The digestive system.
Appendices
(Appendices is the plural of the noun appendix) Most of the statistical information in his book was collected in the appendices. The legislature's appendices to the jobs bill consisted of several dozen pages.
There are five appendices to Constitution of India.
A Year with Swollen Appendices was created in 1995.
appendices starts with a and experimental data with e
A Year with Swollen Appendices has 424 pages.
They belong to the nervous system
How about something like... The appendices of a book list points of reference.
The kidneys belong to the excretory system.
Veins belong to the circulatory system, because they transport blood there for they belong to the circulatory system.
The correct plural form of "appendix" in most contexts is "appendices." However, "appendixes" is also acceptable, particularly in informal usage. In academic or formal writing, "appendices" is preferred.
The eyes belong to the Nervous system.