It's not an arthropod (insect, crustacean, spider, etc). It's not a mollusc. It's not a segmented worm. So what the heck is a Spirulid?
Well, a lot of the difficulty there comes from trying to apply Earth-based concepts to an alien ecosystem. While many biological rules find themselves upheld just as much here as on Earth, there is no fundamental rule of biology that limits the diversity of life to the rather narrow limits of things that have evolved on a single planet over the course of less than a billion years. Thus, any Earth-based terminology we use will, of course, have to be very heavily qualified to make it at all meaningful.
So then, what is a Spirulid? Well, to draw the closest earth equivalent, it's a motile, predatory, diploblastic sponge.
And what the heck does that mean?
Well, let me tell you.
Motile: Spirulids are all capable of moving under their own power. They do so through the use of filaments. These filaments are thin, but very strong threads of living cells. They extend rapidly in length, and their harpoon-like tips dig into or grip onto whatever surface they contact. As the extended spiculin within the cells contracts, the filament shortens, drawing the organism forwards. When in motion, a Spirulus will float, neutrally buoyant in the water, as a team of filaments rapidly fire out, gain traction, and pull it along, releasing at the end of the movement in preparation for their next shot.
Predatory: Spirulids are obligate predators, having no means to gain nutrients and energy beyond consuming other living organisms. Just as they use them to walk, Spirulids can use their harpoon filaments to seize nearby prey items, pulling their catch towards themselves, or pulling their bodies, sophisticated digestive system and all, towards their meal-to-be.
To power their energy-intensive lifestyles, Spirulids all possess digestive systems of significant complexity. As a Spirulid grows throughout its life, its older and smaller filaments, wrapped up near the center of the creature's spiral and no longer useful for hunting or locomotion, undergo a process of apoptosis (deliberate cell death, like the process that turns our embryonic mitts into distinct fingers with spaces between them, or the process by which a growing tree loses its lower branches). The recovered resources are used to grow arrays of digestive organs, which cluster around the posterior areas of the organism, near the spiral's center. While primitive Spirulids simply hold their prey close to these structures, or wrap their spiral tails around digestable material to dissolve them and absorb their nutrients, later and more complex Spirulids would develop a digestive tract, adapting their digestive structures into ancillary stomachs and enzyme-secreting glands. The means by which this tract forms leads us neatly in to the next topic.
Diploblastic: Spirulids possess two of what are called 'germ layers'. Without going into too much detail about just what a germ layer is, I will simply note that it has something to do with the way the cells of an organism have been folded in its development. Humans have three germ layers, comprising their gastrointestinal tract, from mouth to anus (endoderm), of their collection of internal organs (mesoderm) and their skin, skeleton and muscles (ectoderm). This condition is called triploblasty, and all bilaterally symmetrical animals, from fruit flies to squid to ospreys, have it. Sea Jellies have only two layers, an endoderm and an ectoderm, lacking any other internal organs. Sponges have only a single layer, possessing no clear tissue differentiation. Though they do have several types of specialized cell, they are not organized into organs.
Spirulids are diploblast. Upon growing to a thickness where diffusion alone cannot transport nutrients efficiently through the body, an area near the center of the spiral, near the creature's posterior, begins to fold in on itself. This infolding generates a hollow tube, which grows from rear to front, opening at a toothy mouth near the anterior (front) of the organism. Because of this adapation, Spirulids are able to digest much more efficiently than their ancestors, who had to absorb food, digest it in a single chamber, then expel it. A useful metaphor would be to consider a digestive tract to be an assembly line- or rather, a disassembly line- for prey, which is able to perform all stages of digestion simultaneously, while a digestive sac is akin to a single craftsman, who can only work on a single project at once, needing to finish it and clear the workspace before starting anew.
Because of this adaptation, later Spirulids are able to support much greater sizes. While Spirulus was nearly invisible to the naked eye, limited to less than a millimetre in diameter, its close descendant the Tremulus was able to grow to the size of a penny, making it the largest mobile predator to roam its world, at the time of its evolution.
Sponge: At this point, you may begin to wonder 'wait, how can this thing be anything like a sponge? Spirulids are diploblastic, but earlier you said that sponges were monoblasts with no germ layers. The sponges that I know can't move, and they're filter feeders!
This is all true, and just helps to illustrate how hard it can be to apply Earth analogues to other worlds, and why doing so tends to be inaccurate to the point of meaninglessness. However, Spirulids still share many features that put them closer to Earth sponges than to any other group.
The vast majority of cells in a Spirulid are undifferentiated. If you were to shred one by forcing it through a fine mesh, you would find that the great majority of cells (theoretically all those who had not been adapted into sensory organs, neurons and digestive glands) are able to live freely in a single-celled form, much as in a sponge. These tiny fragments will ultimately begin to grow into new Spirulids, with no ill effect caused by their origin. Thus, despite its appearance as a more complex organism, Spirulids have not actually lost any of the colonial traits possessed by their ancestors.
But does this atavism (retention of a primitive trait) hurt them? As anyone who has seen a destroyed Spirulid restore itself, over the course of several days, into several reconstituted offspring, not in the least.
