Absence of frog ribs?

[Stan Dalton]

A zoology student of mine was dissecting a frog last week and asked me 
why frogs do not have ribs.  The three possibilities I came up with 
were: (1) since frogs respired by positive pressure there was no need 
for ribs or a diaphragm; (2) ribs are used for lateral support and since 
a frog's belly was on the ground he didn't need them; and (3) a weight 
reduction adaptation for jumping.
Anyone else have a plausible explanation?
TIA
Stan Dalton