You don't see the big peanut shaped part in the tank since it is usually inside the rock. The feeding part extends out. Here is what one of mine looks like:
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According to what I have read,Fenner & Calfo as well as Shimek, it is beneficial. I have several and they stay in one place mostly coming out to feed on detritus at night although the ones in the shadows of the rock will feed while the lights are on. Here's an article by Dr Shimek that covers them in detail: