This is a cidaroid sea urchin. They live around Antarctica, apparently finding the waters there quite pleasant, thank you - but still brood their babies in a pouch-like hollow on their underside, next to the mouth. Very cute.
(Of course, they're not the only animals to have invented a brood pouch. Marsupials, for example. Also sea horses, pipefish, certain unusual frogs, various kinds of isopod, and trilobites, some of whom had a marsupium in their heads.)
I must here enlighten you about certain details of the sea-urchin internal anatomy - apart from the test and the five-fold jaws called the Aristotle's Lantern, the inside of a healthy sea urchin is pretty full with gonads - a ring of ten of them, joining up to gonopores that open on the top side of the urchin. Were it not for the gonads, the inside of a sea-urchin would look suspiciously like the inside of the new TARDIS console room , complete with roundels where the spines are attached. There's certainly plenty of tubes and wires, since echinoderms use a system of hydraulic tubes in place of muscles.
Interesting fact! Sea Urchin gonads are highly edible, much sought after, and contain the cannabinoid anandamide
But onto the parasite - at least 3 species of these psychrophilic echinoderms are prey to the strange parasite, Echinophyces mirabilis Mortensen 1909. We don't know what it is - it's described as "being of uncertain affinities" which you may know is biologist-speak for "your guess is as good as ours". The best guerss so far is that it's a filamentous fungus. Altho specimens were recently collected so the scientific community can crack open its genes and see what comes out.
Echinophyces produces the drastic effect of turning the reproductive system of the urchin host upside down. In infected urchins the gonoduct is present but atrophied, and no gonopores exist on the genital plates. Instead, another, functional, gonoduct extends from the ovaries down to gonopores around the mouth.
And this makes the baby urchins MORE LIKELY TO SURVIVE - because the embyros don't have to be carefully transferred from the top down the sides to the pouch.
You have to admit this is a pretty drastic change - it's not the only parasite that does horrible things to your reproductive organs - I'll cover Wolbachia later, and just wait till I get to the parasitic barnacles and scrotum maggots - but Echinophyces is the only one I know about that can make a working vagina appear on top of your head.