Annelids, Echinoderms, and Cnidarians! Oh, my!

This past week we had a Marine Biology lab focusing on the characteristics of individual phyla and functional feeding groups. Functional feeding groups are categories, used as way of identifying organisms based on their preferred methods of finding food. For instance, if an animal hunts individual prey items, it can be classified as a predator. Other groups consist of suspension/filter feeders (that filter particles out of the water column), deposit feeders (that sift particles deposited into the sediment), and grazers/browsers (that feed on plants or groups of non-moving animals). The problem with lumping animals into functional feeding groups, is that many times, they can overlap into multiple categories. It is deemed useful, however, as it is one method to begin investigating new and unknown creatures.

Our job for the first part of lab was to examine organisms that were labeled according to their functional feeding groups and identify which phylum they belonged to based on their defining characteristics.

(The videos I included below are just to give you a bit more information or to add a bit of clarification to the things I talk about with each animal. They’re simply there if you’re interested.)

The first critter I looked at was a deposit feeder.


Deposit Feeder (Sea Cucumber)

Lovely looking little things aren’t they? Not having much first-hand experience with the ocean, my initial instinct was that they were a worm of some sort, but then I realized that they had spiny skin. Echinodermata is a phylum derived from the Greek term for “spiny skin.” Echinoderms consist of sea stars, sea urchins, sea cucumbers, and their relatives. Sea cucumber seemed to be the most fitting title to these round little guys. Interestingly, echinoderms have a characteristic radial symmetry, usually in groupings of five. This can easily be seen when looking at a sea star. If you start from the center and work your way around in a clockwise motion, the animal’s body repeats a similar structure five times (notice the five legs).


Sea Star

Another Sea Star

Another Sea Star

Echinoderms also have a water-vascualr system. This system gives them a method of locomotion, allowing for controlled movement. On each of the five repeated segments, echinoderms have hundreds of little tube feet, also known as podia. The water-vascular system is composed of a network of radial canals, which are then joined to the tube feet via some sort of lateral connection. These canals and the tube feet are filled with sea water, and through the expansion and contraction of certain chambers in the water-vascular system, the echinoderms are able to force water into specific tube feet. If done in the correct order, they are able to “walk.” Many of these animals also have several characteristics that make them unique from others in their collective phylum.

My next mystery species was a predator, but it wasn’t much of a mystery! I absolutely love cephalopods, which belong to the phylum Mollusca, and this little guy couldn’t have been more obvious.



Mollusca consists of organisms such as squid, octopi, cuttlefish, scallops, clams, mussels, snails, slugs, limpets, and chitons. In my opinion, cephalopods (squid, octopi, cuttlefish, and nautilus) are the most charismatic group in this phylum.

Mollusks all have the same general body plan, in that they consist of a head, foot, and mass of internal organs, which is typically covered by the mantel. Many members of this phylum also have a shell or some sort of modified shell. They usually have a radula, which is a little extendable structure found in the mouth. It’s used for feeding, whether that be drilling holes in shells or rasping food off a surface. They usually have at least one set of gills for respiration.

Limpets (Mollusca)

Limpets (Mollusca)

Mussel (Mollusca)

Mussel (Mollusca) The shell is cracked open, and you can see the pink, fleshy gill used to siphon out organic particles.

The next creature was a suspension or filer feeder. Though the microscope was initially focused in on an anemone, we were supposed to be identifying a sea sponge. Anemones belong to the phylum Cnidaria. This grouping consists of jellyfish, corals, and sea anemones, which all have very primitive multicellular body structures. All Cnidarians have some form of stinging structure, and their body can either be in the form of a polyp (a sedentary form, like a sea anemone) or medusa (a motile form, such as a jellyfish). Their skeleton is made up of closed compartments containing pressurized fluid. They use their muscles to change the shape of these individual compartments in order to move.

The stinging structures in these organisms function through the use of a venom. Interestingly, many of these venoms are being researched in labs around the word in order to identify their chemical properties and possible usage in the pharmaceutical world. For instance, sea anemones have become of great interest in cancer research.

