Mollusca: The Soft-Bodied Phylum

This past week, our Marine Biodiversity lab focused on Molluscs! The phylum name, Mollusca, originated from the term “molluscs,” the Latin word for soft. Many of the members of this phylum, however, are covered and protected by a calcareous shell. Squid, mussels, slugs, and snails are a few familiar faces from this diverse group of animals. They can be found in all sorts of environments, from land to the salty seas. Though these creatures can be quite different, they all maintain a few characteristics specific to their phylum. In order to move, they all have a strong muscular foot on the ventral side (toward the belly) of their bodies. They usually have gills, called ctenidia, and a radula, or membrane with little “teeth” used for feeding.

We looked at each individual class in order to gain an understanding of their specific characteristics to be able to identify them in the future.

Class Polyplachophora

These guys are the most primitive class of Mollusks. Chitons are found only in marine environments, and have a shell made up of  eight plates covering their body.



They live in rocky intertidal areas under rocks and ledges. Because of the powerful nature of tides and waves in these regions, they have adapted the ability to securely attach themselves to surfaces. They use their flat, broad bodies and muscular foot to crate a vacuum and suction themselves onto the rocks. They lack a well-developed head and don’t have any eyes, preferring to feel around for algae rather than hunt for prey. When you flip them over, you can see the pallial grove lined with ctenidia (gills). The dark red-ish area in the photo below is the pallial groove, and the gills are a bit difficult to see (mostly due to the fact that they were mostly broken off in this ancient specimen).

underside of a chiton

underside of a chiton

To help orient you, mouth of the chiton is the small dot at the bottom of the picture. Water runs under the body along this pallial groove and over the gills, allowing the chiton to extract oxygen. The foot is the large muscular looking mass in the center of the body.  Apparently, as chitons age and grow longer, they have more pairs of gills. Counting the gills can be used as one method to identify or compare the ages of individuals.

Class Gastropoda

Time for some gastropods! This class consists of garden snails, slugs, sea slugs (or nudibranchs), limpets and periwinkles. Most of the species are marine, but there are a few found on land and around freshwater. They’re unique in that they’re the only class of mollusks that show torsion. This basically means that their rear end does a 180˚ as they develop,  ending up above their head. The diagram below helps to illustrate stages of this process:

gastropod torsion Image from

gastropod torsion
Image from

Their heads are well-developed, and they have eyes and tentacles. Most of their bodies are contained within their shell. This coil is usually to the right-hand side and asymmetrical. There are three different subclasses that make up this class:

Subclass Prosobranchia

This is the largest subclass and consists mainly of marine gastropods. They have normal ctenidia and have an operculum (almost like a door) to protect them when they withdraw into their shells. Cominella, a predatory snail species, has osphradia, or olfactory organs. This might help them “sniff” out food particles in their marine environments.


Image from

Subclass Opistobranchia

Sea hares, sea slugs, and planktonic pteropods make up this subclass. They show torsion and they detorsion as adults, where their body returns to its normal position. They have a reduced mantle cavity, which is the area where the respiratory organs can be found. They do not have a fully developed shell; it is partial or lost completely. They have dorsal external gills to breathe instead of the normal ctenidia, and usually have rhinophores, a second pair of tentacles. They’re often brightly colored. A book I was reading recently, “Sex, Drugs and Sea Slime” by Ellen Prager,  had a pretty accurate description:

“It is as if Mother Nature asked a children’s art class to design the sea slug and gave them instructions to use all the colors of the rainbow, to attach any sort of paper cutout for decoration, and most especially, to use their imaginations with abandon.”

Check it out:

Subclass Pulmonata

Our final subclass consists of mostly terrestrial species. Instead of ctenidia, these guys have a vascularized mantle which acts as a lung. They also lack an operculum.

Class Bivalvia

This class of animals are commonly seen upon dinner plates: oysters, mussels, scallops, and clams. The name bivalve comes from the two valves that comprise their shell and are found on the dorsal (or back) surface. They have quite a large mantle cavity but a reduced head. the visceral mass and foot are laterally compressed, or flattened. They do not have a radula but are filter feeders, using their ctenidia (gill) to filter out particles.

