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How does a barnacle molt and grow if it does not leave their outer shell?
Answer 1:

First of all, you might be interested to know that barnacles have confused scientists for a long time. Up until the mid 1800's, it was though that barnacles were mollusks! It was not until careful studies of their free-swimming larvae were conducted that it was realized that they were actually crustaceans.

Barnacles do molt (shed an old cuticle or exoskeleton, as a new one is being made --also called ecdysis). Molting (ecdysis) in barnacles is the same as in other crustaceans, it just is "covered" by calcerous plates (the "shell"). It is a little bit difficult to explain without pictures, but I will try. The growth of the shell is pretty much independent of the growth of the barnacle and molting in terms of timing. The shell steadily grows and the barnacle grows but they are not coupled The cuticle (exoskeleton) is what is "molted" and this is inside the shell of the animal. The calcerous plates are not shed. As the plates (shell) are formed, they cause a pushing out/extension of the older plates, which gives the animal more room to grow (as it molts).

If you can get to a library and look at an invertebrate zoology textbook, you might be able to find some good pictures and diagrams of this. A particularly good reference is a text by Ruppert and Barnes called "Invertebrate Zoology." It is published by Harcourt Brace. Good luck-- and keep asking good questions!

Answer 2:

Humans have skeletons made of primarily calcium.This is an endoskeleton, because all our tissue (heart, liver, etc.) is outside of the skeleton. Other animals have exoskeletons, where all the organs are inside of the skeleton. These animals usually "molt" when they grow. The animal sheds its old skeleton that is too small and grows a new one. A soft shelled crab is essentially a crab that has just molted, where the outer shell hasn't had time to solidify.

Barnacles, like crabs, are crustaceans. They will molt inside of their shell and spit the old skeleton out of the top. The shell is actually a bunch of plates. These plates can expand as the barnacle grows. The barnacle secretes more calcium carbonate to fill in the gaps.

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