Crab Skeletons

Here's a pretty cool little seaside casualty I found the other day. You see these guys pretty often at the beaches here but this one was a bit bigger than most so I cleaned him up and did these sketches.
I hadn't really looked too closely at crab skeletons before and it proved to be a real learning experience. It's cool seeing how these little mammals have adapted to life in the sea. As warm-blooded vertebrates they share a similar basic structure to us but with a few specialised modifications.
The metacarpals and phalanges have simplified and fused into a pair of opposing pincer-like shapes and with their thick epidermal layer they function as highly effective claws.
The wide scapula sits far down the ribcage and forms a strong foundation for anchoring the teres major.
Probably the component most far removed from our own frame is the pelvic girdle which has evolved into a six-socketed form almost unrecognisable to ours.
I painted up a skull study as well - what bizarre little craniums they have! The eye sockets are open with the zygomatic bone being wholly disconnected from the supraorbital foramen - I guess this is due to the stability offered by the hardened epidermis and to allow free movement of the eyestalks.
What appears to be a nasal cavity is actually a food filtering system - the "nasal concha" is in reality a gill-like arrangement for catching too-large fragments in the feeding process. It continues down behind the maxilla.
A remarkable free floating pre-maxilla carries the first battery of tiny teeth with the second, smaller set of mandibles hidden away below.
So there you have it! That was a pretty interesting process for me, learning about these dudes. I find it fascinating how mammals can adapt and transform to live in any environment yet still retain their inherent vertebrate humanity.