We looked at Mytilus, a mussel, trying to identify the dorsal hinge, anterior and posterior ends, and left and right valves.

Insides of Mytilus

Insides of Mytilus

Mytilus is also interesting because they have a slightly smaller foot and byssal threads. These threads are made up of proteins and begin in a liquid form, are secreted by a special gland. Once secreted, it sticks to a surface and hardens. Mytilus secretes hundreds of these threads, securely attaching itself to a surface. You can see the yellow-ish threads in the photo below:

We also looked at Soletellina alba. One of the professors here studied these guys for his PhD, and there are lots of them in the nearby Hopkins River Estuary.

Soletellina alba

Soletellina alba

When in the sediment, Soletellina stick their foot out, turn sideways, and dig down into the sediments with a wiggling motion to bury themselves. It can be a bit of a slow process (depending on how motivated the bivalve is), but this video shows how it works:

Class Cephalopoda

This is the most advanced group of Mollusks and my personal favorite! Octopi, squid, cuttlefish, and nautili all belong to this class. They have a well-developed head and complex eyes. It is believed, however, that these eyes cannot detect color, which is amazing given their awesome color-matching camouflage abilities!

Their “foot” is highly modified and consists of their tentacles and the funnel which they shoot water through for movement. Their shell is reduced and internal, with the exception of nautili.

nautilus shell

nautilus shell

They also have an extremely powerful beak (think parrot) which contains a radula that pulls the food into the mouth. We had the opportunity to dissect a squid to get a better look at their internal organs and structure.

before dissection

before dissection

We started by removing the eye and looking for the cornea, lens, ciliary muscle, iris, retina, and optic nerves.

cephalopod eye diagram

cephalopod eye diagram

Our next project was to flip the squid on its dorsal side (so the funnel was facing us) and slice up the mantle cavity. I’m pretty sure the only thing I’ve ever dissected was a frog in 7th grade, so I was excited to finally use my dissection kit and open up a squid!

open mantel cavity

open mantle cavity

In the middle of the body, you can see the black ink sac. Immediately behind the ink sac is where the stomach is located (you can see it as the white mass sticking out on either side). The two clear-ish structures that stick out from the center of the body are the ctenidia (gills), and the top half of the body contains all of the reproductive organs. I believe the orange coloration was the beginning stages of eggs. We were also able to open up the stomach (nothing but digested orange gunk in ours) and the ink sac. The ink was much more gelatinous than I expected. This video, it’s a slightly different squid from the type we dissected,  points out the structures in more detail:

Squid have eight arms plus two more that are much longer and used to capture prey. Males can be identified by their single copulatory arm that is used to collect and transfer spermatophores to the female.

At the end of our dissection, we removed the beak.

squid beak inside the body Image from

squid beak inside the body
Image from

It was a slimy process, but really neat to see! The two pieces were actually shaped differently to fit together and work like a hinge.


squid beak

After we were done I washed my hands a few times and sanitized, but they still smelled like fish bait for the rest of the day! I can’t say I minded too much though, I dissected a squid! Definitely the highlight of labs so far.

Molluscan Radulae

We also had to look at the different radulae that exist in mollusks. I mentioned earlier that radula a radula is a membrane with little “teeth” used for feeding. It’s shaped differently depending on what type of food the animal eats.

The first type we looked at was relatively smooth. It was from Bembicium, a sea snail that eats algae.

Image from

radula similar to Bembicium           Image from

We also looked at the radula from Nodolitorrina, which was slightly more pointed and toothy.

radula similar to Nodolitorrina Image from

radula similar to Nodolitorrina
Image from

Next up was Cellana, which was even toothier…

radula similar to Cellana Image from

radula similar to Cellana
Image from

And finally, we have the lethal radula of  a cone snail…

cone snail radula Image form

cone snail radula
Image form

You don’t want to mess with these experienced hunters.

Here’s a comparison of all the different types of radula:


Pretty crazy, huh? It’s amazing what you can see sometimes when you put it under a microscope!

We also had a few preserved mollusks to look at and some shells to investigate.





dumbo squid

dumbo squid

Hard to guess how it got its name, huh?



And that was the end of lab! By the end I was feeling pretty comfortable identifying the various parts of the mollusks, now I just have to memorize all those funky class and subclass names!


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