What factors may have triggered some of the large diversification events in protostomes?
Protostomes have diversified in many ways, including the way they accomplish gas exchange, avoid desiccation, feed, move, and reproduce. These changes are a result of the transition from aquatic to terrestrial environments as well as their heterotrophic nature. When animals made the transition to land, they had to adapt to the exposure to air and the presence of gravity. As explained in my post “Themes in Animal Diversity,” a heterotrophic nature also requires digestive, morphological and reproductive adaptations.
To deal with terrestrial environments, some protostomes live in humid soils so gas can be exchanged across their body surface without water escaping. Others developed internal respiratory structures. Internal gills evolved into lungs in some terrestrial snails. Insects are coated in a waxy layer that reduces water loss and respiratory openings can be open or closed in response to the environment.
Protostomes can be suspension, deposit, liquid or food-mass feeders. This is possible due to the variety of mouth parts that have evolved.
Morphologically, these animals have adapted several ways to move. Some aquatic lineages use beating cilia while others have a coelom functioning as a hydrostatic skeleton to generate movement. Worm-like terrestrial organisms with hemocoels accomplish transportation in a similar way. Still others have developed jointed limbs and wings. Mollusks have a structure commonly known as their foot to allow individuals to crawl along a surface by waves of muscle contraction. Cephalopods have even developed a tube called a siphon that propels water to generate movement.
Protostomes may reproduce asexually by budding or through parthenogenesis, which uses mitosis to develop offspring from unfertilized eggs. Sexual reproduction occurs through internal and external fertilization. They are either oviparous or ovoviviparous. Unique to protostomes, metamorphosis may also occur.