54 FROM FISH TO PHILOSOPHER 



formed by the appearance of bone cells (osteoblasts) 

 which crawled into the irregular spaces of the armor and, 

 in the walls of blood and lymph vessels, began to build 

 true bone of calcium phosphate— a structure here mak- 

 ing its appearance for the first time in the animal king- 

 dom. Other bone-destroying cells (osteoclasts) followed 

 them and continually dissolved this bony deposit almost 

 as fast as it was laid down, so that bone could be formed 

 anew and the animal could grow. Internal layers or rods 

 of bone came to be pitted against external plates and 

 scales until this new bony tissue, the precursor of the 

 internal skeleton of the higher animals, became the 

 strongest structural material in the animal kingdom. Be- 

 cause bone could not be used in the embryo, which must 

 grow and constantly reconstitute its anatomy, its place 

 was taken during the embryonic period by soft cartilage, 

 an ostracoderm invention that continues in the higher 

 vertebrates to connect bone to bone and bone to muscle 

 even in the adult animal. 



The cephalaspids also began the transformation of the 

 armor around the head into a skull with cavities for the 

 organs of smell, vision, and hearing, and with a trough 

 to accommodate the rapidly enlarging brain; they in- 

 vented the shoulder girdle by which the thrust of mus- 

 cles could be transmitted to the head in order to steer it 

 and push it into the mud in search of food; and, in feed- 

 ing or in seeking shelter from their enemies among the 

 hghts and shadows, they were guided by a Hght-sensitive 

 eye in the top of the head— the so-called pineal eye,' 

 which is believed to be the precursor of the pineal gland 

 in the higher vertebrates— which looked straight upward 

 and was probably connected by nerve fibers with the 

 pituitary gland. 



The ostracoderms possessed no fins, but in many 

 forms the armor had developed large spinous processes, 

 which, when located in points of advantage, afforded the 

 animal a fulcrum for anchorage. The cephalaspids and 

 pteraspids had single dorsal spines, the cephalaspids and 



