Sunday Rest: urochord.

 A urochord is nothing to do with either urine nor with entries for the Eurovision Song Contest.

So that's a relief.

My Collins Dictionary tells me, entirely correctly, that a urochord is the notochord of a larval tunicate, typically confined to the tail region.

...which gets me not much further forward. And further research reveals that Merriam-Webster is, if anything, even less helpful:

the notochord of an ascidian or tunicate, more conspicuous in the larva than in the adult and confined chiefly to the caudal region

which even manages to replace the simple word tail with the obscure caudal region.

Even more research revels that a notochord is a forerunner of a spine, and is found both in the most primitive of those creatures which have a nervous system based on a string-like structure, and in the embryos of higher vertebrates, from which the spine later develops. 

(A tunicate is a sea-creature, an example being the sea-squirt; an ascidian is the same kind of thing.)

The difference between a notochord and a urochord seems to be that a notochord goes up into the head, and a urochord doesn't.

But I'm not completely sure, even now, and I shall be leaving this word to the experts.

Sunday Rest: urochord. The uro- bit means tail, from the Greek oura. The -chord bit is from the Greek khordē, which means cord, as in string.