First, just because an ADC is 24-bit, that doesn't mean you're getting anywhere near 24 effective bits. My experience with high-res ADCs, especially the sigma-delta's, is that when you go to the data sheets, you'll find that you're actually getting closer to 17 bits- So, you're not getting that nanovolt reading you think you are without some amplification, unless you're taking advantage of an on-ADC PGA. If you're using a PGA with a gain of 256, you're talking hundreds of nanovolts, which may be close. Otherwise, you probably should consider a 32-bit ADC. Preamplification might even be preferred, unless there's a good reason why that isn't possible.
The standard way to protect the inputs is through stacked diodes to the rails.
Don't forget-- you still need to anti-alias filter.