THE PISH SKELETON , 257 



secretions of the soft skin. Meanwhile the animal is sub- 

 ject to all sorts of dangers from its enemies, since between 

 the time of shedding of the old skin and the forming of 

 the new it is wholly unprotected. This is one reason why 

 it so difficult to raise enough lobsters to supply the 

 demands of the people, although the number of eggs 

 hatched each year is enormous. The young are eaten 

 between molts by all sorts of enemies. 



Vertebrate skeletons. — Fishes, frogs, reptiles, birds, and 

 mammals all agree in the possession of a jointed, internal 

 framework, and differ only in the number of the bones, the 

 composition of the bones, and the arrangement with 

 regard to one another. All possess a central column or 

 backbone made up of separate pieces called vertebrae, and 

 on this column are hung the other bones of the body. 



The fish skeleton. — The skeleton of the fish differs from 

 that of the other yertebrates in containing a higher pro- 



Fio. 107 — Skeleton of a flsh ; note the great number of bones. 



portion of cartilage. A few forms show remnants of an 

 external skeleton in the form of the scales which in the 

 garpike are so hard as to offer some protection to the 

 underlying parts. 



EDDY. PHyS. — 17 



