Complex sentence: Caddisfly cases are open at both ends, and larvae draw oxygenated water through the posterior end, over their gills, and out of the wider anterior end which is usually wider and to which they add material as they grow.
Split into simpler sentences: Caddisfly cases are open at both ends, the larva drawing oxygenated water through the posterior end, over their gills, and out of the wider anterior end. The anterior end is usually wider and it is to this end that they add material as they grow.