Hexapterida

Hexapterida (meaning 'six fins') is the clade containing all multicellular organism with a bilateral symmetry body found on the alien planet.

Hexapterid evolved from bulbozoa whose bodies are slimmer and more cylindrical with six fins. This body plan allows them to move through water easily. The muscle fibers develop and stiffen, making their fins strong enough to swim against the water current. Living in a pelagic zone of the ocean grants accesses to large amounts of nutrients, being the nearly inexhaustible microbes, causing them to grow larger.

Hexapterids are very adaptive, able to specialize their diets within thousands of generations, causing them to branch off quickly, with the successful clades are the filter feeding aspidotitans and the predatory gnathopterids that are specialized on hunting small preys.

Anatomy
Hexapterids' body plan is very different than their ancestors, being cylindrical with bilateral symmetry with three pairs of fins, and a long flat tail. These fins provide great thrusting power that allows them to swim against the current at will.

Nervous System and Senses
Hexapterids have an advance nervous system with centralized nerve chord running the full length of the body and a neuron network also presents. Nerve ganglions form at the head front of the creature. The organisms also evolve a pair of simple eyes locating at the front near the mouth, granting a vision in front of them which greatly helps in swimming.

Fins
The creatures' long, cylindrical body and the cephalization that occurred promoted the development of one-directional movement, which is done by the long, posterior end of the body. The body part becomes flat and expands, increasing the amount of water displacement, thus the creature is able to swim freely. Further evolution also causes the creature to develop three pairs of fins, in which the fins are used in stabilizing and turning while swimming.

Digestive Tract and Related Organs
This digestive tract evolved from a frontal groove where the choanocytes heavily concentrate at that spot. As they evolve, the groove carves deeper into the body, eventually connects to the other side of the body, becoming an anus. At the side of the body, around the "cheek", there is also an opening on both sides. These cheek openings are lined with many thin, freshy filaments where the mesohyl extends into it. Because of the water current that passing through this organ, oxygen is easily absorbed and exchanged, increasing the efficiency of respiration.

Hemocoel and Heart
The hemocoel evolved from the mesohyl of their ancestors, and the amoebocytes evolved into cells that function like blood cells. Due to the increased body size, the amoebocytes' self-movement become inefficient. On group managed to evolved a special hollow organ within the hemocoel that automatically beats with several opening line with valves, controlling flow of the hemolymph. The flow created by the organ's beat helps the amoebocytes in efficiently transporting nutrients, oxygen, and wastes from organs to organs. This organ eventually become the heart.

Sexual Reproduction
Similar to the phyllostomes, hexapterids managed to evolve a method of sexual reproduction as well. However, each hexapterid is only able to produce one type of gamete, requiring them to search for mate in order to reproduce. The gametes are released through the same opening as the anus.

Larvae
The juvenile of hexapterids are exactly the same as the adult, with the only different being their smaller size. They are not as strong as the adults, but they can take care of themselves right after they were born.

Behavior
Due to the advanced nervous system, hexapterids are the most intelligent organism during their time. They actively swim around the ocean in search of food and mate.

Reproduction
Hexapterids are mobile organisms, thus they are able to seek mates on their own. When two individual of the opposite sex meet, the sexual organ activates and produces the gamete, which is then released into the water.

Niche
Hexapterids, like phyllostomes, are filter feeders which feed on clouds of planktons, both the microbes and potentially the larvae of phyllostomes and other hexapterids caught within its track. They are being competed for the same food source by other of their species. Hexapterids have a complete digestive tract evolved from the oral groove that grew deeper into their body and connect with the other end. The digestive tract, in addition to the choanocytes, which act as the nutrient absorber within the guts, feature cells that can excrete potent gastric acid to digest food particles. The other end of the digestive tract becomes the anus to excrete the wastes. The digestive tract is surrounded by a hemocoel, which helps in distributing nutrients to the whole body. With the help of their eyes and the developed nervous system, they can potentially adopt a predatory life.

Distribution and Habitats
Similar to the phyllostomes, they can be found all over the ocean, though the population is very scarce near both poles. Hexapterids can also be found in a similar depth as the phyllostomes, though they prefer the shallower water as their main food source is there.