THE COMING OF A BACKBONE 29 



drical body tapering to the solidly muscular tail with its cu- 

 rious fin. All the vital organs are well developed and ex- 

 ceedingly compact. We notice the well marked head, the 

 strange position of the mouth, the keen sense-organs, and 

 the large brain. Indeed the brain looks larger than the needs 

 of a shark require. 



We find only two pairs of lateral appendages, the fore and 

 hind fins used mostly for steering. This is a sharp contrast to 

 the many pairs of appendages of the arthropods, every pair 

 admirably adapted to a special purpose. We will return to 

 this fact later. We find a skeleton in which the ancient noto- 

 chord is being crowded upon by the cartilaginous sheath. 

 This sheath may be infiltrated and stiffened with carbonate 

 of lime, forming calcified cartilage, but no true bone. The 

 skin feels like sand-paper; it is covered with minute pointed 

 scales or denticles composed of bone covered with a layer of 

 enamel. The scales covering the skin of the jaws have en- 

 larged greatly and become triangular teeth, of which there 

 are several or many rows. 



The dog-fish is an admirably framed and fashioned animal: 

 powerful, agile, swift and enduring in locomotion, tough and 

 vigorous, with keen sense-organs and no mean brain. It 

 roams everywhere; especially over the sea-bottom, where food 

 is most plentiful and the struggle fiercest, and is continually 

 meeting new conditions and emergencies. Its eggs are very 

 large, yet it is extremely common. Evidently even the young 

 are well able to care for themselves. No wonder that th§ 

 fishermen hate and curse it, it is the highest tribute to its 

 many virtues. 



Almost or quite as old as the sharks are the ganoid fishes 

 represented by the sturgeons, garpikes and a great host of 

 fossil forms. In the structure of the notochord and sheath 

 they shov7 us all stages up to bony vertebrae. They differ 

 from sharks in one important respect: they have an air-blad- 

 der, a sack connected with the mouth by a tube through which 

 air can be drawn. They have the possibility of using the 

 air-bladder as a lung, and some of them have realized and 



