The vertebral column is 

 developed from the skeletogenous layer, which surrounds the 

 spinal cord together with the notochord and its sheath ; and 

 in the great majority of cases the notochord becomes more or 

 less modified and reduced in the adult. In some cases the 

 notochord remains unmodified and the skeletogenous layer 

 surrounding it is not segmented to form vertebrae, but in 

 every case the neural arches which protect the spinal cord 

 are segmented. The notochord never extends further forwards 

 than the mid-brain. 



All true vertebrates possess a cranium or skeletal box 

 enclosing the brain. 



(I.) CYCLOSTOMATA. 



The mouth in living forms is suctorial and is not supported 

 by jaws. In some fossil forms the character of the mouth is 

 unknown. 



Order I. MARSiPOBRANCHii 1 . 



In these animals limbs and limb girdles are always com- 

 pletely absent. They have no exoskeleton except horny teeth. 



The endoskeleton, excluding the notochord, is entirely 

 cartilaginous or membranous. The axial skeleton consists of 

 a cartilaginous cranium without jaws, succeeded by a thick 



1 See W. K. Parker On the skeleton of the Marsipobranch fishes, 

 Phil. Trans. 1883, London. 



