Radioles or spines … are the appendages articulating upon the tubercles. The different parts of the spines are the socket by which it is articulated to the tubercles; the lower part of the radiole is called the head, and is separated from the neck, which is usually smooth or finely striated, by the milled ring,—a prominent ridge more or less deeply grooved, serving as an attachment for the muscles which are to move the spine; beyond the neck we have the body or shaft of the spine.