Beachtrop
As genus Quadricrus continues its inexorable march onto land, they have adapted further to their terrestrial environment. The Beachtrop has taken residence further up the beach, and even in the surrounding rainforests, where water is plentiful.
With their main food source far beyond their reach, the Beachtrop has changed drastically to make use of a new, and mostly untapped source of food. Their arms have specialized into two distinct sets. The lower three arms are dedicated to feeding and locomotion, while the upper arm is dedicated to reproduction. The lower three arms are pentagonal, with the flat bottom resting parallel to the ground. Protruding from them are a series of fleshy protuberances covered in pores leading into the vein network. The veins within the protuberances can be inflated and deflated at will and allow them to 'walk' across the ground, feeding on any microbe or piece of detritus they step on. This, along with the usual photosynthesis, has become the beachtrop's main means of gathering sustenance.
With the beachtrop now living far from the ocean's waves, moisture is now much harder to find. As a result, the stomata-like pores are restricted to the underside of the beachtrop, where the water stored within the vein network is much less likely to evaporate. Meanwhile, the protuberances will absorb any water they come across, providing more water to the organism.
With the lower three arms being entirely dedicated to feeding and respiration, the upper arm has specialized to meet the beachtrop's reproductive needs. The vein network has a much smaller presence in the upper arm, with it only surrounding the reproductive tube and gamete-producing organs located near the core. The reproductive tube has become larger to allow for greater gamete flow, and will release them during rainstorms. This restricts the beachtrop to very wet environments such as beaches and rainforests.
With the advent of terrestrial pathogens, and the lack of available salt, the beachtrop needed to find a new means of defending itself. The brine sacs that were once present in its ancestor have now been repurposed and specialized into different types. Near the core are sacs dedicated to the production of various chemicals that influence the beachtrop's behavior. This could range from chemicals adapted to regulating protuberance movement to the development of immune cells. The immune cells are produced in sacs located in the lower arms, as this is the area at the highest risk of being invaded by pathogens. Pathogens that are not captured by digestive cells will be hunted down by mobile, ciliated cells with the ability to capture pathogens with a series of tassel-like projections. These immune cells are derived from the digestive cells. The tassels are directly derived from the digestive cells, while the cilia are atavistically derived from the vein cells the feeding cells were derived from. The immune cell-producing glands are derived from brine sacs that had vein tissue grow inside of them. With the obsolescence of the brine sacs, this extra surface area was deemed advantageous and thus, was selected for. As the populations of proto-beachtrops were being decimated by the newly-evolved crystalkillers, some individuals developed digestive sacs with the ability to release some of their digestive cells. These cells proved effective be at capturing pathogens as they travelled across the vein network. As a result, this new immune cell-producing organ was selected for, with the immune cells being derived further to allow them to better detect and capture pathogens. Immune cell production is regulated by hormones, and is spurred into action in the presence of pathogens.