What kind of Crustacean is on the Moon Card?

Laura Borealis

The IJJ is fascinating... what ARE those objects off to the sides? Detached claws? (but the crayfish still has its own....) scorpion stingers? (an odd mix of Scorpio with Cancer...) snails? Escargot!

There's something weird with the musician's legs as well, but that's a topic for another thread I suppose. :)

As for the crayfish on the RWS and older decks: maybe I'm just being simplistic, but I go with the old interpretation that it indicates the water sign Cancer, which is ruled by the Moon. In modern times we usually see Cancer as a crab; historically, it is sometimes a crab but sometimes another crustacean.
Albumasar writes of this sign in the work published in 1489 as a large crayfish. Jakob Bartsch and Stanislaus Lubienitzki, in the 17th century, described it as a lobster. (Wikipedia)
The example card from an Egyptian-style deck with a scorpion instead... that seems to me to be an artist/tarot creator's personal deviation on the usual symbolism. I would have used a scarab instead. It's just as Egyptian and has the added benefit of also being associated with the constellation Cancer.

ETA: I've never really delved into the Egyptian style decks before and was unaware of the extent of their influence on the occult decks. This article gives a good overview. The 1901 book Practical Astrology was written by Edgar de Valcourt-Vermont under the pen name Comte de Saint-Germain. The majors he used actually came from the 1896 Falconnier and Wegener tarot, which in turn was based on Egyptianized descriptions by Paul Christian in 1863 and 1870. "Christian’s presentation of the Tarot is filled with fictitious history, but his stories were taking hold during this time of growing European interest in all things ancient and occult... much of Christian’s elaborate symbolism would find its way into the occult decks that were soon to follow, including those by Falconnier, Papus, Wirth, and even Waite."

In the quoted article, Mr. Filipas uses The Chariot as his example, so it doesn't shed light on the arthropod in question, but does give an idea of the background and influence of these decks. Thank you for bringing this topic to light, lucifall -- it's very interesting.