* ¡Fantástico trabajo! Lo hemos destacado dentro de la carpeta "Featured nº5 o Destacados nº 5" en Special-Groups. (Es la carpeta donde se exhiben los mejores trabajos del grupo). ¡Por favor, queremos ver más trabajos tuyos en el grupo!
* Fantastic work! Featured in Special-Groups in folder Featured nº 5. (Best works of the group). Please, We want to see more works in the group of you!
It has to do more fundamentally with some initial developmental traits. By this, I mean the patterns the very first cells take post-fertilization. We are deuterostomes, the very first opening our developing zygote creates is the anus, and then the mouth comes second. We include Echinodermata (urchins, stars, sea cucumbers etc), Chordata (our line), and a few other minor taxa. Molluscs, arthopods, cnidaria (jellyfish and corals) are protostomes. They make the mouth first, then the anus. There are other cell arrangement and developmental differences, but the orifice one is easier to remember and visualize.
So what fills up the rest of the space inside? Is it mostly intestine? I don't see a lot of musculature being necessary, and heart/liver/gills/etc probably don't take up all that space.
If you hadn't mentioned that it was a sea urchin, it could easily have been mistaken for the internals of an alien. Goes to show you how alien-like terrestrial critters can be (or how terrestrial alien designs actually are).