HEADS AND TAILS. 51 



by thin membrane. They have not only thrust 

 themselves completely into the warp and woof of 

 the skull, but they have in some way strangled, 

 as it were, the development of the cartilage 

 altogether. In the lower jaw we have an 

 admirable object-lesson, showing how the carti- 

 lage is replaced by bone. 



In the sharks, the lower jaw is cartilaginous, 

 and supports numerous specially modified scales 

 called teeth. In the higher fishes this cartilage 

 is invested, or surrounded, by bone, and the teeth 

 have not only undergone considerable change in 

 form, but have entered into complex relations 

 with the bone-sheathed jaw, as we have shown 

 in chapter iv., p. 37. 



Now let us turn our attention to the tails of 

 fishes. The tails of adult fishes may assume one 

 of three forms, known respectively as the diphy- 

 cercal, heterocercal and homocercal forms. How 

 much of meaning there is behind this apparently 

 dry-as-dust information we will now proceed to 

 set forth. 



To begin with, we will take the diphycercal 

 tail (fig. 6), this being undoubtedly the most 

 primitive form. A reference to the somewhat 

 diagrammatic figure will show that the diphy- 

 cercal represents that form of tail wherein that 

 portion of the vertebral column, which forms its 

 support, is continued straight backwards to its 

 farthest extent. Around it are arranged a series 

 of firm rods, which support, in turn, a mem- 

 brane, thus forming the fin. 



The heterocercal tail can be well studied in a 

 dog-fish (fig. 5) or sturgeon. Herein the vertebral 



