Absence of ribs in frogs?

[Stan Dalton]

I'm a biology instructor at a junior college in Miss.  A zoology student 
of mine, as we were dissecting frogs, asked why frogs didn't have ribs. 
Some amphibians do and some (frogs) don't.  The possibilites I came up 
with where: (1) frogs respire with positive pressure differential and 
don't need ribs or a diaphragm (used in negative pressure respiration); 
(2) ribs are for lateral support of musculature and thoracic cavities--a 
frog's belly is on the ground and doesn't need any support; and (3) 
lastly as a possible weight-saving device related to jumping.
Any other thoughts?
TIA
Stan Dalton