Arrowheads
On Sagan IV, the word arrowhead is used to describe a group of worm-like organisms with crustacean-like characteristics that are members of the kingdom Cillognathias. They originated from a tiny, green, worm-like creature, but then they diverged into more diverse armored forms.
Anatomy
The arrowheads have a diverse range of forms, ranging free-swimming predatory speartongues to the bottom-dwelling muckrakers. They all share a dual brain system, with the upper one controlling movement and its active actions such as feeding while its lower brain performs passive actions such as breathing and bodily functions. They also all possess a soft, spongy organ located just behind their head that allows them to "see" the bioelectric byproduct produced by all living things. Finally, all the descendants of the twilight arrowhead possess an organ capable of producing clicks that are used for sonar and have small patches along their sides where they can form small bioelectric currents and "flashes" in order to communicate with one another.
Arrowheads possess a thick chitinous exoskeleton that protects them and provides structure for their insides which are kept in place by a thin endoskeleton. Their connective tissue which attaches their armored plates together is yellow in coloration due to their blood, and is capable of being repaired if damaged.
Behavior
Depending on the group, the behaviors of the arrowheads vary greatly. Amongst the muckrakers, they live along the bottom, slowly crawling about and taking up detritus or scavenging corpses. Their dangerous tails protect them from potential predators as they use their tusks to dig up the sand. The speartongues, however, were active predators, stalking prey amongst the rocky bottom of the sea floor. Once it finds a potential meal, it spears it with its sharpened tongue. The final group, the necarrows, were stationary passive predators that merely waited for prey to come to them, after which they strike. All possess the ability to sense bioelectric currents, and use this trait to sense potential prey, mates, and predators. They use this to successfully find mates, avoid predators, as well as find prey hidden from normal sight.
Breathing and Blood
Arrowheads have colbalt based blood (coboglobins). When without oxygen it appears yellow but when exposed to oxygen it appears pink. They take in oxygenated water through their duel rows of gill holes on their head, in which lay feathery gills. The oxygen is passed into the bloodstream, while the deoxygenated water is sent through its systems. It soon reaches the gill slits at the bottom of the segment just before the tail where it is released alongside waste and spores.
Diet & Energy
All food taken in, be it detritus or flesh, it taken into its body via a multitude of mouth tentacles and, in some species, a powerful jaw. The food is passed into a gizzard-like organ filled with multiple rows of chitinous teeth that grind it into a pulp before passing it on to its stomach. Here it is digested and the nutrients are passed throughout its body. All indigestible material is passed on and eventually released along with deoxygenated water and spores through its gill slits.
Evolution
Locomotion
Depending on the species, their mode of locomotion varies greatly. Some species, either in their larval stage or throughout their entire lives, are free swimming. Others, such as the muckrakers, crawl along the bottom, using their tusks to sift the sand as they do so. Finally, the last ones, the necarrows, become sedentary once they reach adulthood.
Reproduction
All current arrowheads reproduce via spores. While most of the more advanced ones reproduce sexually and are hermaphrodites, some still reproduce asexually due to inability to locate a mate. For those that do reproduce sexually, two members of a species will come together and copulate, pressing their gill slits together. During this both pass genetic material between themselves, fertilizing both in the process. The act complete, they will go their separate ways and release spores periodically from their gill slits.
Senses
Arrowheads possess a variety of senses. First off, they can detect objects via touch. They possess numerous nerves dedicated to this, especially along their back which allows them to know if something has climbed on them, after which they will strike at it with their tail. Next, arrowheads can pick up chemical trails and such by taking in water via their gill holes. In their the chemical markers are detected and identified, allowing the owner to know what it is. A third sense is their sense of taste, though it is very limited and is merely used to identify whether or not something is edible. Their fourth sense is their ability to "see" bioelectric currents produced by all living things, a sense they to their advantage along with their final sense, echolocation, due to their lack of vision-based eyes.
Size
Due to the relative youth of this group, many arrowhead species are less than 10 cm long, though some have grown larger.
Types of Arrowheads
Muckrakers: Bottom-dwelling detritivore that scavenge when possible, they are separated from the other types by their lack of a bottom jaw, which has become a pair of manipulative tusks. They breed rapidly and are very common along the sea-floor. They are capable of swimming only during their larval stage, during which they are a part of the plankton.
Necarrows: Sedentary predators when fully grown, this type of arrowhead's dual brain system has degenerated compared to the other types, yet it is still an effective carnivore. Their mouth tentacles are covered in nematocysts that pierce their prey and inject a neurotoxin that causes paralysis. Their strong jaws are capable of holding prey in place as it is slowly devoured alive.
Speartongues: Active hunters, this serpentine arrowhead type has evolved a deadly hunting tool. Possessing a long spear-like tongue, it uses this to spear its prey before bringing it to its deadly maw filled with nematocyst-lined mouth tentacles. Solitary predators, they used stealth and surprise to their advantage.