76 EVOLUTION 



These are the Ostracoderms, and are regarded by some 

 as a connecting link between the Crustacea (trilobites) 

 and the fishes. As they have no trace of the cartila- 

 ginous rod that represents the early back bone, most 

 authorities deny them this position, and class them as an 

 offshoot of the early Crustacea. The real fish ancestor 

 seems to be found in Palceospondylus, a small back-boned 

 animal, without ribs or limbs, found in the Scotch 

 sandstone a few years ago. Many animals living to-day 

 at the base of the fish world illustrate for us the growth 

 of the back bone. In the young sea-squirt we find a thin 

 rod of cartilage that is lost in the adult form ; in the 

 acorn-headed worm we have slight traces of such a rod ; 

 in the lancelet there is a complete rod from end to end ; 

 and in the lamprey this cartilaginous rod has closed over 

 the spinal cord along the back and spread, as skull, over 

 the brain at the head of it. 



With this appearance of a stiffening rod, which will 

 later turn into bone and break into articulating disks, we 

 get the first vertebrate animal, or an early type of fish. 

 The cilia are replaced as organs of locomotion by fins 

 folds of skin that are worked off in the motion through 

 the water, and then converted into strong paddles by 

 rods of cartilage. The sensitive pits in the skin have 

 slowly developed into eyes and nostrils, and have their 

 telegraphic nerves to the brain. The heart, beginning 

 as a mere pressure bulb in the lower types, develops 

 into a two-chambered pump, and sends a richer supply 

 of blood to the frame. The water that enters the 

 mouth now makes its exit by slits in the gullet and skin, 

 and a fine network of blood-vessels grows over the slits 

 to extract the oxygen from the water (respiration) as it 

 issues. From the evidence of embryology and the 

 illustrations we have in nature to-day we gather that 

 this was the line of evolution of the fish. 



