Allow me to add some things! Annelids, the segmented worms, are woefully underappreciated for all they do. Estimates for segmented worm species range from over 9 000 to over 22 000! That's 1.4 - 3.4 times the amount of mammal species, and so very diverse, you'll find!
(Old annelid collage I made for my fish masterpost)
Tumblr user rosybetta already shows many a beautiful poluchaete worm, but I want to show some more!
This luminous polychaete worm lives several thousand meters deep on the ocean floor, and sports these extremely light-reflecting scales! Each one is hollow and lined inside with hexagonal cylinders which reflect nearly 100% of the light that hits them. The sea mouse's quill-scales create the only documented case of a "complete spectrum photonic crystal" in an organism, that is, a nanostructure that alters the light entering them and can create the whole spectrum of light. It is thought that the quill-scales are so light reflective as to deter predators or to make the sea mouse's silhouette harder to see.
Come on tumblr, you love whalefalls, right? These worms are an important part of the whalefall ecosystem, boring into the bones with acid to feed on them! These peculiar animals lack a stomach and a mouth, instead relying on symbiotic bacteria that break down the bone material, after which they suck up the nutrients with "roots"! The red things you see above are the female worms' gills, as the males are in fact microscopic and live inside the females. These worms are valuable in returning every bit of nutrients back into the biochemical cycle!
Another denizen of the deep that enjoys a diet cooked by symbiotic bacteria. Giant tube worms live near hydrothermal vents, in very hostile conditions with very high sulfur levels! They're one of the fastest growing marine animals, reaching a size of almost a meter (or nearly three feet) in a year and a half!
This deep sea worm is an extremophile if you've ever seen one, it also inhabits hydrothermal vents. They build a tube around themselves and seemingly shield themselves from the elements inside the tube with the help of a "fluffy" coat of sulfur-eating bacteria. What makes them especially fascinating is that their tail end comes to contact with temperatures of up to 80 °C (176°F) while their gills collect oxygen from the outside water that's only 22 °C (72°F)! 60 degrees Celsius of temperature difference to an animal only 13 cm in length, that's bonkers!
These free-swimming polychaetes produce a yellow bioluminescent glow from the appendages at the sides of their body, which is unusual among animals! Gossamer worms have a unique style of swimming from other polychaetes, as they lack many bristles at the ends of their appendages and also the mesenteries in between each body segment. Still they swim, with great flexibility, speed, and maneuverability! It turns out that they retract and extend each appendage to cause differences in drag, causing a wave to undulate through the whole body, which in turn makes their thrust power even stronger. They might serve as inspiration for aquatic robots in the future!
I acknowledge that they are a bit gnarly, with their reputation as bloodsucking parasites! To some, this only makes them cooler. To others, this makes them horrible. But, just so you know, leech saliva contains an anesthetic, which makes their bites painless! Medical leeches have been used in the past and are still sometimes used in a medical context, for example if blood pools in a certain area of the body after a surgery, leeches can be used to prevent tissue death. Leech saliva also prevents blood clots, which helps to keep blood flowing! Leeches are, at the end of the day, a diverse group in and of themselves, inhabiting freshwater, marine and even terrestrial habitats, and some partaking in predatory hunting behaviour instead of parasitism!
I think a lot of people truly underestimate the humble earthworms and what they give us. But, to be fair, it is kind of hard to truly comprehend the title of "(one of the) most important terrestrial detrivore(s)". Their squirming movement aerates the soil and their feeding breaks down organic matter so that they can return back into the biochemical cycle! Various earthworms can create different microhabitats, and they often have a positive impact on plants and fungi! That is already a very important job to do.
I feel like there's this idea of earthworms as mindless animals that kind of do whatever based on instinct, but like every other animal each species behaves uniquely. Take Lumbricus terrestris, the common earthworm! One of my professors is head over heels in love with earthworms and he really opened my eyes to how exciting they are. In the case of the common earthworm, this species digs deep vertical burrows that can descend up to two meters deep! It covers its little burrow in leaf litter, which it also eats. The head of the common earthworm is often darker than the rest of the body, which is an adaptation to camouflage it against the soil so predators will have a harder time noticing it! They enjoy hanging out in their burrows the most and change their position in it depending on external conditions. When they mate, common earthworms first visit a potential partner's home burrow first, likely to tell if their partner-to-be can keep their burrow clean and free of parasites. After both parties making a visit, they embrace each other while mating and both parties become pregnant, as common earthworms are simultaneous hermaphrodites.
Hearing about the life that common earthworms live, well... I really realised that every animal lives a life that is full of purpose and choices. Earthworms were anything but lowly creatures! I knew this about fish and insects already, but never before had I stopped to think about earthworms in the same way. Let alone all other segmented worms! These are animals that hardly live a life of nonexistence and I am a bit sad that I never saw them as anything to think much about before. But... now I do. Now I see them.
And it would be really sweet if you could appreciate them a bit more too, now.
