Ceph arm movement


Larval Mass
I'm not in any way related to this field (at all) but have been browsing the site and have a question...

...the cephelapod arm... how does it move?

I'm guessing it's full of muscle, yeah, but if there's no internal or external skeleton for the muscle to attach to via a tendon then how is the muscle anchored? How does it flex with nothing to flex against.

I've just seen a blog post on another site that shows images of an octopus attacking and retrieving prey using it's arm with a 3-point bend (much the same as a wrist/elbow joint in a human arm) but i can't get a handle on the internal mechanism that allows muscle to contract and relax like this... surely muscled can't work in antagonistic pairs if there's no anchor...

...i'm confused... can someone enlighten me because i'm fascinated by the little fellas!


Yes. They use what's been termed a muscular hydrostatic skeleton, where antagonization of muscles by other muscles provides the equivalent of skeletal support. This paper by Bill Kier (who introduced the concept) and Kathleen Smith (now his wife, I think) is a good introduction to muscular hydrostatic skeletons:

Trunks, Tongues, and Tentacles: Moving with Skeletons of Muscle

A good place to start learning about the muscular system of cephalopods would be here:


That paper is also co-authored by the great Bill Kier. I'd love to summarize it for you, but I'm not sure that I'll have the time. Monty?


Colossal Squid
Staff member
um... said:
That paper is also co-authored by the great Bill Kier. I'd love to summarize it for you, but I'm not sure that I'll have the time. Monty?
I'm frantically running around taking care of things before I catch a plane tonight, so I don't really have time to write up much either... anything I know is from Kier's papers, anyway, though, and he's a good writer, so I don't know that I'd be able to describe it any better. I'm glad he's put a bunch of those on the web, too-- I had to photocopy them at the library!

The 3 minute version is: muscular hydrostats are made up of muscle, and the fluid in them wants to stay the same volume. If we just consider a tentacle stalk, it's a cylinder of some volume. It's flexible, but always wants to stay at the same volume, so if you make it longer, it gets thinner, or if you make it shorter, it gets fatter. The muscles are arranged so that it takes advantage of this "same volume" aspect-- there are big muscles that pull to narrow the stalk that reduce its cross-section, which makes it thinner and causes it to extend-- that's how it shoots out the tentacle to grab prey. There are also muscles that run length-wise that shorten it, to pull the prey back in. Then, there are a number of other, smaller muscles for other types of movement-- some go around the outside edge to cause twisting motions, for example, or sometimes the animal will pull to shrink only on one side of the tentacle, causing it to bend in that direction. Which set of muscles is active can be controlled pretty much continuously along the arm/tentacle as there are a large number of ganglia going along the length, each responsible for a short segment of the muscle control.

I've wanted to make a computer simulation and animation of this for years,

gotta run!

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