Explain two difficulties an ecologist might have in counting a population of migratory birds

What protects the lining of the stomach from acidic gastric juices?

Which flexible structure that supports the nerve cord and led to the backbone

The following image is of a diatom. it lives in water and gets food from photosynthesis. it is encased in a hard shell made of something similar to glass or sand . a diatom is a(n) because it has a) multi-celled organism; specialized parts. b) vertebrate; a skeleton. c) insect; an exoskeleton. d) single-called organism; only one cell.