Framework 6$ 



piece of wood, for it is just one big block of cartilage. 

 Take a tuna's skull, and the most you can do is to knock 

 chips off it with a chisel, for it is composed of bones, with 

 visible joints or sutures between them. The sharks and rays 

 haven't a bone in their bodies — nothing but cartilage all the 

 way through. Whether this is a truly primitive condition 

 in the sense that they were that way originally and never 

 made any improvements, or whether it is a degenerate con- 

 dition to which they have come after once having had bone, 

 is still undecided by scientists, although at present the verdict 

 favors the latter theory. 



There are minor differences in the structure, such as in 

 the gill slits and the fin construction, but the general ar- 

 rangements of the skeleton and the muscular system are so 

 close to those of the bony fishes that nothing more need be 

 said about them here. 



