Crop & gizzard

[Adz795]

Following is what I know about the crop and the gizzard:
Crop: used for storing food
Gizzard: comes after crop and used for grinding hard foods

Well, that is my limited knowledge about these two structures. I want to know more. I have some questions:
1. Are these structures glandular?
2. Any enzyme secretion?
3. Churning should take place in crop, gizzard or both?
4. What animal groups have these? (I think annelids, arthropods and aves. Any others?)
5. How are these structures specialized among various groups? (eg. the avian gizzard is specialized for grinding seeds and nuts. Arthopod, annelid gizzards would be specialized for what?)
6. What role does the stomach play in animals that have crop, gizzard and stomach? (I think if either the crop or the gizzard perform chemical digestion, the stomach would do what... more digestion?)

I would highly appreciate if you could answer one or more of these questions. I have researched the internet but only few sources provide comprehensive information and some of them contradict each other.
Thank you very much.

[JackBean]

what is crop?

[Adz795]

It is a structure in some animal groups.
http://en.wikipedia.org/wiki/Crop_(anatomy)

And wikipedia doesn't provide enough information about it.