Annelids: A Rather Squirmy Lab

Our Marine Biodiversity lab this past Friday focused on Annelids, the worms of the animal kingdom. Our main goal was to become familiar with the various body types and structures, providing us with the tools to identify the various squirming creatures we collected on a field trip a few weeks ago. This blog is as much as a photo study guide for me as to share what we’re learning, so get ready for lots of lovely worm info!

First of all, we had to understand the three different body types of worms: acoelomate, pseudocoelomate, and coelomate. A coelom is a fluid-filled body cavity that serves as a “cushioning” between the outside of an animal and it’s guts. It also allows for separation and compartmentalization of organs and other body features. Animals lacking a coelom, such as Planaria (a flatworm) are considered more primitive and are labeled as acoelomates.

We were able to look at a cross-section of Planaria under the microscope to see this body structure.

You can see that there’s really nothing separating the outside of the outside (epidermal) layer of the animal from the gut cavity. Everything is filled by parenchyma, or the tissues of the various organs.

The next body type is pseudocoelomate, or “false” coelomate. These animals have more of a space in their body cavity, but there is no connective tissue or muscle to support the gut within the fluid of the coelom.  There is still no membrane surrounding the gut and internal organs, or peritoneum. For this reason, it is known as a false coelom.

We were able to look at the cross-section of Ascaris (parasitic nematode worms). You can see that the body structure is slightly more complex.

Finally, coelomates make up the last body type. These animals, like humans, have a lined body cavity that provides support for the coelom.

We examined a polychaete worm known as Nereis, noting the muscle bands, nerve cords, and other internal body structures.

They phylum Annelida is represented by coelomates. Though there are other worms that have aceolomate and pesudoceolomate body structures, they belong to other phylums. Their body is composed of repeating segments, and they have a circulatory system. Our lab focused on the class known as Polychaeta, which are almost entirely marine. These guys have a pari of parapodia (leg-like structures) with many setae (hair-like structures) on each segment. Their head is usually well-developed and contains sense organs.

Okay, here we go!

Family Arenicolidae

These worms, also known as lugworms, do not have appendages, but like to burrow in the sediment, looking for deposited organic materials to feast upon. In their mid-section, you can see the little parapodia and gills on each segment.

Family Orbiniidae

Orbiniidae are also deposit feeders that burrow in the sediment of tidal sand and mud flats. Their bodies, however, are divided more distinctly. The thorax makes up the first section of the body (on the left), and you can see the little parapodia (leg-like structures) coming off of the abdomen (which makes up the rest of the body). The very first segment of the head is known as the prostomium, and the peristomium is the second. The prostomium comes to a point or is truncated, unlike ArenicolidaeThe head also has no palps (appendage-like mouthparts).

Family Eunicidae

Image from

Image from

The name of this polychaete literally means “many bristles,” and they can be found in the crevices and algae holdfasts (or root-like structures) in the temperate rocky reefs of marine environments. As you can see, they have very large jaws. This allows several members of this family to be carnivorous, though there are also many omnivores. A few Eunicidae species can grow to be longer than a meter in length, and they are amongst the largest Polychaetes. They can be identified by their rounded prostomium from which projects five appendages, two palps and three central antennae. Bright red gills also extend from the parapodia. It’s a bit difficult to see since they are very small, but the photo does show little red, feathery gills along each little leg-like structure on the body.

Family Nerididae

Nereididae reminded me of the millipedes we used to carry around in bug boxes in Disney. These guys had lots of little legs that worked together in a wave-like motion. Their main characteristics are their well-developed sense organs. Their prostomium has two anterior tentacles, two jointed palps, and four eyes.

Family Syllidae

This family is the most diverse of the polychaetes and its species are found in all types of marine environments. The main feature used to identify this group is the proventricle (a little round spot that is part of the digestive tract). In the photo above, you can see a little brown spot behind the head– that’s it. They also have three antennae on their first body segment and a toothed pharynx (an organ by the mouth used to eat).

Family Serpulidae

Tube worms are some of my favorites. These guys form calcareous tubes and live together in groups, working collectively to fan in and filter out sediments with their feather-like tops. We were supposed to focus on the various body sections: the head, thorax, and abdomen. The thorax has a sort of collar that distinguishes it from other worms. They also have an operculum, which is used as a sort of “door” when the worm pulls itself inside the tube to hide. A central grove runs along the ventral surface of the body and curves up towards the thorax. As you can imagine, living in a tube makes waste disposal a slightly more complex process, and this grove helps with that.

Family Sabellidae

Representing this family, we had some rather large feather duster worms! These guys are different from Serpulidae because they lack an operculum to protectively “close” their body cavity when they pull inside their tubes. The tubes for this family are made from mucilage (a snot-like substance) and sand from the sediment. If you poke the tube it’s rather squishy and a bit spongy. It’s almost like squeezing a cattail. They’re rather interesting little creatures.

Family Spionidae

Found throughout soft-sediment environments, Spionidae are identified by their  large, grooved feeding palps extending from the top of the head. Their prostomium is also rather prolonged, and you can see it in the photo extending almost like a lengthy nose structure. supposedly, on either side of the head they have notopodia (larger parapodial lobes behind the head) and gills, but I had difficulty seeing these structures.

Family Cirratulidae

I’d describe this family as quite the mess! When I first looked at our specimen, it took me a few seconds to find the body underneath its wriggling mass of tentacles. They like to burrow and crawl in the sediments and are often found in seagrass beds and the enriched sediments of estuaries. Along their body, they have a pair of gills per body segment, and the tentacular filaments may almost cover the body.

Family Terebellidae


I actually took the photo of this one! Spaghetti worms, one of the sci-fi stars of the ocean world (of which there are many)! Those thin little tentacles are non-retractable. They’re deposit feeders, so they sift through organic materials deposited in the sediments and consume them. When I looked under the microscope I could actually see little particles trapped in the tentacles which were slowly being worked towards the mouth. It was freaky and fascinating all at the same time. If one of those spaghetti-like appendages is lost, they can actually regrow them. Handy, huh?

At the end of lab, we had time to identify the worms we had managed to collect in our lab in the weeks prior. My group found very few species in the area we surveyed. I’m not sure if it was because we picked a poor spot or if it was due to some other reason, but we found three worms in our samples overall. Two of them we found to be Syllidae and the other seemed to belong to the family Magelonidae. We didn’t look at this family in our lab, but the worm had characteristics described in the literature that matched this group (a tube-like structure on the bottom half of its body and  large palps). They’re also common to the sandy sediments we were sampling.

At the end of the day I found myself carrying around much more knowledge about worms than I ever thought I would need. Next week we’ll be focussing on another phylum. I think I found myself most fascinated with the spaghetti and feather duster worms this week. Which Annelid was your favorite?


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