The circular muscle sheet is continuous across each interradius
, but is interrupted by a strip of radial, smooth muscle at each perradius.
The oral surface of the dwarf male is pressed against the oral surface of the larger female, and the arms of the male cling to the female at the interradius
The gonoduct exits anteriodorsally from the gonad basis and leads to single or multiple external gonopores located between or near the tentacles in interradius
In Eucidaris tribuloides, we found that the radial nerve sends off two additional nerve cords, that we name jaw nerves, in each interradius