As I mentioned, the organism we were originally supposed to be identifying was a sea sponge. Sponges belong to the phylum Proifera. The main characteristic of these odd creatures is their feeding system. They lack mouths, but instead draw water through microscopic pores on their outer cell walls. The cells filter out any organic particulates for the sponge to feed upon and expels the unnecessary water and waste products through the other side. They have the simplest faunal cellular organization, called cellular-level organization. This means that they have no tissues, but cells are specialized for certain functions.  Flagella, little wavy “arms” that line the surface chambers of the animal drive this water current in its forward, unidirectional motion. Some sponges can actually be carnivorous, though this is an exception rather than the norm.

Sponges are such blob-like organisms. They seem much more plant-like at first glance than animal, but they are animals nonetheless.

The next organism I observed was another that I never would have guessed to be an animal. This photo is as close as I could get to the image I saw under the microscope:

Sea Squirt

Sea Squirt (Image from

I had absolutely no idea what it was. I could see the white blob, that there were two openings (one for feeding and the other for expelling waste), but other than that I was clueless. After asking one of the professors running the lab, I found out it was a sea squirt. Even after that clarification, I still had no idea what it was. Apparently all of my experience studying marine fauna on Lake Michigan was not kicking in (haha).

Sea squirts get their name from their somewhat entertaining ability to squirt water when squeezed. Their basic function is to attach to substances and work as filters, taking in water and particles in one siphon and squirting it out the other. They’re perfectly harmless when their native to an area, but invasive sea squirts can become quite a problem.

Despite their odd structure, sea squirts belong to the phylum Chordata. Chordates consist of many of the animals that are most familiar to the general public. The main characteristics that define this diverse phylum might only be observed during the embryonic stage, modified in development, and essentially “lost” for the duration of its life. For example, humans belong to the phylum Chordata, however, we do not retain the post-anal tail that can be observed during the embryonic stage. Pharyngeal slits (or a row of openings that join the interior of the throat with the outside of the neck), a dorsal nerve cord, and notochord also characterize Chordates. A dorsal nerve chord consists of a group of nerve fibers that runs down the back, connecting the brain to muscles and other organs. The notochord is a cartalage-like support system for the dorsal nerve chord. Somehow, categorizing this little blobby guy and a fish into the same phylum seems a bit odd to me, but the dots start to connect once you draw out the individual characteristics.

Fish (Chordate)

Fish (Chordate)

One of the craziest phyla is Annelida. Some of these wormy creatures are quite alien-like. In short, Annelids are segmented worms such as earthworms, leeches, and polychaetes. We focused mainly on polychaetes, since they’re quite common in marine environments. Many of them are deposit feeders or filter feeders, and they have feathery appendages branching off of their heads that allow them to do so.

Besides having segments, another defining characteristic of annelids is that they have a fluid-filled space between the gut and the body wall. This is called a coelom. By pushing fluid through the coelom through the individual cavities, and using the muscles surrounding each segment, annelids are able to inch along the ocean floor. They use their parapodia (little leg-like structures branching out from each segment) to grip the ground as they move, and have a completely enclosed circulatory system.

And finally, the last phylum we encountered was Arthropoda. This is an extremely diverse group (but then again, which group isn’t?), consisting of animals like tarantulas to lobsters. Barnacles, shrimp, crabs, and lobsters are a few Arthropods you might encounter in the ocean. Like Annelids, they have segmented bodies, but they also have paired, jointed legs and antennae. Their body is covered by a chitinous exoskeleton that must be shed to allow for growth. They have a dorsal nervous system and an open circulatory system, meaning that the circulatory fluid is not always enclosed within vessels.

Some of these guys are pretty intense:

Moreton Bay Bug (Arthropoda)

Moreton Bay Bug (Arthropoda)

For the second part of our lab, we were given similar tasks, but we had the additional task of determining what functional feeding group the organism might belong to based on the mouth parts we observed.

It was an interesting lab and we were able to observe many different animals from a range of phyla. I think I learned more about each individual phylum writing this blog though! There are a lot of interesting creatures out there, and I know we have just begun to scratch the surface.


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