|The inside of a sea urchin is dominated by a large, coiled digestive system which consists basically of a tube joining the lower mouth to the anus on the upper surface. In regular echinoids the mouth opens into an oesophagus (oe) that initially runs through the centre of the Aristotle's lantern. The oesophagus widens into the stomach (st), which then runs anti-clockwise as a loop. Along the entire inner margin of the stomach in most echinoids is a channel, the siphon (si), that either forms a pinched-off groove or a separate tube. The function of the siphon is that it allows water, taken in at the mouth, to bipass the stomach. The stomach recurves near the anterior and becomes the intestine (in) which then does a second complete loop. The digestive system then narrows into a short rectum (re) that leads to the apical anus. The digestive system is suspended from the interior of the test by thin mesenteric strands and there are also mesenteric strands connecting the various loops together.|
In addition to the digestive system, the interior of the test also houses the gonads (go) and the water vascular system (rwv). In regular echinoids there are five gonads, each connected to the exterior by a small opening in the apical disc termed the gonopore. In irregular echinoids the number varies from between two and five. The gonads lie along the centre of each interambulacral zone and are suspended from the interior of the test by thin mesenteric strands.
The water vascular system comprises a circular canal surrounding the eosophagus where it exits the lantern. From this a single tube rises to a perforated plate in the apical disc, the madreporic plate. This is the stone canal, so called because of its spiculated walls. Five other tubes, the radial water vessels (rwv) also branch off the circular canal and pass up the centre of each ambulacrum. Side branches from these vessels give rise to the external tube-feet and their internal ampullae.
The internal organization of organs in irregular echinoids is basically the same. In heart urchins, like Echinocardium shown below, there is an additional caecum (ca) that contains digestive glands.
For a detailed description of digestion and the digestive system in echinoids, see Jangoux, M. and Lawrence, J. M. 1982. Echinoderm Nutrition. A. A. Balkema, Rotterdam.
Upper diagram. Views of interior of lower half (left-hand side) and upper half (right-hand side) of the test of a regular echinoid Glyptocidaris crenularis. Note that the digestive system has been cut through at the point where the stomach recurves and changes into the intestine. From Clark, H. L. 1912. Hawaiian and other Pacific Echini. Memoirs of the Museum of Comparative Zoology, Harvard College, 34, 213-383, pls 90-120 (pl. 90, figs 5, 6).
Lower diagram. A, Interior of the test of the heart urchin Echinocardium cordatum showing the digestive system from the aboral side. B, Digestive system unfolded. From De Ridder, C. and Jangoux, M. 1982. Digestive systems: Echinoidea. Pp. 213-234 in Jangoux, M. and Lawrence, J. M. (eds) Echinoderm Nutrition. A. A. Balkema, Rotterdam.