ABSTRACT
The digestive epithelium consists of a simple columnar to a cuboidal epithelium, according to the region of the gut. The same categories of mucus cells occur in irregular and regular echinoids. Mucus cells are absent in both clypeasteroids and spatangoids. The presence of a second siphon in some species might be related to the requirement for a stronger water circulation to move the sediment through the gut. A well-developed siphon is present seemingly in all irregular echinoids and in most regular echinoids except the primitive forms in which a siphonal groove occur only. In irregular echinoids, which are primarily deposit-feeders, the food bolus consists of more or less compacted sediment. The stomach recurves at radius C, becoming the intestine. The intestine runs clockwise, lying above the stomach all along the latter’s length. The intestine stops at interradius CD, where the gut becomes a narrow vertical rectum which opens externally through the anus.
