OE


To annex is to attach, usually to something larger or more important. To procure is to obtain by some special means. To relinquish is to give up. Since the cells obtain nutrients, and release waste, the word procure is most accurate. If a cell were to annex nutrients, those nutrients would remain attached to the cell surface, rather than incorporated. Thus, procure is the most appropriate choice.
A rudimentary animal is simple with a very basic structure. Primitive suggests a lack of complete development, which does not apply since the sponge is completely developed; it is not a damaged or mutated creature. Vestigial, like primitive indicates the animal is left from another time. Vestigial, however, does not describe the complexity of the organism. A vestigial organism could be quite complex.