40 STRUCTURE OF THE VERTEBRATES 



ordinal differences will be considered in the following discussion 

 of group characteristics. 



Order Chondrostei, the most primitive of the teleostome fish, 

 include the sturgeons and the spoonbill fish. The latter are 

 found in the Mississippi valley and in China. The former are 

 found in the fresh and salt waters of the northern hemisphere. 

 Caviar is the egg roe of the sturgeon, and comes from Russia, 

 the Great Lakes region of America, and Alaska. 



The sturgeon resembles the shark in many respects. The 

 rostrum projects in front of a ventral mouth; the tail is 

 heterocercal ; the digestive tract has a spiral valve; the skeleton 

 is cartilage; and, in the type genus, the spiracle is retained. The 

 bony plates cover the entire head, and the ordinal name refers 

 to the fact that there is a perfect cartilaginous chondrocranium 

 encased in bone. Posterior to the head the plates are arranged 

 in five rows, one dorsal, and two on either side. Between them, 

 and on the lateral-ventral regions, are many dermal denticles. 

 There are no teeth in the recent forms, though the fossil ances- 

 tors had a proper supply. 



This group of fish developed a swim bladder, a structure of 

 great phylogenetic interest. The bladder arises in the animal as 

 a dorsal outpocketing of the end of the pharynx, and the con- 

 nection with the digestive tract is maintained throughout life. 

 It can be inflated or deflated, serving as a hydrostatic organ by 

 lessening the specific gravity of the fish; and in poorly aerated 

 water it has a respiratory function. 



Order Holostei. The holosteans are fish with an entire bony 

 skeleton, but with sufficiently typical characters to set them 

 apart from the following order, although the two merge almost 

 imperceptibly. The most primitive recent form is the Gar-pike 

 (Lepidosteus, not related to the pike or pickerel) which has 

 bony scales covering the body in a tile-like manner. The 

 rhomboidal scales fit edge to edge, and do not overlap as in 

 most fish. The teeth are well developed; the tail is partly 

 heterocercal; the spiracle is only a pit; and the digestive tract 

 retains many primitive characteristics. The swim bladder re- 

 tains its connection with the gut and has a vascular supply. 

 The Bowfin has over-lapping scales, and in all other structures 

 shows the line of evolution toward the following order. 



