The forms of batrachians with which we are acquainted show the vertebral column to have been evolved in the course of time from a notochordal condition with segmented centra similar to that of early bony ganoid fishes (e.g.
The vertebral axis shows a series of broad rings, with distinct neural arches, but no ribs.
LAMPREY, a fish belonging to the family Petromyzontidae (from r rpos and Ww, literally, stone-suckers), which with the hag-fishes or Myxinidae forms a distinct subclass of fishes, the Cyclostomata, distinguished by the low organization of their skeleton, which is cartilaginous, without vertebral segmentation, without ribs or real jaws, and without limbs.
The neck is long and curved, and its vertebrae are remarkable for the position of the canal for the transmission of the vertebral artery, which does not perforate the transverse process, but passes obliquely through the anterior part of the pedicle of the arch.
This is the most perfect arrangement attained by the vertebral column, and is typical of, and restricted to, birds.
In the dorsal fin they have from 17 to 20 rays, and in the anal fin from 16 to 18; there are from 53 to 59 scales in the lateral line and 54 to 56 vertebrae in the vertebral column.
In general structure they all closely resemble human beings, as in the absence of tails; in their semi-erect position (resting on finger-tips or knuckles); in the shape of vertebral column, sternum and pelvis; in the adaptation of the arms for turning the palm uppermost at will; in the possession of a long vermiform appendix to the short caecum of the intestine; in the size of the cerebral hemispheres and the complexity of their convolutions.
Later those pads fuse with the anterior end of the centrum of the vertebra to which they belong; where the vertebral column is rendered inflexible, the disks are ossified with the centra and all trace of them is lost.
This notochord represents the persistent primordial skeletal axis which, in the higher Craniata (though not so in the lower), gives way by substitution to the segmented vertebral column.
It may be added that in the Oreodontidae the vertebral artery pierces the transverse processes of the cervical vertebrae in the normal manner.