Note 2 — Chapter VII · SEA-STARS AND SEA-URCHINS.
Reading edition
Dr. Andrew Wilson, in his “Glimpses of Nature,” impresses upon his readers this “pentarchy” in the building of an Echinus, but curiously describes the plates as being six-sided. He evidently had none but living specimens to refer to when he thus wrote. It is only in a specimen from which the spines and skin have been carefully cleaned that the form of the plates can be seen.