Well, a lot of the difficulty there comes from trying to apply Earth-based concepts to an alien ecosystem. While many biological rules find themselves upheld just as much here as on Earth, there is no fundamental rule of biology that limits the diversity of life to the rather narrow limits of things that have evolved on a single planet over the course of less than a billion years. Thus, any Earth-based terminology we use will, of course, have to be very heavily qualified to make it at all meaningful.
So then, what is a Spirulid? Well, to draw the closest earth equivalent, it's a motile, predatory, diploblastic sponge.
And what the heck does that mean?
Well, let me tell you.
Motile: Spirulids are all capable of moving under their own power. They do so through the use of filaments. These filaments are thin, but very strong threads of living cells. They extend rapidly in length, and their harpoon-like tips dig into or grip onto whatever surface they contact. As the extended spiculin within the cells contracts, the filament shortens, drawing the organism forwards. When in motion, a Spirulus will float, neutrally buoyant in the water, as a team of filaments rapidly fire out, gain traction, and pull it along, releasing at the end of the movement in preparation for their next shot.
Predatory: Spirulids are obligate predators, having no means to gain nutrients and energy beyond consuming other living organisms. Just as they use them to walk, Spirulids can use their harpoon filaments to seize nearby prey items, pulling their catch towards themselves, or pulling their bodies, sophisticated digestive system and all, towards their meal-to-be.
To power their energy-intensive lifestyles, Spirulids all possess digestive systems of significant complexity. As a Spirulid grows throughout its life, its older and smaller filaments, wrapped up near the center of the creature's spiral and no longer useful for hunting or locomotion, undergo a process of apoptosis (deliberate cell death, like the process that turns our embryonic mitts into distinct fingers with spaces between them, or the process by which a growing tree loses its lower branches). The recovered resources are used to grow arrays of digestive organs, which cluster around the posterior areas of the organism, near the spiral's center. While primitive Spirulids simply hold their prey close to these structures, or wrap their spiral tails around digestable material to dissolve them and absorb their nutrients, later and more complex Spirulids would develop a digestive tract, adapting their digestive structures into ancillary stomachs and enzyme-secreting glands. The means by which this tract forms leads us neatly in to the next topic.
Diploblastic: Spirulids possess two of what are called 'germ layers'. Without going into too much detail about just what a germ layer is, I will simply note that it has something to do with the way the cells of an organism have been folded in its development. Humans have three germ layers, comprising their gastrointestinal tract, from mouth to anus (endoderm), of their collection of internal organs (mesoderm) and their skin, skeleton and muscles (ectoderm). This condition is called triploblasty, and all bilaterally symmetrical animals, from fruit flies to squid to ospreys, have it. Sea Jellies have only two layers, an endoderm and an ectoderm, lacking any other internal organs. Sponges have only a single layer, possessing no clear tissue differentiation. Though they do have several types of specialized cell, they are not organized into organs.
Spirulids are diploblast. Upon growing to a thickness where diffusion alone cannot transport nutrients efficiently through the body, an area near the center of the spiral, near the creature's posterior, begins to fold in on itself. This infolding generates a hollow tube, which grows from rear to front, opening at a toothy mouth near the anterior (front) of the organism. Because of this adapation, Spirulids are able to digest much more efficiently than their ancestors, who had to absorb food, digest it in a single chamber, then expel it. A useful metaphor would be to consider a digestive tract to be an assembly line- or rather, a disassembly line- for prey, which is able to perform all stages of digestion simultaneously, while a digestive sac is akin to a single craftsman, who can only work on a single project at once, needing to finish it and clear the workspace before starting anew.
Because of this adaptation, later Spirulids are able to support much greater sizes. While Spirulus was nearly invisible to the naked eye, limited to less than a millimetre in diameter, its close descendant the Tremulus was able to grow to the size of a penny, making it the largest mobile predator to roam its world, at the time of its evolution.
Sponge: At this point, you may begin to wonder 'wait, how can this thing be anything like a sponge? Spirulids are diploblastic, but earlier you said that sponges were monoblasts with no germ layers. The sponges that I know can't move, and they're filter feeders!
This is all true, and just helps to illustrate how hard it can be to apply Earth analogues to other worlds, and why doing so tends to be inaccurate to the point of meaninglessness. However, Spirulids still share many features that put them closer to Earth sponges than to any other group.
The vast majority of cells in a Spirulid are undifferentiated. If you were to shred one by forcing it through a fine mesh, you would find that the great majority of cells (theoretically all those who had not been adapted into sensory organs, neurons and digestive glands) are able to live freely in a single-celled form, much as in a sponge. These tiny fragments will ultimately begin to grow into new Spirulids, with no ill effect caused by their origin. Thus, despite its appearance as a more complex organism, Spirulids have not actually lost any of the colonial traits possessed by their ancestors.
But does this atavism (retention of a primitive trait) hurt them? As anyone who has seen a destroyed Spirulid restore itself, over the course of several days, into several reconstituted offspring, not in the least.
hotosynthesis x3, Drifting x1, Sporogenesis x1
More evolved now 
