Worried About my Brittle Star

[Midus]

Bought it around 3 days ago and he seems to have been doing pretty well. Just took a look at him(Turned the lights off an hour or so ago and he came out fully) and I noticed that hes missing "skin" off of his center. I can see his orangish colored insides with white lining around it. I at first thought it was a mouth, but its on the top. Around half of the top is like this when It was completely whole earlier this morning.

 

[Midus]

Hmmm, just turned the light on. I can see through the whole on top through what I guess is the mouth on the bottom. As soon as I turned on the light, he moved a few legs to cover the hole.