Octopuses: arms vs. tentacles

[lpetrich]

 Cephalopod
The Cephalopoda - at the University of California Museum of Paleontology
Cephalopoda - at the Tree of Life web project

Cephalopods ("head foot") are mollusks, relatives of snails and clams. They have several highly flexible limbs, called arms or tentacles. A tentacle, in the general sense, is a limb that is flexible along its length, and not just at joints, and in that sense, every cephalopod limb is a tentacle.

But there is a complication. Squid and cuttlefish have eight short limbs and two long limbs. But there is a convention in squid anatomy of calling the short tentacles arms and the long tentacles plain tentacles. This convention is often extrapolated to octopuses, making their eight limbs arms instead of tentacles. This convention is also often extrapolated to the extinct belemnoids, making their ten limbs arms instead of tentacles.


But octopus limbs are still tentacles in a general sense, even if not in a squid-anatomy sense. And squid arms are also still tentacles in a general sense.

 

[lpetrich]

Cephalopod limbs are numbered I to V on each side, with I being on the dorsal side and V on the ventral side, where the ventral side is the side with the siphon (jet nozzle). In octopuses, the eyes tend to be on the dorsal side.

Squid and cuttlefish have limbs IV elongated and with suckers only at their ends. Vampire squid have limbs II elongated, and octopuses have lost them. Octopus limbs are often numbered I to IV, making their homologies I - I, II - III, III - IV, IV - V. Bird fingers have a similar reinterpretation, with homologies I - II, II - III, III - IV. Those homologies are apparent in bird embryos, with birds initially growing all five fingers, and then getting rid of I (thumb) and V (little finger). Fingers II, III, IV (index, middle, ring) are then reinterpreted as I, II, and II. However octopus embryos have no evidence of ancestral limbs II.


Other notable possessors of tentacles are cnidarians (sea anemones, jellyfish, etc.) and ctenophores (comb jellies).


Closer to home, there is a kind of tentacle that is usually not called a tentacle, but that fits the general definition very well: an elephant's trunk.

 

[Jokodo]

Cephalopod limbs are numbered I to V on each side, with I being on the dorsal side and V on the ventral side, where the ventral side is the side with the siphon (jet nozzle). In octopuses, the eyes tend to be on the dorsal side.

Squid and cuttlefish have limbs IV elongated and with suckers only at their ends. Vampire squid have limbs II elongated, and octopuses have lost them. Octopus limbs are often numbered I to IV, making their homologies I - I, II - III, III - IV, IV - V. Bird fingers have a similar reinterpretation, with homologies I - II, II - III, III - IV. Those homologies are apparent in bird embryos, with birds initially growing all five fingers, and then getting rid of I (thumb) and V (little finger). Fingers II, III, IV (index, middle, ring) are then reinterpreted as I, II, and II. However octopus embryos have no evidence of ancestral limbs II.


Other notable possessors of tentacles are cnidarians (sea anemones, jellyfish, etc.) and ctenophores (comb jellies).


Closer to home, there is a kind of tentacle that is usually not called a tentacle, but that fits the general definition very well: an elephant's trunk.

what about the tongue? doesn't it too fit the general definition of a tentacle?

 

[lpetrich]

Yes, though usually a very stubby one.

 

[untermensche]

So in some cases a tentacle is not called an arm.

It is called a tongue.

This is not a "complication". Unless we imagine nature should fit into nice little categories for us.

 

[James Brown]

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Source

 

[Gun Nut]

these are not anatomical descriptions.. they are functional descriptions... human babies use their tongues to explore objects, which is why toddlers tend to put anything and everything into their mouths. In this case, the human tongue is like a tentacle. As our senses develop over time, and also with our developing sense of "contamination", we stop using our tentacle-tongues to explore objects, and rely on other senses... reserving our tongues for the all-important function of speech, as the brain catches up to the task.

 

[skepticalbip]

these are not anatomical descriptions.. they are functional descriptions... human babies use their tongues to explore objects, which is why toddlers tend to put anything and everything into their mouths. In this case, the human tongue is like a tentacle. As our senses develop over time, and also with our developing sense of "contamination", we stop using our tentacle-tongues to explore objects, and rely on other senses... reserving our tongues for the all-important function of speech, as the brain catches up to the task.

The tongue is used as a tentacle when eating. It moves and continually positions the food we are chewing so the teeth can properly grind the food to make it suitable for swallowing. It would be almost impossible to chew food without the tongue. It is also used as a tentacle like in licking an ice cream cone... it reaches out and collects a bit of ice cream and returns it to the mouth for ingestion.

An alternate use of the tongue is identifying foods through taste to determine if it is edible.

But then that tentical is often used to poke out at others to express our disagreement...



or by the Maori to demonstrate their fierceness.

 

[Politesse]

Love ammonites! When I was young, I drew them all over everything. Lost my mind the first time I saw a chambered nautilus in person at the Monterey Bay Aquarium. My dad still has one of my adolescent-years ink sketches of an ammonite pinned to his office wall, next to one I did on a restaurant napkin at age 7.

 

[lpetrich]

Let's count up the tentacles:

• Ctenophores (comb jellies) -- sometimes two long ones that may be branched
• Cnidarians -- tentacles around the mouth
• Cephalopods -- tentacles around the mouth
• Vertebrates -- tongue: a tentacle inside the mouth
• Proboscideans -- trunk: the nose elongated into a tentacle

For coleoid cephalopods, one must be careful about two senses of "tentacle"

• General sense -- all cephalopod limbs are tentacles
• Short-vs-long sense -- short ones are arms, long ones are tentacles, a shorthand for distinguishing limb length

 

[Tharmas]

Love ammonites! When I was young, I drew them all over everything. Lost my mind the first time I saw a chambered nautilus in person at the Monterey Bay Aquarium. My dad still has one of my adolescent-years ink sketches of an ammonite pinned to his office wall, next to one I did on a restaurant napkin at age 7.

My area of the country was under the sea during much of the Cretaceous Era, so there's a lot of chalk, and in places you can pick up ammonite casts off the ground and take them home. Some still have bits of shell stuck to them. The actual shells are much harder to come by of course.

 

[Peez]

Bryozoans, star-nosed mole, some polychaetes,some caecilians, some snails, a few cestodes, perhaps barnacles...


Peez