<span>The spicules have an important purpose for loosely constructed animal such as sponge. First of all, spicules give structure which is vital due to loose construction. Secondly, it protects from predators. This is similar to porcupine quills in serving to protect.</span>
The exchange takes place in the millions of alveoli in the lungs and the capillaries that envelop them. As shown below, inhaled oxygen moves from the alveoli to the blood in the capillaries, and carbon dioxide moves from the blood in the capillaries to the air in the alveoli.
Nuclear forces, according to my textbooks! Hope this helps!
