I think this thread has wandered way into the weeds of taxonomy and nomenclature. It may be best to remember the target audience: high schoolers who mostly don’t have a strong scientific background and who may be thinking about these ideas for the first time :)
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My students definitely have that misconception! I once said that the mosquito was the deadliest animal, and a student said, “A mosquito is an insect, not an animal!” Well, we had to have a conversation about that one.
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I was told that an important derived characteristic that mollusks share is that they have a shell, whether inside or outside. Even a squid has a shell, it’s just the pen on the inside. The only “exception” are slugs.
But, it sounds like it might not be helpful to use shells as a defining characteristic if there are so many types of slugs.
Would you consider a squid pen to be a shell? And do octopuses have the same thing?
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Some land slug families have no shell, external or internal, but they have a small sac of calcareous granules inside their backs.
Many families of sea slugs have no shell at all, not even a remnant, and neither does the octopus.
I guess I could call a squid pen a “shell” of some sort, but usually it is just referred to as a pen.
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So… Was Ogden Nash not completely wrong when he said
“Come crown my brow with leaves of myrtle!
I know the tortoise is a turtle.
Come carve my name in stone immortal!
I know the turtoise is a tortle”?
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According to my 1974 Barnes Invert. Zoo. text, octopuses have completely lost their shell. I don’t know what modern thinking about Mollusks, but the whole group Phylum struck me at the time (and still does), as a sort of ‘shoehorn’ group. But I really don’t know much about that Phylum.
I feel the same way about the Phylum Arthropoda.
This is helpful. For some reason I was thinking an octopus had a pen, but I guess it’s only the squid!
I’m not sure I know what you mean by a “shoehorn” group? Could you please explain?
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Very interesting that slugs have calcareous granules! I did not know that!
What would you say are the “defining characteristics” of the mollusk phylum then? I’m looking particularly at how to communicate these at a high school level
I think they just share a common ancestor. it might be a good place to discuss modern vs. older ways of categorizing animals.
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I assumed @mamestraconfigurata meant they seemed like they were “shoehorned” into the group.
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And @elkins456 Yes. That is what I meant. I believe they have some ancestral characteristics, but sometimes these Phyla can seem like many things are crammed into one group on the basis of some dodgy logic! But hey, I’m not a professional!
Okay, yes, I certainly see what you mean and agree! In the end, our groupings and taxonomy are just models and ways that are helpful to understand the world. It’s like the equator-- it’s an imaginary line to some degree, but it’s very helpful to model our maps around. We are drawing imaginary lines around certain animals (slugs and octopi belong together!) but those groups are just based on (1) factors we decide are important and (2) language that is helpful for us to use as we discuss a complex and surprising world!
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Won’t someone think of the arachnids? Poor things either probably get assumed as Insects or Animals randomly or just plain not thought about :p
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I would say, the mantle. And in most mollusks (but not in bivalves), the radula.
Other sources say: "They have a soft unsegmented body and live in aquatic or damp habitats, and most kinds have an external calcareous shell.
Or you can go more technical and say:
Characteristic Features of Phylum Mollusca
- They are bilaterally symmetrical.
- They are triploblastic, which means their bodies basically have three layers.
- They show organ system grade of organisation.
- The body is soft and unsegmented.
- Body is divisible into three regions – head, a visceral mass, and ventral foot.
- Body is covered by a mantle and usually a shell.
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The fate of most invertebrate species unfortunately. And spiders are also in that catch all Phylum, Arthropoda.
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Thanks!
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Keep in mind that although broad groups of organisms, such as phyla, were historically arranged based on morphological similarities, genetics plays a bigger role in groupings today. Also, the body plans of adult organisms within a phylum or class may look radically different but their early developmental stages (in the egg, as an embryo, fetus, larva, or nymph, etc.) might be quite similar. The morphological divergence takes place during development as certain genes are switched on or off at different times, leading to very different looking creatures as adults. Don’t know if evolutionary-developmental biology (evo-devo) is beyond high school biology, but the concept is really rather simple and opened my eyes to how divergence among organisms can happen.
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Check out the modern arrangement for the Order Artiodactyla https://www.inaturalist.org/taxa/152870-Artiodactyla
I still have a hard time reconciling that whales and antelopes are grouped together at the Order level.
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Depends how you measure it!
That might be an exciting lesson on its own – asking the students to come up with as many measurable concepts of “largest” as they can, and revealing which organisms are largest in that specific way.
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Betta are a type of gourami, many people think that it’s a separate type of fish altogether.
Additionally the word “fish” is taxonomically ambiguous as “fish” like coelacanths and lungfish are actually more closely related to us than they are to other fish
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